NORMAL EXTENSION AND RETRACTION - DESCRIPTION AND OPERATION

** ON A/C NOT FOR ALL

1. General
The Landing Gear (L/G) normal extension and retraction system has electrical control and hydraulic operation, to extend and retract the Main Landing Gear (MLG) and doors (Ref. AMM D/O 32-10-00-00) and the Nose Landing Gear (NLG) and doors (Ref. AMM D/O 32-20-00-00). The electrical control system has:

a L/G control-lever
two Landing Gear Control and Interface Units (LGCIU)
a gear electro-hydraulic selector valve
a door electro-hydraulic selector valve
a L/G electro-hydraulic safety valve
32 proximity sensors and their related targets
a set of indicator lights (Ref. AMM D/O 32-61-00-00).

The electrical control system has two sub-systems, SYSTEM 1 and SYSTEM 2 which operate independently of one another. Each sub-system has:

an LGCIU
16 proximity sensors (and their related targets)
isolated electrical supplies.

The SYSTEM 1 gets electrical power from the Essential busbar and the SYSTEM 2 gets electrical power from the Normal busbar.

Each sub-system connects to the L/G control lever and to the gear and door selector valves. To keep the two sub-systems electrically isolated, each has its own electrical connections. The selector valves have two solenoid windings, for extension of the L/G and two for retraction of the L/G. The L/G control lever has two sets of switches for each selection position.
The SYSTEM 1 and the SYSTEM 2 sub-systems connect to the same solenoid (for the baulk mechanism) on the L/G control lever. They have diode protection.

The primary hydro-mechanical components in the normal extension and retraction system are:

three L/G actuating cylinders
three door actuating cylinders
three gear uplocks
three door uplocks
three door by-pass valves (door ground opening function)
a NLG downlock release actuator
two MLG lockstay actuating cylinders (Ref. AMM D/O 32-11-00-00).

The Green hydraulic system (Ref. AMM D/O 29-11-00-00) supplies the hydraulic power to operate the landing gear system.
When the L/G control lever is moved, the LGCIU sends a signal to the electro-hydraulic valve assembly. The LGCIU makes the selections of the valve assembly to retract/extend the L/G and move the doors in a given sequence. The proximity sensors send signals to the LGCIU to make sure that the L/G operates in the correct sequence.
The Engine/Warning Display (Ref. AMM D/O 31-66-00-00) and the Systems Display WHEEL page (which together make the ECAM) show system data. The L/G downlock indicator lights show:

green when the L/G is extended and locked down
red when the L/G is unlocked.

The red light on the L/G control-lever 6GA comes on if the aircraft is in a landing configuration and the gear is not locked down.

The Master Warning/Caution lights and the aural warning also give indications of specified failures (Ref. AMM D/O 32-61-00-00).

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2. Component Location

Cockpit - Component Location ** ON A/C NOT FOR ALL

Main Landing Gear and Doors ** ON A/C NOT FOR ALL

MLG - Component Location ** ON A/C NOT FOR ALL

MLG - Component location ** ON A/C NOT FOR ALL

NLG - Component Location ** ON A/C NOT FOR ALL

Landing Gear Control-Lever ** ON A/C NOT FOR ALL

Safety Valve ** ON A/C NOT FOR ALL

NLG Door Closing/Opening Safety Valves ** ON A/C NOT FOR ALL

Landing Gear Control and Interface Unit (LGCIU) ** ON A/C NOT FOR ALL

Cockpit - Component Location ** ON A/C NOT FOR ALL

MLG - Component location ** ON A/C NOT FOR ALL

NLG - Component Location ** ON A/C NOT FOR ALL

Landing Gear Control-Lever ** ON A/C NOT FOR ALL

Safety Valve ** ON A/C NOT FOR ALL

NLG Door Closing/Opening Safety Valves ** ON A/C NOT FOR ALL

Landing Gear Control and Interface Unit (LGCIU) ** ON A/C NOT FOR ALL

Main Landing Gear and Doors ** ON A/C NOT FOR ALL

MLG - Component Location ** ON A/C NOT FOR ALL

Landing Gear Control and Interface Unit (LGCIU)

FIN	FUNCTIONAL DESIGNATION	PANEL	ZONE	ATA REF
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5GA1	LGCIU-1	93VU	121	32-31-71
5GA2	LGCIU-2	94VU	122	32-31-71
6GA	LEVER-L/G NORM CTL	400VU	212	32-31-11
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8GA	PROX SNSR-R L/G UPLK, SYS 1		148	32-31-73
9GA	PROX SNSR-L L/G UPLK, SYS 1		147	32-31-73
10GA	PROX SNSR-R L/G UPLK, SYS 2		148	32-31-73
11GA	PROX SNSR-L L/G UPLK, SYS 2		147	32-31-73
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12GA	PROX SNSR-NLG UPLK, SYS 1		124	32-31-73
13GA	PROX SNSR-NLG UPLK, SYS 2		123	32-31-73
14GA	PROX SNSR-R L/G DNLK, SYS 1		148	32-31-73
15GA	PROX SNSR-L L/G DNLK, SYS 1		147	32-31-73
16GA	PROX SNSR-R L/G DNLK, SYS 2		148	32-31-73
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17GA	PROX SNSR-L L/G DNLK, SYS 2		147	32-31-73
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18GA	PROX SNSR-NLG DNLK, SYS 1		711	32-31-73
19GA	PROX SNSR-NLG DNLK, SYS 2		711	32-31-73
20GA	PROX SNSR-R L/G EXT, SYS 1		148	32-31-73
21GA	PROX SNSR-L L/G EXT, SYS 1		147	32-31-73
22GA	PROX SNSR-R L/G EXT, SYS 2		148	32-31-73
23GA	PROX SNSR-L L/G EXT, SYS 2		147	32-31-73
24GA	PROX SNSR-NLG EXT, SYS 1		711	32-31-73
25GA	PROX SNSR-NLG EXT, SYS 2		711	32-31-73
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26GA	PROX SNSR-R L/G DOOR CLOSED, SYS 1		148	32-31-73
27GA	PROX SNSR-L L/G DOOR CLOSED, SYS 1		147	32-31-73
28GA	PROX SNSR-R L/G DOOR CLOSED, SYS 2		148	32-31-73
29GA	PROX SNSR-L L/G DOOR CLOSED, SYS 2		147	32-31-73
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30GA	PROX SNSR-NLG DOORS CLOSED, SYS 1		123	32-31-73
31GA	PROX SNSR-NLG DOORS CLOSED, SYS 2		123	32-31-73
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32GA	PROX SNSR-R L/G DOOR OPEN, SYS 1		148	32-31-73
33GA	PROX SNSR-L L/G DOOR OPEN, SYS 1		147	32-31-73
34GA	PROX SNSR-R L/G DOOR OPEN, SYS 2		148	32-31-73
35GA	PROX SNSR-L L/G DOOR OPEN, SYS 2		147	32-31-73
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36GA	PROX SNSR-NLG R DOOR OPEN, SYS 1		124	32-31-73
37GA	PROX SNSR-NLG L DOOR OPEN, SYS 1		123	32-31-73
38GA	PROX SNSR-NLG R DOOR OPEN, SYS 2		124	32-31-73
39GA	PROX SNSR-NLG L DOOR OPEN, SYS 2		123	32-31-73
49GA	SAFETY VALVE-L/G SYS ISOLATION		140	32-31-17
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2505GM	SIDE STAY ASSY-MLG, L		731	32-11-16
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2520GM	UPLOCK ASSY-MLG DOOR, R	744	148	32-31-33
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2503GM	ACTG CYL-MLG, L		731	32-31-46
2504GM	ACTG CYL-MLG, R		741	32-31-46
2506GM	SIDE STAY ASSY-MLG, R		741	32-11-16
2509GM	UPLOCK ASSY-MLG, L		140	32-31-44
2510GM	UPLOCK ASSY-MLG, R		140	32-31-44
2517GM	BYPASS VALVE-MLG DOOR GROUND OPENING, L		140	32-31-13
2518GM	BYPASS VALVE-MLG DOOR GROUND OPENING, R		140	32-31-13
2519GM	CHECK VALVE B		147	32-31-36
2521GM	UPLOCK ASSY-MLG DOOR, L		147	32-31-33
2522GM	ACTG CYL-MLG DOOR, R		744	32-31-35
2523GM	ACTG CYL-MLG DOOR, L		734	32-31-35
2524GM	MANIFOLD ASSY-L/G AND DOOR SELECTOR VALVES		140	32-31-31
2525GM	CHECK VALVE B		147	32-31-36
2527GM	ACTG CYL-NLG		711	32-31-22
2529GM	CHECK VALVE B		147	32-31-36
2530GM	UPLOCK ASSY-NLG		123	32-31-21
2531GM	ACTG CYL-NLG DOOR		124	32-31-16
2533GM	BYPASS VALVE-NLG DOOR GROUND OPENING		120	32-31-15
2534GM	UPLOCK ASSY-NLG DOOR		123	32-31-14
2535GM	RESTRICTOR VALVE-NLG DOOR OPENING		123	32-31-57
2536GM	RESTRICTOR-NLG DOOR CLOSING		123	32-31-58
2537GM	CHECK VALVE B		147	32-31-47
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2505GM	SIDE STAY ASSY-MLG, L		210	32-11-16
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2520GM	UPLOCK ASSY-MLG DOOR, R		148	32-31-33
** ON A/C ALL
2629GM	SAFETY VALVE-NLG DOOR CLOSING		120	32-31-19
2630GM	SAFETY VALVE-NLG DOOR OPENING		120	32-31-19
2691GM	RESTRICTOR VALVE-NLG EXTENSION		124	32-31-56

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3. System Description
The MLG and NLG are hydraulically operated and retract into bays in the fuselage. When the L/G is retracted, the L/G doors and the fairings align with the adjacent structure to make an aerodynamic contour.

The MLG retracts inboard, into the bays to the rear of the wing spar box, and into the fuselage (to the rear of the hydraulics bay).

The NLG retracts forward into a bay in the fuselage.
The L/G normal extension and retraction system divides into these parts:

an electrical system
a hydraulic system
the mechanical components.

A. Electrical System

L/G Electrical Control - Block Diagram

The electrical system controls the operation of the L/G extension and retraction sequences. The system has two independently connected sub-systems, identified SYSTEM 1 and SYSTEM 2, that are electrically isolated from each other. Each sub-system contains an LGCIU and 16 proximity sensors. Each sub-system connects to:

the L/G control lever (6GA)
the L/G door selector valve (41GA)
the L/G selector valve (40GA)
the L/G isolation safety valve (49GA).

The two LGCIUs (which are the same) are identified as LGCIU1 and LGCIU2, and can be in SYSTEM 1 or SYSTEM 2. The LGCIU in SYSTEM 1 independently controls the operation of a circuit for the L/G downlock indicator lights. When an LGCIU is installed in SYSTEM 2 the independent circuits for the L/G downlock indicator lights are not used.

The LGCIU's are continuously supplied with power, but only one LGCIU controls the extension/retraction sequence at one time. The control changes from one LGCIU to the other after each retraction/extension cycle (when the L/G control lever is moved away from the DOWN position) or when one LGCIU becomes unserviceable.
The LGCIU gets position signals from the proximity sensors and the L/G control lever. It uses these position signals to set the L/G door and the L/G selector valves in the correct position. The LGCIU calculates the necessary door and gear configuration signals and compares them with the configuration requested from the L/G control lever. It then sends the necessary signals to operate the selector valves.
The LGCIU has secondary functions to:

control the operation of the baulk solenoid in the L/G control lever
supply L/G and door position and system configuration data to other systems
interface with the Centralized Fault Display System (Ref. AMM D/O 31-30-00-00) and the Central Warning System (Ref. AMM D/O 31-50-00-00)
monitor the control circuits to find failures and do automatic tests (BITE).

The proximity sensors are electronic position indicators and are installed on the mechanical components of each L/G. There are two sensors for each function. One sensor sends signals through SYSTEM 1 and the other sends signals through SYSTEM 2. The sensors show the configuration of these components:

each L/G uplock
each L/G downlock
each L/G door uplock
each L/G door (open position).

