ALTERNATE BRAKING WITH ANTI SKID - DESCRIPTION AND OPERATION

** ON A/C NOT FOR ALL

1. General

Alternate Braking with Anti Skid ** ON A/C NOT FOR ALL

A. The Alternate braking with anti skid associates the Yellow high pressure with the anti skid regulation. You get this mode automatically:

if Green pressure is not available or falls during braking,
or if certain failures occur on the Normal system (Ref. AMM D/O 32-42-00-00).

A hydraulic selector causes changes between the Green and Yellow systems automatically.
You control only with the pedals. An auxiliary low-pressure hydraulic-system transmits the orders. The pressure supplied to the left and right brakes is shown on a brake Yellow-pressure triple-indicator installed on the center instrument panel.
The anti skid system and related indicating are operative.
This braking mode uses the second set of pistons of the brake unit.

** ON A/C NOT FOR ALL

2. Component Location

Dual Valve - Alternate Braking - Component Location ** ON A/C NOT FOR ALL

Reservoir / Master Cylinder - Brake Control - Component Location ** ON A/C NOT FOR ALL

Servovalve / Safety Valve - Alternate Braking - Component Location ** ON A/C NOT FOR ALL

FIN	FUNCTIONAL DESIGNATION	PANEL	ZONE	ATA REF
** ON A/C NOT FOR ALL
40GG	SERVO VALVE N A 40GG EQ002DEQ0040GG	LVE N	671	32-43-27
41GG	SERVO VALVE N A 41GG EQ002DEQ0041GG	LVE N	571	32-43-27
42GG	SERVOVALVE-ALTN BRK WHEEL 4	LVE N	671	32-43-27
43GG	SERVOVALVE-ALTN BRK WHEEL 2	LH N	571	32-43-27
2577GM	DUAL VALVE-ALTN BRK DISTR		148	32-43-14
2614GM	MANIFOLD-ALTN BRK SERVOVALVE,R		671	32-43-31
2615GM	MANIFOLD-ALTN BRK SERVOVALVE,L		571	32-43-31
2620GM	SAFETY VALVE-ALTN BRK, WHEEL 1		731	32-43-16
2621GM	SAFETY VALVE-ALTN BRK, WHEEL 4		741	32-43-16
2622GM	SAFETY VALVE-ALTN BRK, WHEEL 2		731	32-43-16
2623GM	SAFETY VALVE-ALTN BRK, WHEEL 3		741	32-43-16
2624GM	HYD RESERVOIR-BRK CTL		122	32-43-11
2625GM	MASTER CYL-BRK CTL,L		122	32-43-12
2626GM	MASTER CYL-BRK CTL,R		122	32-43-12

** ON A/C NOT FOR ALL

3. System Description

A. Auxiliary Low-Pressure Control-System

(1) Brake-control hydraulic reservoir

Brake-Control Hydraulic Reservoir ** ON A/C NOT FOR ALL

The reservoir supplies the master cylinders of the Alternate system with hydraulic fluid to make allowance for:

the changes in the volume of the fluid,
possible leakages.

A spring-loaded piston pressurizes the reservoir lightly (1.5 bars absolute).
The reservoir has a filling valve and two calibrated valves:

one which limits the internal pressure,
the other which removes fluid if an overpressure occurs.

(2) Master cylinders

Master Cylinder ** ON A/C NOT FOR ALL

Reservoir / Master Cylinder - Brake Control - Component Location ** ON A/C NOT FOR ALL

There are two master cylinders (one for the left side, one for the right side).
The master cylinders are used to:

control the distribution dual valve of the Alternate brake system by displacement of a volume of fluid from chamber C1,
give artificial feel at the pedals by means of an internal spring and the load resulting from the pressure that increases in the control circuit.

Master Cylinder Load in Relation to Travel ** ON A/C NOT FOR ALL

When the pedals are released an internal valve opens to connect the chamber C1 to the upper chamber C2 which is then connected to the hydraulic reservoir.
The chamber C2 contains a safety reserve of fluid.

(3) Alternate-brake distribution dual-valve

Dual Valve - Alternate Braking - Component Location ** ON A/C NOT FOR ALL

Alternate-Brake Distribution Dual-Valve ** ON A/C NOT FOR ALL

Alternate-Brake Distribution Dual-Valve-Schematic ** ON A/C NOT FOR ALL

The distribution dual valve includes two independent pressure-reducing valves, one for each main gear. A master cylinder operates each valve.
The distribution dual valve is installed in the hydraulics compartment.

Alternate-Brake Distribution Dual-Valve-Schematic ** ON A/C NOT FOR ALL

A transparent drain tube connected to the cover of the distribution dual valve, allows to check for possible presence of water.
A cap closes this tube.
Each pressure reducing valve includes a control piston which operates a distribution slide valve through a spring R1 and a rocker arm.
When the pedal is released, the Yellow pressure at A is shut off and the brake port C is connected to the reservoir return B.
When the pedal is pushed in, the volume of fluid moved by the master cylinder causes the displacement of the piston which then operates the rocker arm and the slide valve; B is shut off and the pressure port A is connected to the brake port C.
When line C is filled, the pressure pushes on the end of the slide valve (chamber D) which, through the rocker arm, causes the spring R1 to compress; the control piston remains in the same position.
A check valve is installed to rapidly move the fluid from the chamber D to the brake port C: thus the slide valve can be in the fully open position as soon as the pedals are pushed in and the brakes are filled without delay.
In balanced state, the pressure supplied is in proportion to the compression of the spring R1 and consequently to the displacement of the control piston.
The pressure increases in the control line as a result of the compression of the return springs of the control piston.
The delivery pressure (decreased) in relation to the control piston travel and in relation to the control pressure is shown on the graphs.
The maximum pressure supplied is 175 bars (2537 psi).

