CONFIGURATION AND OPERATIONAL SPEED COMPUTATION - DESCRIPTION AND OPERATION

** ON A/C NOT FOR ALL

1. General
The Flight Augmentation Computer (FAC) has different functions independently of the engagement status of the FLT CTL/FAC pushbutton switches.
These functions are necessary for:

The control of the speed scale on the PFDs
The adaptation of gains of the Flight Management and Guidance Computer (FMGC) and the Elevator Aileron Computer (ELAC)
The distribution of signals for the FMGC control laws
The protection of the Flight Envelope (FE) in automatic flight (speed limits for the FMGC and alpha-floor for the autothrust)
The display of the slat/flap maneuver speed
The windshear warning (pin program activation)
The low energy warning
The display of the positions of the control surfaces.

The FAC therefore calculates:

The weight and the Center of Gravity (CG)
The characteristic speed data
The aerodynamic flight-path angle and the potential flight-path angle
The alpha-floor protection
The position of the rudder trim for the ECAM system
The position of the rudder travel limiter for the ECAM system.

The Flight Augmentation Computer (FAC) has different functions independently of the engagement status of the FLT CTL/FAC pushbutton switches.
These functions are necessary for:

The control of the speed scale on the PFDs
The adaptation of gains of the Flight Management and Guidance Computer (FMGC) and the Elevator Aileron Computer (ELAC)
The distribution of signals for the FMGC control laws
The protection of the Flight Envelope (FE) in automatic flight (speed limits for the FMGC and alpha-floor for the autothrust)
The display of the slat/flap maneuver speed
The windshear warning (pin program activation)
The low energy warning
The display of the positions of the control surfaces
The Runway Overrun Prevention System (ROPS) (pin program activation).

The FAC therefore calculates:

The weight and the Center of Gravity (CG)
The characteristic speed data
The aerodynamic flight-path angle and the potential flight-path angle
The alpha-floor protection
The position of the rudder trim for the ECAM system
The position of the rudder travel limiter for the ECAM system
The aircraft distance-to-stop on the detected runway.

** ON A/C NOT FOR ALL

2. System Description

A. Presentation of characteristic speed data

Interconnections Between FAC and Users

The characteristic speed data are shown on the PFDs with the Display Management Computers (DMC).
In normal operation:

FAC1 transmits data to the CAPT PFD
FAC2 transmits data to the F/O PFD.

The transmitted data are validated from:

The status matrices of the transmitted labels
The FAC HEALTHY hard-wired discretes.

If a failure has an effect on minimum one label or the computer itself, the related DMC is automatically connected to the opposite FAC.
If there is a DMC failure, the related PFD is connected to DMC3.
Since the FAC processes Air Data and Intertial Reference System (ADIRS) data (Ref. AMM D/O 22-65-00-00), this makes sure that the characteristic speed data stay shown:

At first detected or undetected failure of the ADIRS by the same FAC
At second detected failure of the ADIRS by the FAC related to the other ADIRS.

The FAC processes the display logic of the different speed data specially with the positioning of the label status matrices:

Failure warning: source change sequence
Non Computed Data (NCD): cancelation sequence for the used label.

The information transmitted to the FMGC (speed data, weight, CG, flight-path angle, alpha-floor) is processed from:

The correct status matrices
The FAC HEALTHY hard-wired discretes.

B. Definition and symbols

(1) Definition and presentation of speed data on the PFD

FAC Data on the Speed Scale of the PFD (Sheet 1/2)

FAC Data on the Speed Scale of the PFD (Sheet 2/2)

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

! Speed ! Definition ! Presentation ! Presentation on the PFD !

!--------!---------------------!------------------!---------------------------!

! VSW ! For the ELAC ! After lift-off ! Red checkered tape at !

! ! alpha-floor ! in direct law ! the bottom of the scale !

! ! protection ! only ! !

!--------!---------------------!------------------!---------------------------!

!VALPHA ! Speed related ! After lift-off ! Amber and black stripe at !

!PROT ! to ELAC alpha-floor ! in normal law ! the bottom of the scale !

! ! protection ! ! !

!--------!---------------------!------------------!---------------------------!

!VALPHA ! Minimum speed ! After lift-off ! Red stripe at the bottom !

!LIM ! related to ! in normal law ! of the scale !

! ! ELAC alpha-floor ! ! !