The L/G control lever has two positions, UP to retract the L/G, and DOWN to extend the L/G. Signals are sent to the LGCIUs when a selection is made. The L/G control lever has a baulk device which stops movement of the lever to the UP position, when the aircraft is on the ground. One of the LGCIUS sends a signal to release the baulk device, to let the L/G control lever move (to the UP position). It is not necessary for the LGCIU to be in control when it sends the signal.
The L/G isolation safety valve, the L/G selector valve and the L/G door selector valve are solenoid operated valves in the hydraulic system. Signals from the LGCIUs control the operation of the selector valves. A signal from the Air Data/Inertial Reference System (ADIRS) (Ref. AMM D/O 34-10-00-00) and the L/G control lever, controls the operation of the safety valve.

B. Hydraulic System

The hydraulic system has:

actuators that move the mechanical L/G components
electro-hydraulic valves to control the operation of the actuators
mechanically operated valves to let the doors to be opened on the ground
mechanically operated valves to operate the free fall extension system.

Each L/G has these actuators:

a L/G retraction-actuator
a L/G downlock actuator
a L/G uplock actuator
a door actuator
a door uplock actuator.

Pressurized hydraulic fluid is sent to the actuators through the electro-hydraulic valves to move the related mechanical components. The electro-hydraulic valves that follow are installed in the system:

a L/G isolation safety valve 49GA
a selector valve and manifold assembly 2524GM, which includes a L/G door selector valve 41GA and a L/G selector valve 40GA.

The sequence of operations is controlled by the LGCIU.
The L/G isolation safety valve 49GA is a two-position valve that isolates the L/G hydraulic supply from the Green hydraulic system, (when its solenoid is de-energized). This stops extension of the L/G when the aircraft speed is more than 264 kt to prevent damage.
The selector valve and manifold assembly 2524GM controls the flow of hydraulic fluid to:

the door open lines
the door close lines
the L/G extend lines
the L/G retract lines.

The L/G door selector valve 41GA and the L/G selector valve 40GA operate independently from each other.

To open the L/G doors (for access on the ground) each L/G bay has a mechanically-operated door-bypass valves. These valves isolate the door actuator from the doors-close pressure-line and hydraulically connect the two sides of the actuator together. This prevents movement of the doors if the system is pressurized, and makes the system safe during maintenance. The doors will only move, if the ground-opening-handle is moved to the closed position.
To let the L/G extend (with gravity), the Free Fall Extension System (Ref. AMM D/O 32-33-00-00) has mechanically operated cut-out and vent valves to prevent a hydraulic fluid lock in the actuators.

The system also has safety valves and restrictor valves.
The safety valves keep hydraulic fluid upstream of the safety valve if a leak occurs downstream.

The restrictor valves are installed in the selector valve and manifold assembly to control the rate of fluid flow through the L/G extend lines.

C. Mechanical Components

Each L/G has these related mechanical components:

a L/G uplock
a L/G door uplock
a L/G downlock.

The L/G uplocks are installed in each L/G bay to lock the L/G in the retracted position. The MLG uplocks have spring struts which keep the uplock in a set position. They also decrease the loads on the airframe structure when the MLG uplock pin goes into the lock.
The L/G door uplock locks the L/G doors in the closed position. The L/G doors can be opened for maintenance on the ground (Ref. AMM D/O 32-12-00-00) and (Ref. AMM D/O 32-22-00-00).

The lockstay locks the L/G in the extended position (Ref. AMM D/O 32-11-00-00) and (Ref. (Ref. AMM D/O 32-21-00-00). The lockstay holds the L/G in the extended position because of its geometric (overcenter) position.

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4. Power Supply

A. Electrical Power Supply

Normal Extension and Retraction - Block Diagram

B. Hydraulic Power Supply

The Green Main Hydraulic-Power System (Ref. AMM D/O 29-11-00-00) supplies hydraulic power to operate the system.

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5. Interface

Normal Extension and Retraction System - Interface

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6. Component Description

A. L/G Control Lever FIN: 6-GA

Landing Gear Control-Lever

The L/G control lever is installed on the First Officer's side of the center instrument panel to control the L/G extension and retraction (normal extension and retraction mode). The front face of the unit has a control lever and two arrows identified 'UP' and 'DOWN'.
The control lever has a knurled wheel on the end of an arm. The arm moves in a slot in the face of the unit. Internal locks in the unit keep the arm in the fully UP or the fully DOWN position. The arm moves switches inside the unit, which supply electrical signals to the LGCIUs. A solenoid-operated baulk mechanism prevents an UP selection if the L/G shock absorbers are not fully extended. The LGCIUs supply 28V DC to energize the baulk solenoid and release the mechanism, when the shock absorbers are fully extended. Internal white lighting shows through the UP and DOWN legends and the outline of the two arrows.
To operate the L/G control lever, you pull it away from the face of the unit. Then move it to the UP or DOWN position. The lower part of the DOWN arrow shows red when the aircraft is in a landing configuration (but the L/G is not locked down).

B. Safety Valve 49GA

Safety Valve

Safety Valve - Schematic

The primary components of the valve are:

a valve body
a solenoid and electrical connector
a pilot valve.

The safety valve is installed on a manifold in the RH MLG bay.

The pilot valve is a spool valve that is connected to the main valve body by two screws.

The safety valve body has three ports that are identified A, B and C. The valve body has holes to connect the ports and faces to install the solenoid and connector.
The safety valve is an electrically-operated, two-position valve that isolates the Green hydraulic supply to the L/G system. When the computed airspeed is more than 264 kts the safety valve closes (solenoid de-energized). The safety valve opens (solenoid energized) when the computed airspeed is less than 260 kts and the L/G control-lever is selected DOWN.

A signal from LGCIU-1 (5GA1) keeps the safety valve open during maintenance. The LGCIU sends this signal when the LH and RH MLGs are compressed or, when ground power is connected to the aircraft.

The safety valve gets its electrical supply from the essential busbar (401PP), during maintenance it can be supplied from the ground service busbar (601PP).
With the solenoid energized the pressure inlet A is connected to the supply outlet C and the return outlet B is closed. This connects the Green hydraulic system to the L/G system.

With the solenoid de-energized the pressure inlet A is closed and the supply outlet C is connected to the return outlet B. This isolates the Green hydraulic system from the L/G system.

C. Selector-Valve Manifold-Assembly (2524GM)

Manifold Assembly - Selector Valve (2524GM)

Selector Valve - Schematic

Restrictor Valve (2538GM)

Check Valve (2537GM)

The selector valve manifold assembly is an electrically-operated hydraulic unit that controls the flow of hydraulic fluid to and from the L/G hydraulic components. The primary components of the unit are:

a manifold 2524GM which contains a restrictor valve 2538GM, and a check valve 2537GM
two selector valves 41GA and 40GA. The selector valve 41GA controls the operation of the L/G doors and the selector valve 40GA controls the operation of the L/G.

The two selector valves are mounted on the manifold block. Each valve has two operating solenoids, each with two coil windings, one for each of the L/G control sub-systems.

When electrical or hydraulic power is not available the solenoids are de-energized and the spool is centralized by spring tension.

(1) Manifold Block

The manifold has drillings which connect to external ports and hydraulic connectors. Other drillings connect specified ports together. The manifold has two faces on which the selector valves are installed. The faces have fluid ports which connect to the selector valve ports (annuli).
To centralize the spool, hydraulic pressure is supplied to the pressure port and passes through annuli 3 and the pilot valve to the two end caps. Clearances between the spool lands and the main valve annuli 2 and 4 connect the service ports to the return port.

(a) Solenoid

When the door solenoid 'A' is energized, the ball valve moves away from its seat to close the pilot valve orifice. Hydraulic fluid is then sent through annuli 5 to return. Because the pressures in the end caps are different, the spool moves to the right to open annuli 4. Ports 'D' and 'L' are connected to the pressure supply directly across the open spool. The return from the service ports 'C' and 'K' goes through annuli 2 and annuli 1 to the return gallery. The return gallery connects to annuli 5 and the pressure return line.
When the door solenoid 'A' is de-energized and hydraulic pressure is available, the ball valve is moved on to its seat to close the valve. This stops the flow of return fluid. Hydraulic fluid is then sent through the pilot valve to the end cap. The spring tension and fluid pressure return the spool to the central position. The two solenoids operate the same.

(2) Restrictor Valve (2538GM)

The restrictor valve decreases the fluid pressure to decrease the MLG extension speed. When the MLG is retracted the restrictor valve lets the return fluid flow fully.
The restrictor valve has a cylindrical body with threaded connections. The body holds the valve plunger and a spring. The spring pushes the valve plunger onto an internal seat in the valve body. The valve plunger has holes to let fluid flow through it.

Hydraulic fluid pressure (in the direction 'X') moves the valve plunger away from the seat and lets the fluid flow through the body. When fluid flows in the opposite direction, the valve closes and lets fluid flow only through the valve plunger. This decreases the flow through the body.

(3) Check Valve (2537GM)

The check valve keeps the supply pressure to the door actuator when the L/G is in operation.
The check valve has a cylindrical body which has hydraulic connections (of different dimensions) at each end. The body contains a spring that holds a poppet against a face in the body. When the check valve is in the closed position, the metal-to-metal contact between the poppet and the valve seat makes the internal seal.

The hydraulic fluid pressure on the inlet side of the valve pushes against the face of the poppet. When the pressure of the hydraulic fluid is more than the pressure of the spring, the poppet moves away from the body face. The hydraulic fluid passes through the valve and goes out through the outlet. If the pressure on the outlet side of the valve is more than that at the inlet side, the spring force closes the poppet. This makes sure that hydraulic fluid can only flow in one direction.

An arrow on the body shows the direction in which the fluid flows.

(4) Selector valve 41GA, 40GA

The selector valve is a three-position electrically-operated valve and has:

two solenoids A and B
two pilot valves
a spool valve which contains a spool, a sleeve and two springs.

Each solenoid, A and B, has two coil windings. Each coil winding is independently connected to two electrical connectors identified SYSTEM 1 and SYSTEM 2. Usually only one circuit is used, but one circuit can operate the solenoids if the other is not available.
When energized, each solenoid moves a plunger which operates a spool-type pilot valve. The pilot valve opens or closes a hole that connects to one end of the spool valve.

The spool valve has a sleeve in which a spool moves. Two springs keep the spool in the center position of the sleeve when there is no pressure in the valve.
The sequence of operation of the two selector valves during retraction and extension are as follows:

Landing gear UP selection

-------------------------------------------------------------------------------

ACTION EFFECT RESULT

-------------------------------------------------------------------------------

The door solenoid The pressure port is The doors open, which

'A' is de-energized, connected to the ports completes the electrical

and the door solenoid C and K. The ports D circuit to energize the

'B' is energized. and L are connected gear solenoid 'B'. The

to the return port. door solenoid 'B' stays

energized until all the

gears are locked in the

selected position.

The gear solenoid The pressure port is The gear retracts

'B' is energized. connected to the ports and locks up, which

E and M. The ports F completes the electrical

and N are connected circuit to energize the

to the return port. door solenoid 'A'.

The gear solenoid 'B'

stays energized until

all of the doors are

locked in the closed

position.

The door solenoid The pressure port is The doors close and

'A' is energized. connected to the ports lock. The door

D and L. The ports C solenoid 'A' stays

and K are connected energized and the

to the return port. gear solenoid 'B'

de-energizes.

Landing gear DOWN selection

-------------------------------------------------------------------------------

ACTION EFFECT RESULT

-------------------------------------------------------------------------------

The door solenoid The pressure port is The doors open, which

'A' is de-energized, connected to the ports completes the electrical

the door solenoid C and K. The ports D circuit to energize the

'B' is energized. and L are connected gear solenoid 'A'. The

to the return port. door solenoid 'B'

stays energized until

all the gears are locked

in the selected position.

The gear solenoid The pressure port is The gear extends and

'A' is energized. connected to the ports locks down, which completes

F and N. The ports E the electrical circuit to

and M are connected to energize the door solenoid

the return port. 'A'. The door solenoid 'B'

de-energizes.

The door solenoid The pressure port is The doors close and

'A' is energized. connected to the ports lock. The door solenoid

D and L. The ports C 'A' stays energized

and K are connected to whenever electrical

the return port. power is applied.

The gear solenoid 'A'

de-energizes as soon

as the first door

moves away from the

fully open position.