B. Brake System

(1) Automatic Selector (Alternate Mode)
The primary function of the automatic selector is to select the Normal hydraulic system (Green) (Ref. 32-42-00, paragraph 2.D) or the Alternate system (Yellow) for the supply of the brakes.
The Alternate system is used if the Green pressure is not available (selector valve de-energized or faulty, or selector valve energized but with the Green hydraulic system not available).
The automatic selector includes a primary stage which selects the system.
This primary stage includes:

a differential section piston which operates a slide valve, for Normal braking,
a set of valves for Alternate braking.

Thus the two systems are isolated.
When you change from Normal to Alternate, in the middle position, the piston which the Yellow pressure at D (Yellow pressure plus accumulator back-up) operates:

cuts off the return line (E to H),
then pressurizes the Alternate brake system (D to H).

The automatic selector also includes a secondary stage. This secondary stage is used to cut off the return from the servovalves of the Alternate brake system if:

the Yellow system pressure is lost. The brake Yellow-pressure accumulator is used; as the Alternate servovalve return is isolated, this prevents leakage,
the Parking brake is used. As the return is isolated, this causes the inhibition of the Alternate servovalves. The release of the brake during the electrical test of the servovalves is no longer possible.

The spring and the Yellow system pressure at G keep the secondary stage in the return open position.
The secondary stage changes to the return closed position:

when the Yellow system pressure is no longer applied at G (loss of the Yellow system): PG less than or equal to 115 bars (1667 psi), or
when a pressure is applied at J (Parking brake applied): PJ more than or equal to 138 bars (1968 psi).

(2) Dual Shuttle Valve

Dual Shuttle Valve ** ON A/C NOT FOR ALL

The dual shuttle valve includes two valves, one for each main gear, installed in a single block adjacent to the Alternate-brake distribution dual-valve. Each valve is used to select either the Alternate system (E or D) (spring biased position) or the Parking system (A) to supply the brake system of the corresponding gear (C or B).

(3) Alternate Brake-System Pressure-Transmitters and Brake Yellow-Pressure Triple-Indicator

Yellow Pressure Transmitters ** ON A/C NOT FOR ALL

Two pressure transmitters measure the pressure at the outlet of the dual shuttle valve, i.e. the pressure supplied either by the Alternate-brake distribution dual-valve or by the Parking-brake pressure limiter.
The pressure transmitters send the information to an indicator (brake Yellow-pressure triple-indicator) in the cockpit. The indicator also shows the pressure of the brake Yellow-pressure accumulator.

(4) Alternate Brake Servovalves-Safety Valves

(a) Servovalves

Alternate Brake Servovalve - Location ** ON A/C NOT FOR ALL

Alternate Brake Servovalve - Schematic ** ON A/C NOT FOR ALL

One servovalve supplies one brake only.
The servovalves are installed on a manifold installed in the shroud. The manifold includes:

2 servovalves,
2 safety valves with integral filter,
1 filter in the manifold pressure supply.
The servovalves are supplied with pressure when the pedals are pushed in.
When there is no brake regulation, the pressure applied to the brakes is equal to the pressure supplied to the servovalves.
The regulation system decreases the pressure at the brake.
The current moves a flapper located between two jets. Two coils are installed around the core which is mounted in the gap of a permanent magnet (only coil 1 is used).
The attraction of the core by the poles of the magnet is function of the current which passes through the coil.
The movement of the flapper modifies the sections of the fluid passage at the jets. Consequently the pressure in the control chambers of the slide valve is modified. The differential action of the pressure moves a slide valve. This fills or drains the brake depending on whether the current decreases or increases.
The slide valve is in a balanced position under the action of the pressure supplied on one of the opposite faces of the slide valve.
With zero current in the coil, the pressure supplied is equal to the pressure supplied to the servovalve.

(b) Safety valves

Alternate Brake Safety-Valve P/N C24757000 ** ON A/C NOT FOR ALL

Alternate Brake Safety-Valve P/N C25243002-2 ** ON A/C NOT FOR ALL

The principle of operation is the same as that of the Normal brake safety-valves (Ref AMM 32-42-00 Page Block 1).

** ON A/C NOT FOR ALL

4. Operation/Control and Indicating

A. Pedal Artificial Feel

(1) Control
A hydro-mechanical system transmits control inputs at the pedals. This hydro-mechanical system is installed between the master cylinders and the distribution dual valve (auxiliary low-pressure control-system).
The pressure delivered to the Alternate brake servovalves is in relation to the pedal load/angle.
Before you get the pressure,

the free travel of the pedals, corresponding to the closure of the internal valve of the master cylinders,
the displacement of the distribution slide valve to shut off the return and then let the pressure in,
and the compressibility of the fluid in the low pressure system must be taken up.

Incomplete bleeding of the low pressure system results in a decrease in the load and an increase in the free travel.
The brake Yellow-pressure triple-indicator shows the pressure supplied by the distribution dual valve to the LH and RH brakes. This indicator thus shows that the system is supplied with the Yellow pressure.
The servovalves can decrease the pressure at the brakes.

(2) Anti Skid Regulation
The regulators used for the Normal system are also used for anti skid regulation.

Alternate Braking with Anti Skid - Principle ** ON A/C NOT FOR ALL

The servovalve gets brake release orders.

[Rev.10 from 2021] 2026.04.01 12:29:46 UTC