! ! protection ! ! !

!--------!---------------------!------------------!---------------------------!

! ! 1.13 Vs 1g takeoff ! ! Amber stripe at the !

! VLS ! 1.23 Vs 1g elsewhere! Ditto ! bottom of the scale !

! ! 0.2 g/buffeting ! ! !

! ! 1.28 Vs 1g in clean ! ! !

! ! configuration ! ! !

!--------!---------------------!------------------!---------------------------!

! ! Drift down speed ! Clean configura- ! O !

! VMAN ! ! tion ! Green dot !

!--------!---------------------!------------------!---------------------------!

! V3 ! 1.18 Vs 1g of 18/10 ! Flaps extended ! F !

! ! ! except full ! !

! V4 ! 1.18 Vs 1g of 0/0 ! Only slats ! S !

! ! ! extended ! !

!--------!---------------------!------------------!---------------------------!

! VFE ! Max flap and slat ! Flaps or slats ! Red checkered tape at the !

! ! extended speed ! extended ! top of the scale !

! VLE ! Max landing gear ! Landing gear ! !

! ! extended speed ! extended ! !

!--------!---------------------!------------------!---------------------------!

! VC ! Airspeed tendency ! After ! Pointer moves to the !

! TREND ! ! lift-off ! calculated airspeed symbol!

!--------!---------------------!------------------!---------------------------!

! VM0 ! VM0 + MM0 ! Ditto ! Red checkered tape at the !

! ! ! ! top of the scale !

!--------!---------------------!------------------!---------------------------!

! VMAXOP ! 0.2 g related ! Ditto except if ! Not shown !

! ! to buffeting ! VMAX OPP<VM0 ! !

!--------!---------------------!------------------!---------------------------!

! ! Predictive VFE at ! Z<20,000 ft. ! Two amber stripes !

! VFEN ! next slat/flap ! Not shown in ! !

! ! position ! full position ! !

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

The definition of the different speed data is given as follows:

VSW is for the stall warning speed.
VALPHA PROT is for the speed related to the Angle Of Attack (AOA) that the operator gets when the ELAC alpha-floor protection is activated.
VALPHA LIM is for the minimum speed which the operator gets in the in ELAC alpha-floor protection.
VLS is for the lower selectable speed for a given configuration.
VMAN (green dot) is for the maneuvering speed.
This speed shows the drift down speed which agrees with the optimum speed (maximum lift-to-drag ratio) if there is an engine failure.
V3 and V4 is for the minimum flap and slat retraction speed:
V3(F) is for the minimum flap-retraction speed.
V4(S) is for the minimum slat-retraction speed.
VMAX is for the maximum allowable speed.
The speed must not be more than this maximum value limit. For the different configuration, these smallest values are shown:
VFE is for the maximum flap and slat extended speed.
VLE is for the maximum landing-gear extended-speed in clean configuration.
VM0/MM0 is for the maximum operating-limit speed.
VMAXOP is for the maximum selectable speed.
VC TREND is for the airspeed tendency.
It agrees with the speed increment in ten seconds with the correct acceleration of the aircraft.
VFEN is for the landing phase, it agrees with the VFE at next slat/flap position.

** ON A/C NOT FOR ALL

3. Operation

A. Speed calculation principle

Speed Computations - General Architecture

The calculation principle is in relation to the fact that most of the speed data given are related to the weight.
As the weight changes slowly, this parameter becomes stable when there is a modification of the configuration (for example, speedbrakes or control surfaces extended, deceleration, turn) to prevent transients on the speed presentation.
On the ground, weight and XG initializations are made with the Flight Management System (FMS).
In cruise phase (Zp >15,000 ft and Vc >250 kts), the engine consumption laws give approximate updating calculations in the FAC.
The sequence of the calculations (which gives the characteristic speed data) and the parameters is shown in the figure.
The calculation is started from the curves Cz max and from the conditions of equilibrium of the aircraft with thrust and balance correction. This lets the operator get the stall warning speed Vs 1g.
From stall warning speed Vs 1g, the FAC calculates the aircraft weight with:

The equilibrium incidence
The equilibrium speed
The thrust
The CG
The altitude.

The calculation of the weight is stable above the calculation range.
The weight is aligned again with a correcting fuel-used term specified in the FAC.
The data that follows is given from the weight calculation:

Calculation of the CG with the stability plane, altitude, configuration and speed.