D. NLG Door Closing/Opening Safety Valves (2629GM, 2630GM)

NLG Door Closing/Opening Safety Valves

NLG Door Closing/Opening Safety Valve (2629GM, 2630GM)

Safety Valve (2629GM,2630GM) - Schematic

The safety valve is installed in the open and close lines between the NLG door-actuator and the L/G door selector-valves. The safety valve closes the line if there is a hydraulic leak.

The safety valve will only operate at a flow rate equal to or greater than 11 liter/minute.

(1) Valve Body

The valve has a body with two hydraulic ports B and C. The body is machined and holds the slide valve and valve seat. The end fitting (installed in the upper end of the body) holds:

the jet
the spring
the spring seat
the spring pin
the bleed screw.

The slide valve has drilled ports to let the hydraulic fluid move through the valve. The wire installed on the spring pin goes through the jet to prevent it from being blocked.
The spring pushes against the spring seat to keep the valve in the open position.

The valve seat is installed at the lower end of the valve body and is held in position with a screwed ring sealed by packing rings.

(2) Hydraulic Fluid

In usual operation, the hydraulic fluid goes into port B, goes through the slot in the slide valve and then goes out through port C.

When a leak occurs downstream of the safety valve (with a flow rate more than 11 liter/minute), the slot in the slide valve stops the increase of flow. This causes the valve to start to close. At the same time the spring is compressed and a cavitation effect in chamber A slows the movement of the slide valve.
The hydraulic fluid goes through the small hole in the slide valve and fills chamber A. When the chamber A is full, the effect of the cavitation is removed and the valve closes. The speed at which chamber A fills controls the speed at which the valve will operate.

When the NLG door-actuator is operated it momentarily increases the hydraulic flow rate and causes cavitation in chamber A. When this occurs, the spring returns the slide valve to the open position.
The safety valve will stay in the closed position when the supply pressure decreases to that of the Green system reservoir.

To reset the safety valve it is necessary to depressurize the Green system reservoir or to open the bleed screw installed on the end of the safety valve.

E. MLG Actuating Cylinder (2503GM,2504GM)

MLG Actuating Cylinder (2503GM,2504GM) ** ON A/C NOT FOR ALL

MLG Actuating Cylinder - Schematic

MLG Actuating Cylinder - Schematic ** ON A/C NOT FOR ALL

(1) Description

The MLG actuating cylinder is installed on the main fitting at the piston rod end. Two lugs attach the body of the actuating cylinder to the wing rear spar.

The actuator is operated hydraulically and has rate control devices. The primary components of the unit are:

a cylinder
a piston rod
two valve housings.

The cylinder contains the piston rod and a sealed piston head. Each valve housing contains a restrictor and a restrictor valve. The restrictor decreases the flow of fluid in each direction and the restrictor valve decreases the flow of fluid out of the cylinder.

(a) Actuating Cylinder

The body of the actuating cylinder has a housing bolted to each end that are connected by an external pipe. Each housing contains a restrictor and a restrictor valve. These components control:

the speed at which the actuating cylinder operates
the end of travel damping, which occurs in each direction of travel.

To give protection against the high fluid pressures (caused during end of travel damping) the walls of the actuating cylinder are thicker at the piston rod end.

(b) Piston Rod

The piston rod moves in the cylinder assembly and has:

a plug at the internal end
an integral eye-end (with a spherical bearing) at the external end
a vent valve.

If pressurized fluid gets into the piston rod, the vent valve blows out and prevents an increase in pressure in the piston rod. To prevent corrosion a small quantity of hydraulic fluid is put into the piston rod during assembly.
With hydraulic pressure supplied to:

the full area side of the piston, the piston rod extends and retracts the MLG
the annular side of the piston, the piston rod retracts and extends the MLG.

The piston rod range of travel is controlled by:

the side stay during the MLG extension cycle
the uplock during the MLG retraction cycle.

(c) Restrictor Valve

The restrictor valve is closed by hydraulic fluid flow when the L/G is operated. This decreases the flow of fluid and controls the speed of the piston rod. Near the end of the piston rod travel the piston closes a control orifice. This causes:

1 The remaining fluid to return through the closed restrictor valve to the hydraulic system return.

2 An increase in pressure (on the return side of the piston) which damps the end of the piston rod travel.

The shape of the piston causes a smooth increase in the damping effect.

(2) Operation

(a) Retraction Sequence (start)

When the piston rod is in the extended position, the piston blocks the control orifice of the restrictor valve at the piston rod end. The increased internal diameter of the cylinder prevents the orifice of the one-way restrictor being fully blocked.

When a selection is made to retract the piston rod, port A is open to the hydraulic system pressure and port B to the hydraulic system return. The hydraulic fluid enters port A and pressure is transmitted directly to the valve housing at the piston rod end. The fluid cannot go through the control orifice, so the valve of the one-way restrictor is moved to permit full fluid flow. The initial piston movement is slow because the control orifice of the restrictor valve is closed by the piston.

Once the piston has moved past the control orifice of the restrictor valve, the flow is increased and the piston accelerates.

(b) Damping (start)

At the other end of the retraction actuator, the hydraulic system return fluid goes through the piston end valve housing to port B. The fluid is forced through the control orifice of the restrictor valve and the orifice of the one-way restrictor valve, which is moved to decrease the fluid flow. As the piston blocks the control orifice, the fluid flow is decreased and then stopped. Fluid which returns through the one-way restrictor causes a pressure build-up in the piston end of the actuator. This decelerates the piston movement and dampens the end of travel stroke.

(c) Extension (start)

The two restrictor valve sub-assemblies on the retraction actuator are the same in operation. Thus the extension of the actuator is opposite to the retraction.

F. NLG Actuating Cylinder (2527GM)

NLG Actuating Cylinder (2527GM)

NLG Actuating Cylinder (2527GM) - Detail

NLG Actuating Cylinder - Schematic

(1) Description

The NLG actuating-cylinder is a two-acting type. The body has a valve housing at each end, each of which contains a restrictor.

The actuating cylinder assembly includes these components:

a piston-rod assembly
a cylinder assembly
a rod extension body
a rod retraction body
the hydraulic external fittings.

(a) Piston-Rod Assembly

The piston-rod assembly has:

a rod which moves in a spherical bearing
a piston with a seal.

The piston is connected to the rod by a lockwasher and a nut, which are locked with a bolt and a nut. The piston has a ring which prevents sideways movement of the piston on the rod.

The piston movement is kept in specified limits by a seal, and preformed packing and back-up rings.

(b) Cylinder Assembly

The cylinder assembly has:

a cylinder
a spherical bearing
a bearing
a retaining ring
a plate.

The cylinder has a spherical bearing at one end that connects the cylinder to the aircraft structure. The bearing, with a seal and scraper, is held in the opposite end of the cylinder with a ring and plate.
The exterior of the cylinder contains ports for the hydraulic connections.

(c) Extension Body

The extension body is installed on the upper end of the actuating cylinder. The body has three hydraulic unions and contains two internal annuli. One of these annuli is connected to the actuator-supply port. The other annuli contains the restrictor.

Two annuli in the body, aligned with two annuli in the cylinder, supply the hydraulic connection between the components.

(d) Retraction Body

The body assembly contains a hydraulic union and an annuli. The annuli contains a restrictor.

An annuli in the body, in line with an annuli in the cylinder, supplies the hydraulic connection between the components.

(e) External Hydraulic Fittings

The actuating-cylinder extension-body has connections for the extension and retraction hydraulic-fluid supply-hoses. The extension and retraction bodies are connected together by a hydraulic pipe attached to the cylinder assembly.

(2) Operation

(a) Retraction

To retract the NLG, pressurized hydraulic fluid goes through the restrictor into the annular side of the actuating-cylinder. This causes the piston rod to retract to move the NLG. The hydraulic fluid on the other side of the piston goes to the hydraulic system return through the main return orifice and the restrictor. As the piston gets near to the end of its travel, it closes the return orifice in the actuating cylinder. This decreases the flow of hydraulic fluid to the hydraulic system return, which damps the end of piston-rod travel.

(b) Extension

During the extension of the NLG, the aerodynamic loads, and the weight of the NLG, cause the NLG to extend. The restrictor on the annular side of the piston decreases the return flow of the fluid from the actuating-cylinder. This causes a damping-pressure on the annular side of the piston and thus controls the rate of NLG extension.
A restrictor valve (2691GM) keeps a limit on the damping-pressure. To do this, the valve decreases the flow of pressurized fluid to the extend side of the actuating-cylinder.

G. MLG Door Actuating Cylinder (2523GM, 2522GM)

MLG Door Actuating Cylinder (2523GM,2522GM)

MLG Door Actuating Cylinder - Schematic

(1) Description

The door actuator is a hydraulic actuator with rate control devices. The piston and piston rod are contained in a cylinder. A damping piston, on each side of the piston, goes into a damping bore at the end of the extend and retract strokes.

The door-actuating cylinder attaches to the center-fuselage keel-structure at the fixed end, and to the hinged door at the piston end.

(a) Actuating Cylinder

The actuating cylinder has a:

cylinder assembly
restrictor housing
piston.

The cylinder assembly has a cylinder closed at one end by a fixed end fitting. At the other end it is closed by a gland housing and a piston. The fixed end fitting and the gland housing are internally machined to shape damping chambers. The piston is shaped to go in the damping chambers at the end of the assembly.

(b) Damping Chamber

The damping chamber in:

the fixed end fitting has two diametrically opposite damping holes and two diametrically opposite axial feed slots
the gland housing has two damping holes that are axially displaced to give gradual damping.

The restrictor housing is installed on the fixed end fitting. It has two port connectors that contain restrictors to control fluid flow. A service line connects port B on the restrictor housing to the gland housing.
The actuating cylinders range of travel is controlled by:

the door stops and the door uplock when the door closes
an internal stop (the piston head touches the internal face of the gland housing) when the door opens.

(2) Operation

(a) Doors Closed

When main gear doors 'closed' is selected, hydraulic fluid from port B passes through the restrictor and the damping holes. The hydraulic pressure is applied to the piston flange and the annular face of the piston. The piston retracts slowly (due to the restricted fluid flow) until it opens the second damping hole which increases the piston speed. The speed increases until the piston is free of the damping chamber.

(b) Doors Open

When main gear doors 'open' is selected, hydraulic fluid from port A passes through a restrictor and the axial feed slots. At the same time the hydraulic fluid passes through the damping holes, the piston then extends slowly due to the restricted fluid flow. Piston speed increases until the piston goes into the gland-housing damping chamber.

(c) Damping

The end of stroke damping occurs in the two directions. The damping effect occurs when the piston head goes into the damping chamber, which causes the removed fluid to pass through the damping holes. On extension, the damping effect increases when one damping hole is closed by the piston.

H. NLG Door Actuating Cylinder (2531GM)

NLG Door Actuating Cylinder (2531GM)

NLG Door Actuating Cylinder - Schematic

(1) Description

The door actuator is a hydraulic actuator with rate control devices. The piston and piston rod are contained in a cylinder. The shape of the internal bore, the position of the return orifices, and the restrictor valve, control the damping.

(a) Actuating Cylinder

The NLG door actuating-cylinder is a two-acting type. A valve-housing on the body of the actuating-cylinder contains a check-valve. The movable part of the check-valve has a slot in the face that touches the valve seat.

The actuating-cylinder connects:

the door control-rod bellcrank at the piston-rod end
the roof of the landing gear bay at the fixed end.

(b) Restrictor

An in-line restrictor (2536GM) is installed in the line to the annular side of the actuating cylinder. It controls the speed at which the piston-rod operates.
The actuating cylinders range-of-travel is controlled by:

the NLG door uplock as the NLG doors close
an internal stop as the NLG doors open.

(2) Operation

(a) Doors Close

With the NLG doors set to CLOSE, the pressurized hydraulic fluid goes into the annular side of the actuating-cylinder. The piston-rod retracts and closes the NLG doors. On the other side of the piston, the hydraulic fluid goes to return through the main return orifice and the slot in the closed check-valve. Near the end of the piston-rod travel, the piston closes the main return orifice. This causes a large decrease in the flow of fluid to return, which damps the end of the piston-rod travel.

(b) Doors Open

With the NLG doors set to OPEN, the aerodynamic loads help the doors to open. The restrictor (2536GM) decreases the return flow of fluid from the actuating-cylinder. This causes a damping-pressure on the annular side of the piston to control the speed at which the NLG doors open. The restrictor-valve (2535GM) keeps a limit on the damping-pressure. To do this, the valve decreases the flow of pressurized fluid to the extend side of the actuating-cylinder.

I. MLG Uplock (2509GM,2510GM)

MLG Uplock (2509GM,2510GM)

MLG Uplock - Schematic

(1) Description

The MLG uplock is a mechanical device that automatically locks the main gear in the retracted position. The uplock is closed mechanically and opened hydraulically in the normal extension and retraction mode. The uplock can also be opened mechanically in the free fall extension mode (Ref. AMM D/O 32-33-00-00). The primary components of the uplock are:

a casing
a hydraulic actuator
two proximity sensors with their related targets
a locking mechanism
a free-fall mechanical-release mechanism.

(a) Casing

The casing has two halves, which are connected by bolts. The hydraulic actuator has a piston, and is connected to the casing. A spring keeps the piston retracted when there is no hydraulic pressure. The actuator has two hydraulic ports A and B. The casing has two holes for rigging pins and are positioned for these functions:

one pin hole is for a rigging check of the position of the mechanism for the Free Fall Extension System
one pin hole locks the uplock in the closed position and is for a rigging check of the proximity sensors.

(b) Proximity Sensors

The proximity sensors are attached to the casing and the targets to the latch assembly. They continuously supply an independent electrical signal to show the uplock configuration (uplock closed = 'target near' or uplock open = 'target far').

The primary components of the locking mechanism are a latch assembly and a hook. The latch assembly has a bearing and two targets. The bearing turns on a pin at its center, as do the latch assembly and the hook. Tension springs connect the latch assembly to the hook. The hook has a cam which touches the bearing.

The mechanical release mechanism has a release lever which is connected to a splined drive shaft. This drive shaft connects to the Free Fall Extension System (Ref. AMM D/O 32-33-00-00).

The uplock is connected to the airframe by a rod end and two support lugs.

(2) Operation

(a) Gear Extend

To move the L/G to the extended position, the L/G selector-valve is energized. Hydraulic pressure is then applied through port A. This extends the piston and compresses the actuator spring. The piston strikes the latch assembly, which pivots to release the bearing from the hook cam. The hook opens because of the tension springs and L/G pin forces, and touches the stop bolt. When the L/G has fully extended, the two hydraulic ports are opened to system return and the actuator spring releases the piston.

(b) Gear Retract

To lock the L/G in the retracted position, the L/G selector-valve is energized. Hydraulic pressure is applied through port B to make sure that the piston is fully released. The L/G retracts and the L/G pin strikes the upper hook jaw. This causes the hook to pivot and the bearing to move across the hook cam face. When the hook touches the stop bolt, the tension springs pull the latch assembly over the hook cam, this locks the L/G in position.

(c) Proximity Sensors

During the operation of the locking mechanism, the target to proximity sensor relationship changes. This positional change causes an uplock 'locked' or 'unlocked' signal to be supplied to the LGCIU's.

(d) Hydraulic Failure

When hydraulic pressure is not available, the uplock is released by mechanical operation of the release lever (free-fall system). When the free-fall extension-handle is rotated, the splined shaft rotates. Rotation of the splined shaft causes the release lever to release the bearing from the hook cam. The hook opens because of the tension springs and L/G pin-forces.

The reset of the extension handle resets the release lever (free-fall system), which returns the uplock to its 'normal' open position.

J. NLG Uplock (2530GM)

NLG Uplock (2530GM)

NLG Uplock - Schematic

(1) Description

The NLG uplock is a mechanical device that automatically locks the NLG in the retracted position. The uplock is closed mechanically and opened hydraulically in the normal extension and retraction mode. The uplock can also be opened mechanically in the free-fall extension mode (Ref. AMM D/O 32-33-00-00). The primary components of the uplock are:

a casing
a hydraulic actuator
two proximity sensors and their related targets
a locking mechanism
a free-fall mechanical-release mechanism.

(a) Casing

The casing has two halves which are connected with bolts. The hydraulic actuator has a piston, and is connected to the casing. A spring keeps the piston retracted, when there is no hydraulic pressure. The actuator has two hydraulic ports A and B.

Two internal attachment lugs are used to install the uplock to the airframe.

A rigging pin hole is provided for initial installation and subsequent adjustment of the free fall system.

(b) Proximity Sensors

Two proximity sensors are attached to the casing and the targets are attached to the lock lever. They continuously supply an independent electrical signal to show the uplock configuration (uplock closed = 'target near' or uplock open = 'target far').

(c) Lock Mechanism

The primary components of the lock mechanism are a lock lever and a hook. The lock lever has two bearings and two targets. Each bearing turns on a pin at its center. The lock lever and the hook also turn on pins at their centers. Springs tension the lock lever and the hook. The hook has a cam which touches the lower bearing on the lock lever.

The mechanical release mechanism has a release cam attached to a splined drive shaft. This shaft connects to the Free-Fall Extension System (Ref. AMM D/O 32-33-00-00).

(2) Operation

(a) Gear Extend

To move the L/G to the extended position, the L/G selector-valve is energized. Hydraulic fluid goes through port 'A' which extends the piston and compresses the actuator spring. The piston strikes the lock lever, which pivots and releases from the hook cam. The hook opens due to return spring and L/G pin forces. The two hydraulic ports are opened to system return, and the actuator spring releases the piston. When the L/G has fully extended, the hydraulic ports open (to system return) which causes the actuator spring to retract the piston.

(b) Gear Retract

To lock the L/G in the retracted position, the L/G selector-valve is energized. Hydraulic pressure is applied through port 'B', to make sure that the piston is fully retracted.

The L/G retracts and when the L/G pin strikes the upper hook jaw, it causes the hook to pivot to the closed position. The lock lever return spring causes the lock lever to pivot and engage the hook cam. This locks the hook in the closed position.
During the operation of the lock mechanism, the target to proximity sensor position changes. This positional change causes an uplock 'locked' or 'unlocked' signal to be supplied to the LGCIU's.
When hydraulic fluid pressure is not available, the uplock is released by the mechanical operation of the free-fall release-cam. When the free-fall extension-handle is rotated, it rotates the cam shaft. Rotation of the cam shaft causes the release cam to disengage the lock lever from the hook cam. The hook pivots open due to the return spring and land gear pin forces.

The reset of the extension handle, resets the release cam, which returns the uplock to its 'normal' open position.

K. MLG Door Uplock 5224GA(5225GA)

MLG Door Uplock (2520GM,2521GM)

MLG Door Uplock - Schematic

(1) Description

The MLG door uplock is a mechanical device that automatically locks the MLG doors in the closed position. The uplock is closed mechanically and opened hydraulically in the normal extension and retraction mode. The uplock can also be opened mechanically in Free Fall Extension (Ref. AMM D/O 32-33-00-00) and the Ground Door Opening (Ref. AMM D/O 32-12-00-00) modes. The primary components of the uplock are:

a casing
a hydraulic actuator
two proximity sensors and their related targets
a locking mechanism
a free fall mechanical release mechanism
a ground door opening release mechanism.

The door uplock is attached to the airframe by two integral mounting lugs.

Rigging pin holes are provided for initial installation and adjustment of the free fall/ground door opening systems.

(a) Casing

The casing has two halves which are connected by bolts. The hydraulic actuator has a piston, and is attached to the casing. A spring keeps the piston retracted when there is no hydraulic pressure. The actuator has two hydraulic ports A and B. The casing has three holes for rigging pins which are used to:

lock the uplock in the closed position to do a rigging check of the proximity sensors
permit a rigging check of the mechanism for the Free-Fall Extension System
permit a rigging check of the position of the mechanism for the Ground Door-Opening System.

(b) Locking Mechanism

The primary components of the locking mechanism are a latch assembly and a hook. The latch assembly has a roller and two (proximity) targets. The roller, the latch assembly and the hook turn on pins at their centers. Tension springs connect the latch assembly and the hook to the casing. The hook has a cam which touches the roller.

(c) Release Mechanism

Each mechanical release mechanism has a release lever connected to a splined drive shaft. One connects to the Free Fall Extension System (Ref. AMM D/O 32-33-00-00) and the other to the Ground Door-Opening System (Ref. AMM D/O 32-12-00-00).

(d) Proximity Sensors

The proximity sensors are installed on the casing, and the targets are installed on the latch lever. They continuously supply an independent electrical signal to show the uplock configuration (uplock closed = 'target near' or uplock open = 'target far').
In the open condition, the bearing on the latch assembly is against the top face of the hook cam. The tension springs hold the latch assembly and hook in this position. The targets are not near the proximity sensors.

(2) Operation

(a) Doors Open

To open the L/G doors, the L/G door selector-valve is energized. Hydraulic pressure is applied through port A, which extends the piston and compresses the actuator spring. The piston strikes the latch assembly, which pivots to disengage the roller from the hook cam. The hook opens, because of the tension springs and door pin forces, and touches the stop bolt. When the MLG doors are fully open, the two hydraulic ports open to system return and the actuator spring releases the piston.

(b) Doors Close

To close and lock the L/G doors, the L/G door selector-valve is energized. Hydraulic pressure is applied through port B, to make sure that the piston is fully released. The door closes and the door pin strikes the upper hook jaw. This causes the hook to pivot and the roller to move across the hook cam face. When the hook touches the stop bolt, the tension springs pull the latch assembly over the hook cam, and locks the L/G door in position.

(c) Proximity Sensors

During the operation of the locking mechanism, the target to proximity sensor position changes. This positional change causes an uplock 'locked' or 'unlocked' signal to be supplied to the LGCIU's.

(d) Mechanical Release

When hydraulic fluid pressure is not available, the door uplock is released by mechanical operation of the manual release lever (free-fall system). When the free-fall extension handle is rotated, it rotates the splined shaft. Rotation of the splined shaft causes the release lever to disengage the roller from the hook cam. The hook opens because of the tension springs and door pin forces.

The reset of the manual release lever returns the door uplock to its 'normal' open condition.

(e) Ground Door Opening

The ground-door opening-system uses an independent cam but operates the same as the free-fall system.

L. NLG Door Uplock 2534GM

NLG Door Uplock (2534GM)

NLG Door Uplock - Schematic

(1) Description

The NLG door uplock is a mechanical device that automatically locks the NLG doors in the closed position. The uplock is closed mechanically and opened hydraulically in the normal extension and retraction mode. The uplock can also be opened mechanically in the Free-Fall Extension (Ref. AMM D/O 32-33-00-00) and the Ground Door-Opening (Ref. AMM D/O 32-22-00-00) modes. The primary components of the uplock are:

a casing
a hydraulic actuator
two proximity sensors and their related targets
a locking mechanism
a free fall mechanical release mechanism
a ground door opening release mechanism.

Two integral attachment lugs attach the uplock to the airframe.

Rigging pin holes are supplied for initial installation and subsequent adjustment of:

the free fall system
the ground door opening system.
the proximity sensors.

(a) Casing

The casing has two halves which are connected by bolts. The hydraulic actuator contains a piston and is connected to the casing. A spring keeps the piston retracted, when there is no hydraulic pressure. The actuator has two ports A and B. The casing has holes for rigging pins, which carry out these functions:

a rigging check of the position of the mechanism for the Free-Fall Extension System
a rigging check of the position of the mechanism for the Ground Door-Opening System
a rigging check of the proximity sensors.

(b) Proximity Sensors

The proximity sensors are installed on the casing, and the targets are installed on the lock lever. They continuously supply an independent electrical signal to show the uplock position (uplock closed = 'target near' or uplock open = 'target far').

(c) Lock Mechanism

The primary components of the lock mechanism are a lock lever and a hook. The lock lever has two rollers and two targets. Each roller turns on a pin at its center. The lock lever and the hook also turn on pins at their centers. Springs tension the lock lever and the hook. The hook has a cam which touches the lower roller on the lock lever.

The mechanical release mechanisms have release cams attached to the splined drive shafts. The shafts connect to the Free Fall Extension System (Ref. AMM D/O 32-33-00-00) and the Ground Door Opening System (Ref. AMM D/O 32-12-00-00).

(2) Operation

(a) Doors Open

To open the L/G doors, the L/G door selector-valve is energized. Hydraulic pressure is supplied through Port A, to extend the piston and compress the actuator spring. The piston strikes the lock lever, which pivots and disengages from the hook cam. The hook opens due to return spring and NLG door pin forces.

(b) Doors Close

To close and lock the L/G doors, the L/G door selector-valve is energized. Hydraulic pressure is supplied through Port B, to make sure that the piston is fully released. The NLG doors close and when the door pin strikes the upper hook jaw, it causes the hook to pivot to the closed position. The lock-lever return-spring causes the lock-lever to pivot and engage the hook cam, which locks the hook in the closed position.

(d) Hydraulic Failure

When the hydraulic system is not available, the door uplock is released by the mechanical operation of the free-fall release-cam. When the free-fall extension-handle is rotated, it rotates the cam shaft. Rotation of the cam shaft causes the release cam to disengage the lock lever from the hook cam. The hook pivots open due to the return spring and door pin forces.

The reset of the free-fall release-cam (free-fall system) resets the uplock to its 'normal' open position.

(e) Ground Door Opening

The ground door opening system uses a separate release cam which operates in the same way as the free-fall release cam.

M. MLG Lockstay Actuator

MLG Lockstay Actuator

The lockstay actuator is a hydraulic unit with rate control devices. The main components of the actuator are:

a cylinder
a piston
a two way restrictor valve
a pressure relief valve.

The lockstay, that moves to an overcenter position, locks the L/G in the fully extended position.

A more detailed description and operation is given in (Ref. AMM D/O 32-11-00-00).

N. NLG Downlock Release Actuator

NLG Downlock Release Actuator

NLG Downlock Release Actuator - Schematic

(1) Description

The lockstay actuator is a double acting hydraulic actuator.

The lockstay, that moves to an overcenter position, locks the NLG in the fully extended position.

(2) Operation

(a) NLG Extension

When you extend the NLG , the two lockstay-actuator hydraulic-ports (A and B), are open to return. As the NLG extends, the two lock springs force the lockstay and drag strut to straighten. The lockstay is pulled to an overcenter position by the lock springs. When the NLG doors start to close, the full area of the lockstay actuator is pressurized, through port B. This makes sure that the lockstay is in the overcenter position. After the NLG has extended, the pressure remains to keep it in the locked position (if the landing-gear control-lever is in the DOWN position).

(b) NLG Retraction

When you retract the NLG, hydraulic pressure acts on the annular face of the piston, through port A. The actuator then retracts. As the actuator retracts, it breaks the overcenter lock, the lockstay and side stay fold into position against the tension of the locksprings.

(c) Restrictor Valve

The restrictor valve is installed in the actuator assembly , and decreases the speed of operation of the downlock actuator. This causes a decrease in the speed of operation of the lockstay and the NLG.

(d) Proximity Sensors

During the operation of the lockstay assembly, the related position of the target and proximity sensor changes. This change causes a NLG 'locked' or 'unlocked' signal to be supplied to the LGCIU's.

O. In-line Check Valves

Landing Gear System - In-line Check Valve

(1) General

There are five in-line check-valves installed in the L/G hydraulic system.

The check-valves have a cylindrical body, made of stainless steel or aluminum alloy, which contains a poppet and a spring. There are threaded hydraulic connections (of different dimensions) at each end of the body, that connect the check valve to the hydraulic system. An internal poppet is held against the valve seat (in the closed position) by a spring to make a metal-to-metal seal.

When the pressure on the inlet side is more than that on the outlet side, the poppet opens against the spring compression. When the pressure on the outlet side is more than (or equal to) that on the inlet side, spring compression keeps the poppet against the valve seat.

(2) MLG Free Fall Check-Valve 2519GM

The check valve is installed in a return line of the L/G system. This line connects the return line of the MLG Vent Valve (2516GM) to the extend line of the MLG Actuating Cylinders (2503GM/2504GM).

This check valve operates when free-fall extension of the L/G is selected. Its function is to prevent the cavitation of the MLG actuating cylinders. As the L/G extends :

(a) The pressure in the extend sides of the MLG actuating cylinders decreases below that of the retract sides.

(b) The check valve opens.

(c) The system fluid flows (through the open MLG vent valve) from the retract sides into the extend sides of the MLG actuating cylinders.

(3) L/G Return Isolation Check-Valves 2537GM (2748GM)

These check valves 2537GM (2748GM) are installed in the return lines of the L/G system that connect to the Green LP manifold system. If pipe lines in the Green LP system are damaged when an engine fails the check valves will close. This is caused by the drop in pressure in the Green LP manifold circuit. Operation of the check valves prevents loss of oil from the return lines of the L/G system.

(4) MLG Door Close Check-Valves 2525GM (2529GM)

Each check valve is installed in a MLG 'doors close' return line. These lines connect the MLG Vent Valve (2516GM) to the MLG Door Actuating Cylinders (2523GM/2522GM).

If the pressure increases in one of the MLG 'doors close' return lines, the check valve in the other 'doors close' return line closes. This isolates the 'doors close' return line of each MLG door actuating cylinder from the other.

During a free fall extension of the landing gear:

(a) The fluid removed from the 'doors close' side of the MLG door actuating cylinders causes the check valves to open.

(b) The open check valves let the removed fluid flow (through the open MLG vent valve) into the Green LP manifold circuit.

P. In-line Restrictor Valves

Landing Gear System - In-line Restrictor Valve

(1) General

There are two in-line restrictor valves installed in the L/G extension and retraction system. They are a NLG door-opening restrictor-valve, and an NLG extension restrictor-valve.

The restrictor valve has a cylindrical body made of stainless steel, which contains a poppet and spring. There is a threaded hydraulic connection at one end of the restrictor valve, and a union nut on the other. These connect the restrictor valve to the hydraulic system. The restriction orifice in the poppet restricts the flow of hydraulic fluid when the poppet is in the closed position.

When the hydraulic pressure on the inlet side is more than that on the outlet side, the poppet opens against spring compression. This lets more hydraulic fluid through the valve. When the hydraulic pressure on the outlet side is equal to (or more than) that on the inlet side, spring compression keeps the poppet against the valve seat. Thus the closed poppet restricts the flow of fluid through the valve.

(2) Restrictor Valve - NLG Door Opening (2535GM)

The restrictor-valve (2535GM) is installed in the 'doors-open' line to the NLG door actuating-cylinder (2527GM).

The function of this valve is described in the description of the NLG Door Actuating Cylinder (2531GM).

(3) Restrictor Valve - NLG Extension (2691GM)

The restrictor valve (2691GM) is installed in the 'extend' line to the NLG actuating-cylinder.

The function of this valve is described in the description of the NLG actuating cylinder (2527GM).

Q. MLG and NLG Door Ground-Opening

MLG Door Ground Opening

NLG Door Ground Opening

NLG/MLG Door Ground Opening By-Pass Valves (2517GM, 2518GM, 2533GM)

With the aircraft on the ground, the MLG and NLG doors can be opened independently to get access to the wheel bays for maintenance operations.

The MLG and NLG door ground-opening by-pass valves (2517GM, 2518GM, 2533GM) have a rigging pin hole for:

initial installation
adjustment of the ground door-opening system.

A control lever with two lockable positions is installed in each wheel bay. When the control lever is set to the OPEN position, it causes the mechanical system to:

operate the door by-pass valve (2517GM, 2518GM, 2533GM)
release the door from its uplock.

(1) Operation

The operation of the door by-pass valve isolates the door actuating cylinder from the hydraulic supply.
The door, when released from its uplock, opens because of gravity or may be manually assisted.

Each by-pass valve (2517GM/2518GM/2533GM) has three hydraulic connections marked A, B and C:

Port A : door open line
Port B : door close line from the selector valve
Port C : door close line to the door actuator.

For normal operation, Port A is closed with Port B connected to Port C.

For door ground opening, Port B is closed before Port A is connected to Port C.

A locking plunger (baulk) in the by-pass valve stops the movement of the control lever from the OPEN to the CLOSE position. Before selection can be made, a hydraulic pressure greater than 70 bar (1015 psi) must be supplied to port B (doors close line) to retract the locking plunger.

A more detailed description and operation of the L/G door ground-opening is given in (Ref. AMM D/O 32-12-00-00).

R. Landing Gear Control and Interface Unit (LGCIU), (5GA1,5GA2)

Landing Gear Control and Interface Unit (LGCIU)

LGCIU - Block Diagram

LGCIU - L/G Retraction Control Logic

LGCIU - L/G Extension Control Logic

LGCIU - L/G Door Close Control Logic

LGCIU - L/G Door Open Control Logic

LGCIU - Baulk Solenoid Logic

LGCIU - Cargo Door Control Logic

LGCIU - L/G Control Logic

LGCIU - Cargo Door Control Logic

(1) General

There are two Landing Gear Control and Interface Units (LGCIUs):

SYSTEM NO 1 LGCIU (5GA1), located in rack 90VU (Shelf 93VU)
SYSTEM NO 2 LGCIU (5GA2), located in rack 90VU (Shelf 94VU).

There are two types of LGCIU:

part number 664700500A4x (x is A, B, C or D), which is the first type of LGCIU
part number 80-178-02-880xx, which is the newest type of LGCIU.

The two types of LGCIU are interchangeable.
LGCIU part number 80-178-02-880xx has an interlink ARINC bus between the LGCIUs. This lets proximity sensor states be checked across the LGCIUs.
This will let the two LGCIUs identify the sensor that gives an incorrect state when the systems do not agree.
For this function to work:
each LGCIU must be part number 80-178-02-880xx
the interlink must be installed for ARINC 429 communication.

The aircraft can operate with one of each type of LGCIU or without the interlink ARINC bus. If the interlink ARINC bus is not installed, the LGCIU part number 80-178-02-880xx and the LGCIU part number 664700500A4x will operate the same. The proximity sensor states are not checked across the LGCIUs.

The function of each LGCIU is to:
control the operation of the landing gear (L/G) and doors
find the position and status of the L/G and doors
supply L/G and door position, and status information to other aircraft systems
process proximity sensor inputs from the Flap disconnect system (Ref. AMM D/O 27-51-00-00)
supply data to the Slat and Flap Control Computer (SFCC) (Ref. AMM D/O 27-51-00-00)
process proximity sensor and microswitch inputs from the Cargo Door Control System (Ref. AMM D/O 52-35-00-00)
supply a control logic circuit for the Cargo-Compartment Door-Systems (Ref. AMM D/O 52-35-00-00)
monitor LGCIU system operation and report component/system failures to the Flight Warning Computer (FWC) (Ref. AMM D/O 31-50-00-00) and Electronic Centralized Aircraft Monitoring System (ECAM) (Ref. AMM D/O 31-60-00-00)
store details of failures and to supply this information to the Centralized Fault Display System (CFDS) (Ref. AMM D/O 31-30-00-00)
supply BITE and self-monitoring functions
simulate different landing gear configurations for maintenance tests and to find possible failures.

The two LGCIUs are interchangeable, but the aircraft wiring in SYSTEM NO 1 is different to that of SYSTEM NO 2. Thus the installed LGCIUs will supply different interfaced aircraft systems with L/G system position information (Ref. AMM D/O 32-62-00-00).

The aircraft can operate satisfactorily when one of the LGCIUs does not operate. When this occurs, the serviceable unit must be installed in SYSTEM NO.1 (Shelf 93VU).

Only one LGCIU is in control of the L/G system at one time. Control of the L/G system automatically changes to the other LGCIU on:

each de-selection of DOWN of the L/G control-lever
if a fault is found in the LGCIU/L/G system of the LGCIU which has control of the L/G.

To let one LGCIU change to the other, each LGCIU supplies a status signal to the other.

Each LGCIU is connected to the other LGCIU by two pairs of wires that supply LGCIU status signals. For each LGCIU, one pair of wires is for the output status signals to the other LGCIU. The second pair of wires is for input status signals from the other LGCIU.

LGCIU part number 80-178-02-880xx has an ARINC 429 interlink bus. This supplies more information between the two LGCIUs for better trouble shooting.
For this function to work:

each LGCIU must be part number 80-178-02-880xx
the interlink must be installed for ARINC communication.

Although only one LGCIU is in control of the L/G system at one time, each LGCIU continues to supply L/G position and status information to the interfaced aircraft systems.
Each part number 664700500A4x LGCIU consists of an ARINC 600 4 MCU case, that has:
7 Printed Circuits Boards (PCB)
an On Board Replaceable Memory Module (OBRM)
a Power Supply Unit (PSU).

The 7 PCB's are:

2 Proximity Signal Conditioning cards
1 Output/Control and Indication card
3 Interface cards
1 combined OBRM/Microprocessor card.

Two 28V d.c. inputs supply each LGCIU. One input is connected to the PSU which is connected to:

the two Proximity Conditioning cards
two of the Interface cards
the Microprocessor card.

The other input is connected to the Output/Control and Indication card and the Interface card that remains.

Each part number 80-178-02-880xx LGCIU consists of an ARINC 600 4 MC case that has four Circuit Card Assemblies (CCA).

The four CCA's are:

Filterboard
Motherboard
Control Card
Driver Card.

(2) For LGCIU part number 664700500A4x only:

Data is received and transmitted by each LGCIU in ARINC 429 and discrete form. The inputs to each LGCIU are:

two L/G control-lever (6GA) UP selection signals
two L/G control-lever DOWN selection signals
an 'Active' or 'on Standby' signal from the other LGCIU
proximity 'target near' or 'target far' signals from:
16 L/G-system proximity-sensors
2 Flap-disconnect proximity-sensors (1 L/H) (1 R/H)
2 Cargo-Door Safety-Mechanism proximity-sensors (1 FWD) (1 AFT) SYS 1 only
1 Cargo-Door Locking-Handle proximity-sensors (1 FWD) (1 AFT) SYS 1 only
2 Cargo-Door Locking-Shaft proximity-sensors (1 FWD) (1 AFT) SYS 2 only
a ground circuit for the Semi-Automatic Cargo Loading-System
maintenance data from the Centralized-Fault Display-Unit (CFDIU)
a Ground Power 'applied' or 'not applied' signal from the External Power Contactor (3XG)
a wheel speed signal from the Brakes and Steering Control Unit.

Each LGCIU supplies output signals to the:

landing-gear and door selector valves (40GA/41GA)
safety valve (49GA) (SYS 1 only)
L/G position indicators (8GB/9GB/10GB) (SYS 1 only)
L/G position relays SYS 1 (11GB) SYS 2 (12GB)
L/G control lever (6GA) UP baulk-solenoid
FWC and ECAM
CFDIU
SFCC
interfaced aircraft systems for necessary L/G position information (Ref. AMM D/O 32-62-00-00)
Cargo-Door Interlock relays (6MJ/7MJ)
Cargo-Bay Heating Temperature-Controllers (1HC/10HC)
an 'Active' or 'on Standby' signal to the other LGCIU.

(3) PCB Functions

(a) Proximity Conditioning Cards

The Proximity Conditioning cards change the proximity sensor 'Target Near' or 'Target Far' input signal into two user status signals Normal and Complementary. Thus:

Target Near = Normal logic off, Complementary logic on
Target Far = Normal logic on, Complementary logic off.

These Normal and Complementary logic signals are supplied to the PCB's as follows:

Normal and Complementary to the Microprocessor card on an ARINC 429 data bus
Normal and Complementary to the Output/Control and Indication card (L/G system only)
Complementary only to the Interface Cards.

(b) Output/Control and Indication Card

The Output/Control and Indication card supplies the control logic circuits for the L/G system and the L/G position-indicator lights. The indicator-light logic-circuit is only used by the LGCIU installed in SYSTEM NO.1. This card also supplies:

a 28V DC output and an earth return to energize the solenoid windings of the L/G and door selector valves
a 28V DC output to energize the baulk solenoid in the L/G control-lever.

The L/G-system control-logic is divided into two parts. Each part is different in design and one part functions inversely to the other.

One part of the control logic receives:

one of the UP selection signals from the L/G control-lever
one of the DOWN selection signals from the L/G control-lever
all of the L/G system Normal signals from the proximity conditioning cards
an enable or disable signal from the microprocessor card.

The other part of the control logic receives:

the other UP selection signal from the L/G control-lever
the other DOWN selection signal from the L/G control-lever
all of the L/G system Complementary signals from the proximity conditioning cards
an enable or disable signal from the microprocessor card.

The L/G control-logic processes these input signals and when:

each of the two logic parts supply the correct output signal
the microprocessor controlled enable/disable line supplies an enable signal.

Then the L/G control-logic will let a 28V DC output and an earth return to be supplied to the (L/G or door selector-valve) solenoid-windings.

The L/G system retraction and extension cycles are controlled by electrical power applied to the (L/G and door selector-valve) solenoid-windings in the correct sequence.

A separate logic circuit controls the L/G position indicator lights. This logic uses only the L/G-system Normal-signals from the proximity conditioning cards. The output signals from this logic supply the indicator-light relays. These relays are located on the Annunciator-Light Test and Interface Board (9LP).

NOTE: Only the logic of the LGCIU in SYSTEM No1 controls the three indicator lights. The change in control of the L/G has no effect on the logic.

(c) Interface Cards

The Interface cards provide L/G position-information, (i.e L/G downlocked, shock absorber compressed) to the interfaced aircraft systems (Ref. AMM D/O 32-62-00-00). One of these cards also contains the logic circuitry for control of the Cargo-Door Open/Closing System.

The Interface cards process L/G-system Complementary input-signals and supply L/G position-information output-signals in discrete form.

(d) OBRM/Microprocessor card

The main components of the OBRM/Microprocessor card are:

an On Board Replaceable Memory Module (OBRM) which contains an Electronically Erasable Programmable Read Only memory (EEPROM)
an Intel 8031 microcontroller
a Harris 3282 ARINC 429 transceiver
an Intel 8251 RW232C interface device
a Non Volatile Random Access Memory (NOVOL RAM)
Random Access Memory (RAM).

Contained in the OBRM is the software used by the microcontroller. The software is mainly programmed in Intel PL/M-51 high level programming language and has specific routines programmed in ASM-51 Assembly language.

The software performs these main functions:

1 System Status (position) Information

The software prepares and transmits LGCIU system status data (ie L/G downlocked, LH flap disconnected) on the ARING 429 output data buses. This data is transmitted to:

the FWC (for ECAM display)
the SFCC
the CFDS
other systems (Ref. AMM D/O 32-62-00-00).

Those systems that require high integrity data either receive discrete data alone (Interface cards) or receive discrete data against which ARINC 429 data is checked.

2 'Active'/'Standby' Determination

In a serviceable LGCIU, the software receives input signals from the L/G control-lever and the other LGCIU. From these signals the software (changeover logic) makes the decision if the LGCIU should be 'Active' (in control of the L/G system) or 'On Standby'.

When the decision is 'Active' the software transmits:

an enable signal to the L/G control-logic (Output/Control and Indication card) so that the L/G and door selector-valve solenoid-windings can be energized
an 'Active' signal to the other LGCIU.

When the decision is 'On Standby' the software transmits:

a disable signal to the L/G control-logic so that the L/G and door selector-valve solenoid-windings cannot be energized.
an 'On Standby' signal to the other LGCIU.

With both LGCIU systems serviceable, LGCIU 1 Active and LGCIU 2 ON Standby:

a de-selection of DOWN will cause LGCIU 1 to become 'On Standby' and LGCIU 2 to become 'Active' (and take control of the L/G system)
at the subsequent de-selection of DOWN, LGCIU 1 will become 'Active' and LGCIU 2 will become 'On Standby'.

The software continuously monitors the performance of the LGCIU/system hardware. If a failure occurs that affects the control of the L/G system, the microprocessor (changeover logic) transmits:

a disable signal to the L/G control logic so that the L/G and door selector- valve solenoid-windings cannot be energized
an 'On Standby' signal to the other LGCIU.

Thus with:

a LGCIU 1 in control of the L/G, LGCIU 2 Fails.
LGCIU 2 sets its status to 'On Standby'. LGCIU 1 observes that LGCIU 2 will not become 'Active' and maintains control of the L/G system even after the de-selection of DOWN.

b LGCIU 1 in control of the L/G, LGCIU 1 Fails.
LGCIU 1 sets its status to 'On Standby'. LGCIU 2 observes that LGCIU 1 is 'On Standby' so it becomes 'Active' and takes and maintains control of the L/G system.

c Both LGCIU's Failed.
If both of the LGCIU's should fail, then both of the LGCIU's will set their status to 'On Standby'. Thus none of the two LGCIU's will have control of the L/G system.

3 Failure Monitoring

The software continuously monitors the performance of the LGCIU/system hardware. The main functions monitored are:

proximity sensor outputs
L/G control-logic outputs
L/G control-lever inputs (UP and DOWN)
electrical power supplies
discrete (transistor) outputs
ARINC 429 transceiver inputs and outputs exited in the correct sequence.
memory failure (EPROM, RAM, NOVOL).

To detect failures in the hardware the software uses a comparison technique for example:

it compares the two output signals from each proximity sensor
it compares each discrete output signal with the proximity-sensor input-signal.

When a failure is detected that does not affect the control of the L/G system, the LGCIU continues to be/become the 'Active' unit. The details of a failure are stored in the NOVOL memory and reported to the CFDS.

The software is monitored for failure by itself and a Watchdog Timer.
The software monitors itself by the use of an audit trail technique. This is a software procedure that makes sure that each software process is entered and exited in the correct sequence. The Watchdog Timer's function is to check the software for serviceability at regular intervals. A serviceable software causes the Watchdog Timer to be reset. Should the software fail, the Watchdog timer will not be reset and this causes the LGCIU to change its status to 'On Standby'.

4 System Status/Maintenance Data Transmissions

The software continuously monitors the performance of the LGCIU and the inputs from the proximity sensors. The data collected is processed by the microprocessor and transmitted on the ARINC 429 data buses to:

the CFDS
the FWC (for ECAM display)
the SFCC's
the Cargo Door system
other systems connected by ARINC 429 data buses (Ref. AMM D/O 32-62-00-00).

This is the function which also forms the ARINC 429 words that contain the system status and maintenance data.

5 Fault Logging

This software function stores failure and maintenance data in the NOVOL memory for later transmission to the CFDS. Failure data can be transmitted to the CFDS up to 64 flights after the failure occurred.

6 BITE

The software provides a Built in Test Equipment (BITE) function which is additional to the continuous failure monitoring function. The BITE test is started by manual use of the CFDIU (Menu Mode) and takes approximately 7 seconds to complete (Ref. AMM D/O 32-69-00-00).

The BITE test is inhibited:

at aircraft wheel speeds greater than 70kts
during flight.

7 Landing Gear (L/G) Configuration Simulation

The L/G Configuration Simulation function is able to simulate any or all of the landing gear configurations. This function is provided for the use of the maintenance crew during test and failure investigation tasks. It is inhibited during flight.

A L/G Configuration Simulation is selected by manual use of a CFDIU (Menu Mode (Ref. AMM D/O 31-32-00-00). While the LGCIU is in a L/G simulation mode it will continue to supply both ARINC 429 and discrete output signals.

8 Intel 8251 RS232C Interface Device

This interface device supplies an interface for the connection of a Maintenance Computer.

(e) LGCIU Flap Disconnect Function

The LGCIU's are used to convert the discrete input signals from the flap disconnect-system into ARINC 429 data for transmission to the SFFCs (Ref. AMM D/O 27-51-00-00). This LGCIU function is the same as that as described in PCB Functions.

(f) LGCIU Cargo-Compartment Doors-System Function

The LGCIU performs two separate functions for the cargo door system (Ref. AMM D/O 52-35-00-00). Each LGCIU provides one of these functions:

the LGCIU in System 1 (5GA1) is used to process the cargo door warning data that is supplied to the FWCs
the LGCIU in System 2 (5GA2) provides a logic circuit for the electrical control system of the cargo doors.

1 Cargo Door Warning

The cargo door warning system uses proximity sensors to monitor the position of the cargo doors. The LGCIU converts the discrete output signals from these proximity sensors into ARINC 429 data. This data is sent to the FWCs and the ECAM system. This LGCIU process is the same as that described in paragraph (2) subparagraphs (a) and (d).

2 Cargo Door Control

The electrical control system of the cargo doors uses limit switches and proximity switches to control the door hydraulic system. The LGCIU2 changes the signals from the limit switches, the proximity switches and the manual selector valves internal proximity switch into a signal. This signal controls a time delay relay 6MJ. This relay controls the yellow electric pump. The manual selector valves control pressure from the yellow hydraulic system into the door actuators.

(4) For LGCIU part number 80-178-02-880xx only:

(5) Circuit Card Assemblies (CCA)

(a) Filterboard

The filterboard supplies an electromagnetic compatibility (EMC) filter for all aircraft inputs and LGCIU outputs. It also gives lightning and transient voltage protection for the proximity sensor inputs.
It sends signals from the ARINC 600 connector to the control card and the driver card through the motherboard.

(b) Motherboard

The motherboards primary purpose is:

lightning and transient voltage protection of the LGCIU discrete outputs
lightning and transient voltage protection of the solenoid driver signals
an EMC filter for all ARINC interfaces.

It also supplies the interconnect between the filterboard, control card and driver card.

microprocessor (software) and peripherals
hardware based control and indication logic
proximity sensor interface
discrete input interfaces
internal power supply
ARINC 429 interfaces
interfaces with off-aircraft equipment.

A MPC565 microcontroller monitors the system, while five Programmable Logic Devices (PLD) supply the hardware based control and indication logic.
Seven MA47 ASICs and their related circuits make up the proximity sensor interface. Converters change the +5V supplied by the driver card to +3.3V, +2.6V and +/- 15V. A power supply reset signal is made as a function of the internal supplies.
The ARINC 429, ARINC 615-3, RS422 and RS232 interfaces are all supplied by the control CCA.

(d) Driver Card

The driver card supplies:

all the aircraft discrete and solenoid drive outputs
a filter for the LGCIU +28V power supply.

It has a high side and a low side driver for each of the selector valve solenoids. All other discrete output circuits have one high side driver or one low side driver. Each driver supplies output status and fault status signals for use by the monitoring software. A switching power supply changes the filtered +28V power to +5V.

(6) External Connections

Connections to the aircraft wiring are made with an ARINC 600 connector on the rear panel of the LGCIU. This connector supplies connections for the interfaces that follow:

(a) Inputs

changeover discrete input
proximity sensor inputs
discrete inputs
pin programming inputs
aircraft power supplies (+28 VDC, OV).

(b) Outputs

changeover discrete output
landing gear and landing gear door solenoid drive signals
landing gear status discrete inputs
system status
landing gear baulk solenoid
cockpit indication discrete outputs
cargo door relay drive signal
cargo door status.

ARINC 429, aircraft systems
ARINC 429, inter-LGCIU
RS232, RMS/MICBAC (shop use only).

(7) Aircraft Interfaces

(a) Proximity Sensor Interface

The Proximity Sensor Interface (PSI) gives the condition of all connected proximity sensors. The PSI monitors 28 proximity sensor channels. It has seven, four channel Application Specific Integrated Circuits (ASIC) installed on the control card. The proximity card supplies the proximity sensor with an approximate 1.4KHz peak to peak sine wave. The proximity card measures the amplitude and phase of the returned current to get the target near/far status.
The status of each channel can be accessed using TARGET (near/far) and FAULT discrete outputs or through the four user-selectable bus interfaces that follow:

8-bit parallel data bus
ARINC 429 serial data bus
Serial Peripheral Interface (SPI) bus
UART serial data bus.

The LGCIU hardware based control logic interfaces with the PSI and the TARGET and FAULT discrete outputs. The LGCIU software receives its proximity sensor data from the PSI's SPI bus. The software uses the proximity data for:
BITE
ARINC 429 transmissions to other aircraft systems
inter LGCIU ARINC 429 transmissions.

(b) Discrete Inputs Interface

The Discrete Input Interface (DII) accepts inputs from the aircraft. It supplies logic level signals for use by the LGCIU functions together with analog monitor signals in proportion to the input voltages.
The analog level for each discrete input is sent to the Analog Digital Converters (ADC) on the microprocessor. The software checks each signal to make sure it is in the correct operating range. It uses the checked values for monitoring functions.
The logic level discrete signal is sent to the PLD's. The LGCIU controls Ground/Open and 28V/Open discrete inputs.

landing gear and system status
Landing Gear Selector Lever (LGSL) baulk solenoid (LBS) high side drive
cockpit indicator lamp commands
cargo doors relay command and cargo door status.

The logic for the discrete status and indication outputs is supplied across all the PLD's. This will make sure that one PLD does not control all the outputs related with a discrete output function group.

The proximity sensor and discrete inputs are isolated into three groups. This is for safety and reliability and to make sure that a failure of one group will not cause a failure of the other two groups.

The PLD's supply signals for the discrete output drivers on the driver card. Signals are then sent out of the ARINC 600 connector through the motherboard and filterboard. Each discrete output circuit has one high side driver or one low side driver. Each driver gives output status and fault status which is used by the BITE. The status outputs are sent back to the monitoring software on the control card through an SPI bus. Many of the discrete outputs have multiple output signals caused by a single driver.

Discrete outputs are defaulted to their fail-safe states by the PLD's if a related proximity sensor channel fails. If a proximity sensor interface fault is found that effects the outputs of all proximity sensor channels, all discrete outputs are defaulted to fail-safe states.
The landing gear and system status signals are used by different aircraft systems to sense if the aircraft is in flight or on the ground.
The landing gear status signals supplied by the LGCIU include:
indication if the landing gear shock absorbers are extended or compressed
if the landing gear is downlocked or uplocked
if ground power is connected.

The LGCIU will also report its operational status. It uses discrete output and control cockpit indicator lamps to show if the landing gear is locked down or unlocked.

(d) Landing Gear Solenoid Valve Driver Interface

The driver card has a high side driver and a low side driver for each of the landing gear valve solenoid driver circuits as follows:

high side landing gear valve solenoid
high side landing gear door valve solenoid
low side landing gear valve solenoid
low side landing gear door valve solenoid.

The high side driver and low side driver for each solenoid are controlled by different control signals. The signals are supplied by the high side and low side PLDs.
The proximity sensor inputs and discrete inputs are isolated into three groups. This is to keep the signal isolation necessary for safety and reliability.

The system, control and door PLD's, supply the proximity sensor signals and the discrete signals to the high side and low side PLDs. The PLDs will then give the landing gear commands. To give protection against accidental operation of the landing gear, the LGCIU has different procedures to stop control of the landing gear. The software and hardware can each stop the solenoid valve driver circuits. The software stops the solenoid valve driver circuits by a signal sent directly to the driver card. At the same time, the hardware stops the signals sent to the solenoids by use of logic in the control PLD.

The hardware and software will stop solenoid drive outputs on power up. One of the LGCIU's will become the 'active' LGCIU if it does not have a fault. It will then start the solenoid driver outputs after a specified time.
The solenoid drive outputs are stopped by hardware and software if:
simulation mode is set
or
a reset occurs
or
the LCGIU is not on.

If faults on the landing gear or landing gear door proximity sensor channels are found, the PLD's will immediately stop landing gear control. These faults are:
landing gear or landing gear door proximity sensor faults
selector lever faults
proximity sensor interface faults.

The PLD's will start the landing gear control again if the fault is corrected. If a control fault is found by the software that effects the LGCIU control of the gear or doors, the solenoid drive outputs are stopped.
The software will start the solenoid drive outputs again if the control fault is corrected and the fault is cleared.

(e) ARINC 429 Interface

There are three ARINC 429 inputs to the LGCIU. These are:

1 A low-speed bus from the CFDS that gives aircraft system status and BITE signals to the LGCIU.

2 A low-speed bus from the other LGCIU used by the BITE function. This bus is opto-isolated to make sure that if one LGCIU fails, the other LGCIU will continue to operate.

3 A high-speed bus sent from an ARINC 615-3 data loader. The high-speed bus will upload software to the LGCIU while it is in data load mode.

(f) The LGCIU has three ARINC 429 transmitters. These are used to drive five ARINC 429 output buses. These are:

1 A transmitter that supports three low-speed ARINC 429 output buses. The three buses supply LGCIU status and maintenance data to the CFDS and other aircraft systems.

2 A transmitter that drives the inter-LGCIU low-speed bus. This low-speed bus is used only to supply the system status to the other LGCIU.

3 A transmitter that supports a high-speed bus that is used only to interface with a data loader through the ARINC 615-3 interface.

Each of the low-speed ARINC 429 transmit buses is attached to an ARINC 429 receiver so that it is monitored.

(g) ARINC 429 Inputs from other systems

The LGCIU receives the data that follows from the CFDS ARINC 429 input bus:

Label 125 - Time
Label 126 - Flight Phase
Label 155 - Printer Installation
Label 227 - BITE Commands
Label 260 - Data
Label 301, 302, 303 - Aircraft Identifier.

This data is for report and logic functions.

(h) ARINC 429 Outputs to other systems

The LGCIU transmits its status, maintenance data and identification information to aircraft systems through seven ARINC labels as follows:

Label 020 to 023 - LGCIU System Status
Label 354 - System Identifier (contains part number and serial number)
Label 356 - Maintenance Data
Label 377 - Equipment Identifier.

The flight software will output the four system status labels based on the proximity sensor status data, discrete inputs and BITE status at 100 millisecond intervals. The LGCIU system identifier, equipment identifier and fault status are sent with the system status labels.

(i) ARINC 429 Inter-LGCIU

The LGCIU transmits its system status to the other LGCIU across the inter-LGCIU ARINC 429 bus. Data sent across the inter-LGCIU bus is used for fault isolation. The LGCIU will compare its internal signals with those from the other LGCIU. It will then use algorithms to find the possible cause of the errors. The cause will be local proximity sensors or a failure of the mechanical system.

(8) LGCIU Functions

(a) Landing Gear Control Logic

The LGCIU controls the extension and retraction of the Nose Landing Gear (NLG) and doors and the Main Landing Gear (MLG) and doors. The landing gear has two isolated hardware control paths. The high side and low side of the landing gear and landing gear solenoid valves are independently switched by high side and low side drivers. These drivers are independently controlled by the high side and low side PLDs. To keep the risk of common mode design errors causing accidental valve operation to a minimum, each PLD uses a different state machine. To sequence the landing gear control, each state machine uses:

the state of the Landing Gear Selector Lever (LGSL)
proximity sensors
discrete inputs.

(b) Changeover

Each LGCIU has a select-in discrete input and a select-out discrete output. These control the 'change-over' of landing gear control with the other LGCIU. The select-out from one LGCIU connects to the select-in of the other LGCIU.

The flight software will make a decision if the LGCIU will be in control from the information that follows:

active/inactive status before the last power cycle
status of the LGSL discrete inputs
fault status of the LGCIU
select-in status.

With this data, the software will set its select-out enable signal to the correct state on power-up when:
a fault is found
or
its select-in signal has no stimulus (for example if the other LGCIU is switched off)
or
the LGSL moves away from the down position.

The control PLD receives the software generated select-out enable signal. It then supplies a control signal for the select-out driver. The driver will go to the 'inactive' state when the system is in reset or all watchdog lives are lost. If not, the driver will be set by the active/inactive status set by the software.
The LGCIU that is in control, will give control to the other LGCIU when the LGSL moves out of the down position. The LGCIU will continue to supply the solenoid valve drives until the other LGCIU accepts control by setting its select-out signal to active.
If the LGCIU stops its control because of a control fault, it will immediately disable the landing gear solenoid valve drive outputs. This will make sure it does not give incorrect commands to the landing gear or landing gear doors.

(c) Failure Monitoring and BITE

The software has a Built In Test Equipment (BITE) function. LGCIU Built In Test (BIT) has four parts:

power-up BIT (PBIT)
reset BIT (RBIT)
continuous BIT (CBIT)
initiated BIT (IBIT).

PBIT is done on recovery from a power-on reset. RBIT is done on recovery from all resets and includes power-on resets. CBIT is done by the software during usual operations and uses data supplied by the hardware. IBIT is done through the CFDS during interactive maintenance mode.

The LGCIU has a built in test function that finds and isolates the fault conditions that follow:
configuration pin programming failures
proximity sensor/wiring failures
sensor/target unreasonableness failures
aircraft ARINC 429 input interface failures
discrete input failures
selector valve solenoid failures.

The LGCIU also supplies a built in test function to find and isolate internal fault conditions that affect the LGCIU interfaces that follow:
selector valve solenoid control outputs
discrete status and indication outputs
ARINC 429 outputs.

A built in test failure that has an effect on how the LGCIU controls the landing gear and doors is a control fault. When a control fault is found, the LGCIU will set its select-out signal to inactive and disable its solenoid valve driver circuits. This will prevent the LGCIU that failed from controlling the landing gear and doors. If the other LGCIU can operate, its select-in signal will go inactive and it will start to control the landing gear and doors.
Data related with logged faults is transmitted to the CFDS through the ARINC 429 Interface (Ref. AMM D/O 32-69-00-00).

(d) Watchdog Timers

The LGCIU has three watchdog timers as follows:

two Software (SW) watchdogs, one internal and one external
a proximity sensor interface watchdog.

If one of the two SW watchdog timers expires, the hardware does a hard reset. The watchdog reset count will increment by one. A subsequent power-on reset will clear the reset count. When the reset count gets to four, the LGCIU will stay in reset mode. It will not operate until power is cycled.

(e) System Resets

The LGCIU has three different resets as follows:

power-on reset
hard reset
soft reset

1 A power-on reset is done after the hardware has made sure that the internal power supplies are sufficient after a power interruption. The PLD's, proximity sensor interfaces and microprocessor all reset after a power-on reset.

2 On a hard reset, only the microprocessor is reset. The control PLD causes a hard reset when one of the conditions that follows occurs:

the internal or external software watchdog operates
the data loader other internal microprocessor resets.

A hard reset on the microprocessor will operate the soft reset signal. This signal is used by the PLD's to increment the software watchdog timer count.

3 A soft reset that is operated independently of a hard reset, will only reset the internal logic of the microprocessor. Soft resets are only for debug and test functions. If the software finds a soft reset when flight software is in operation, it will cause a fault.

The select-out signal is made inactive by the control PLD when the system is in reset mode. This will make sure that the other LGCIU will control the landing gear, if it is in operation.

(f) Operating Modes

The LGCIU has the operating modes that follow:

Normal Mode
Interactive Mode
Simulation Mode
Data Loading
Other Modes.

1 Normal Mode

The LGCIU operates in normal mode when power is applied and the LGCIU has completed its initial set-up. The LGCIU continues to operate in normal mode until the mode is stopped by:

a reset
or
entry into interactive mode.

2 Interactive Mode

Interactive mode lets the operator get access to more maintenance data and to test procedures. The LGCIU software stops normal mode and goes into interactive mode when the software finds the correct aircraft conditions. The conditions are BSCU wheel speed less than 70 knots and (L/G compressed or External Ground Power selected). This is done from the CFDS by the operator through the MCDU.
When interactive mode is on, the LGCIU will transmit a CFDS menu page. From this, the operator can do or get:

an initiated test (IBIT)
ground scanning
a display of fault reports
LGCIU identification data.

ARINC 429 system status transmissions continue as usual while the LGCIU is in interactive mode.

The LGCIU interfaces with the CFDS during interactive mode. It uses the aircraft ARINC 429 interface. The CFDS sends an instruction to the LGCIU on Label 227. The LGCIU sends a message to the CFDS instruction on Label 356 (Ref. AMM D/O 32-69-00-00).

3 Simulation Mode

Simulation mode lets the operator override the LGCIU landing gear and landing gear door proximity sensor inputs to test status/indication outputs. The operator does this through the CFDS when you access the landing gear simulation mode.
Simulation mode is accessed from the interactive mode main menu or during IBIT. This is done when the test option is selected from the interactive mode main menu.

The landing gear proximity sensor values are overridden for test and maintenance during simulation mode. The discrete outputs and ARINC 429 status outputs give the state driven by the proximity sensor override values, not the specified proximity sensor states.

During simulation mode, the LGCIU outputs to other systems will change state. Other systems can start and cause danger to personnel and the aircraft. The correct flight configuration precautions must be done (Ref. AMM D/O 32-00-00-00).

When simulation mode is in operation during the IBIT, the SSM of the ARINC 429 labels 020-023 is set to normal operation. The landing gear solenoid outputs stop to prevent accidental operation of the landing gear when simulation mode is on.
The software supplies the simulation commands and stops the solenoid drivers when in simulation mode. If the software or control PLD finds the simulation is active, they will stop control of the landing gear.

4 Data Loading

The data loading function uploads flight software with an ARINC 615-3 compliant data loader. This connects to the front panel of the LGCIU. Data loading starts during power up if an ARINC 615-3 data loader is connected and the aircraft is on the ground.
When data loading is approved, data transmits to the LGCIU. This is done on a high speed ARINC 329 bus with the automatic mode specified in the ARINC 615-3 specification. If failures occur during the upload procedure, the recovery will be to re-initiate the upload from the start of the procedure.

5 Other Modes

Resident maintenance software and fault insertion functions is accessed when the LGCIU is in shop mode. Shop mode is accessed by connecting two pins on the RS422 connector to the ground. A power-on or hard reset is performed.

S. Proximity Sensor

Proximity Sensor and Target ** ON A/C NOT FOR ALL

Proximity Sensor - Operating Characteristics

(1) Description

The proximity sensor is an electronic position indicator. The main components of the sensor are a sensor body, a sensor coil and an electrical connector. The sensor has a related target.

The sensor body contains the sensor coil. The coil is connected to the electrical connector.

Each proximity detection circuit contains:

a proximity sensor
a sensor target
a channel of a proximity-sensor electronics card.

The proximity-sensor electronics-card has 14 channels and is located in the LGCIU.

(2) Operation

The proximity sensor continuously transmits an electrical signal to the proximity-sensor electronics-card. As the target moves into or out of the proximity-sensor's actuation area it causes a change in the proximity sensor's electrical property. The electrical signal to the proximity sensor electronics card also changes. The proximity-sensor electronics-card converts these electrical signal changes into user status signals. The LGCIU control logic modules use these user status signals to control the extension and retraction of the L/G.

** ON A/C NOT FOR ALL

7. Operation/Control and Indicating

A. Operation/Control

Hydraulic Schematic - L/G UP Selection, Door Movement to Open ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G UP Selection, Gear Movement to Up ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G UP Selection, Door Movement to Close ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G DOWN Selection, Door Movement to Open ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G DOWN Selection, Gear Movement to Down ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G Down Selection, Gear Movement to Down ** ON A/C NOT FOR ALL

Hydraulic Schematic - L/G DOWN Selection, Door Movement to Close ** ON A/C NOT FOR ALL

Landing Gear System - Sequence of Operations

The L/G control system is electrically controlled and hydraulically actuated. The Green system supplies the hydraulic pressure. The L/G and doors are related systems and operate in the sequence that follows:

door uplocks released and doors open
L/G locks released and L/G retracts or extends
doors close.

The position of the L/G control lever controls the extension and retraction of the L/G. The L/G control lever cannot be moved to the UP position (the baulk mechanism prevents this) if:

the MLG is not fully extended
the NLG shock absorber is not fully extended
the nosewheels are not in the center position.

When the baulk mechanism of the L/G control lever is released, the lever can be moved to the UP position.

(1) L/G Retraction-Sequence

With the L/G down and locked, and the doors closed and locked, on selection of UP, the LGCIU will:

(a) Signal the doors to open.

(b) When all the doors are fully open:

signal the L/G to retract
hold the doors open signal to keep the door open line pressurized.

signal the doors to close
maintain the L/G retract signal to keep the L/G up-line pressurized.

(d) As the last door closes and locks, the operations that follow will occur:

the L/G retract signal will cancel
the L/G selector valve will de-energize
the L/G retract lines will de-pressurize.

When the calculated airspeed increases to 264 kts, a signal from the Air-Data Inertial-Reference Units (ADIRU) causes the safety valve to close. This isolates the Green hydraulic system from the L/G hydraulic system. When the pressure has decreased sufficiently, a spring in the door selector valve will adjust the valve to center. This causes the door close line to depressurize.

(2) L/G Extension Sequence

When the calculated airspeed decreases to 260 kt or less, the ADIRUs will function. This lets a "valve open" signal go to the safety valve when you make a DOWN selection. The safety valve then opens and connects the L/G system to the Green system hydraulic pressure.
With the L/Gs up and locked and the doors closed and locked, on selection of L/G DOWN, the LGCIU will:

(a) Signal the doors to open.

(b) When all the doors are fully open:

signal the L/G to extend
hold the door open signal to keep the door open-line pressurized.

signal the doors to close and pressurize the lockstay actuating cylinder.

(d) As the first door moves from the fully open position:

cancel the L/G extend signal.

(e) Maintain the doors closed signal and the L/G lockstay actuating cylinder pressurized.

(3) Sequence Control

The sequence control is such that:

it is not possible to move the L/G unless all the doors are fully open
it is not possible to close the doors unless all the L/Gs are locked in their selected position.

With the L/Gs locked up and the doors locked closed, the doors will not be signalled to open if a door becomes unlocked. This is only if the control lever is selected to the UP position.

(4) LGCIU Control

Only one LGCIU controls the operation of the system at any one time. The LGCIU in control is the "Active" unit and the unit not in control is the "Standby" unit. Each LGCIU has a non-volatile random access memory (NOVOL RAM) which makes a record if it was "Active" or "Standby" at the last retraction and extension sequence. Control of the L/G system automatically changes from one LGCIU to the other when:

the L/G control lever is moved away from the DOWN position
a failure occurs in the "Active" LGCIU.

Each LGCIU sends a status signal to the other whether it is "Active" or "Standby". It also sends position and status data to other aircraft systems. If one LGCIU fails, it sends a "Standby " status signal to the other LGCIU. The serviceable LGCIU becomes "Active", if it was not already, and stays "Active" for all the subsequent control sequences.

If a failure occurs in an LGCIU that does not affect the control of the system, the LGCIU can still be "Active". The failure is stored in the NOVOL RAM.

B. Indicating

(Ref. AMM D/O 32-61-00-00)

The System Display shows the position of the landing gear and related doors. This is shown as part of the WHEEL page.

The landing-gear downlock indicator-lights show green lights when the landing gear is downlocked and red lights when it is not locked in the selected position.

The red light of landing gear control-lever 6GA comes on if the aircraft is in a landing configuration and the gear is not locked down. The flight warning computer controls this function (Ref. AMM D/O 31-52-00-00).

** ON A/C NOT FOR ALL

8. BITE Test

MCDU System Displays - LGCIU

The LGCIU contains hardware and software to complete these functions:

to continuously monitor the L/G Normal Extension and Retraction System for failures and defects
to store failure and defect data and also to send this data to the Centralized Maintenance System
to do an automatic test of parts of the system during power-up
to output simulated L/G configurations during ground maintenance
to do an automatic test of the system when a selection is made on a Multipurpose Control and Display Unit (MCDU) (Ref. AMM D/O 22-82-00-00).

Refer to (Ref. AMM D/O 32-69-00-00) for the description and operation of these functions.
The monitor and failure/defect storage functions are completed all the time the LGCIUs are supplied with power and are serviceable. The automatic test selection on the MCDU cannot be selected at wheel speeds of more than 70 knots or during flight. The simulation function is also not available during flight.

A. Power-up Tests Initialization and Cockpit Effects.

(1) Conditions of power-up tests initialization:

de-energization time of computer: >3mS
A/C configuration:
- whatever the A/C configuration on the ground.

(2) Progress of power-up tests:

duration: 210mS
cockpit effects directly connected to power-up test done (other effects can occur related to the A/C configuration but these can be ignored):

(a) LGCIU 1:

LDG gear panel.

(b) LGCIU 2:

none.

(3) Results of power-up tests (cockpit effects):

(a) Correct test results:

1 LGCIU 1:

LDG Gear Panel: No ECAM warning, no visible indication other than normal operation.

2 LGCIU 2:

no ECAM warning, no visible indication other than normal operation.

(b) Incorrect test results:

1 LGCIU 1:

ECAM warning: L/G LGCIU 1 FAULT.

2 LGCIU 2:

ECAM warning: L/G LGCIU 2 FAULT.

[Rev.10 from 2021] 2026.04.01 04:51:57 UTC