The FAC uses its weight and CG to calculate characteristic speeds (VLS, VMAN (green dot), V3 (F), V4 (S)).

Speed Computation - Use of FM Weight/CG ** ON A/C NOT FOR ALL

List of Abbrevations:
Az1 is for the vertical acceleration in relation to aircraft centerline.
AOA is for the voted angle of attack.
Vc is for the corrected airspeed.
N1L and N1R are for the left or right engine thrust.
Phi is for the bank angle in relation to aircraft centerline.
Teta is for the pitch attitude.
Z°° is for the vertical acceleration in relation to a given reference.
VZBI is for the baro inertial vertical-speed.
M is for the mach number.
S/F is for the slat/flap position.
CG is for the center of gravity.
Z is for the altitude.
AB is for the speed brakes.
mFMS is for the weight given by FMS.
t(s) is for the time.
MLGS is for the landing-gear shock absorber compressed.
Delta N1 is for the delta between N1 command and N1 actual.
m(t) is for the weight as a function of time (consumption law).
iH is for the Trimmable Horizontal Stabilizer (THS) position.
PS is for the static pressure.
The sequence of the calculations (which gives the characteristic speed data) and the parameters is shown in the figure.
The calculation is started from the curves Cz max and from the conditions of equilibrium of the aircraft with thrust and balance correction. This lets the operator get the stall warning speed Vs 1g.
From stall warning speed Vs 1g, the FAC calculates the aircraft weight with:

The equilibrium incidence
The equilibrium speed
The thrust
The CG
The altitude.

Calculation of the CG with the stability plane, altitude, configuration and speed.

To make the FAC and FMS characteristic speeds (shown on the PFDs and on the Multipurpose Control and Display Units (MCDUs)) (basic or optional) agree, the FAC uses the Flight Manual (FM) weight and the CG to calculate characteristic speeds (VLS, VMAN (green dot), V3 (F), V4 (S)).

Speed Computation - Use of FM Weight/CG ** ON A/C NOT FOR ALL

B. Calculation of aerodynamic Flight Path Angle (PFA) (gamma-a) and potential PFA (gamma-T)

FAC - Computation of Aerodynamic Flight-Path Angle (Gamma-a) and Potential Flight-Path Angle (Gamma-T)

These data which are necessary for the FMGC control laws are calculated and duplicated in the FAC.

C. Calculation of alpha-floor protection

Alpha Floor _ Computation and Interface

FAC - Alpha Floor

The alpha-floor protection is calculated in the FAC but not duplicated.
This function lets the operator:

Prevent the aircraft against too much AOA.
To do this, a comparison is made between the aircraft AOA and the given thresholds function of configuration.
Above the thresholds, the FAC transmits a command signal to the autothrust which will apply full thrust.
Prevent the aircraft from longitudinal wind differences in approach with the wind acceleration which the operator calculates (from the difference between ground acceleration and air acceleration).

At the second detected or not detected failure of the ADIRS, the alpha-floor signal is not available at this time.
When the FAC or the ELAC makes the first detection, the ELAC direct calculation of the alpha floor is directly used.

D. Rudder trim position

ECAM - RTL / Rudder Trim Position

The correct position of the rudder trim (rudder position) is sent to:

The rudder trim indicator
The ECAM system
Label 313.

This position is validated with the parameters that follow:

The status matrix changes to:
. NCD if the RUD TRIM function is not available
. Failure warning if there is a computer failure.
The FAC HEALTHY hard-wired discretes.

A flag comes into view if the signal is not available.

E. Rudder travel limiter
The correct position of the rudder-travel limitation unit is sent to the ECAM system ("F/CTL" page).
The status matrix validates this position which is Normal Operation (NO) only if the Rotary Variable Differential Transducer (RVDT) sensor monitoring is correct.
In NO, the Travel Limitation Unit (TLU) indexes are shown in green, if they are not shown.

F. Windshear warning

(1) General
The windshear is a sudden change in the wind direction and/or speed for a short distance in the atmosphere. This can have an effect on the aircraft performance during the takeoff and landing phases.
In windshear conditions, the principle is to decrease the detection threshold in relation to the detected windshear to do a Go-Around (GA) maneuver in a short time.

Windshear Detection Principle

(2) Warning

Windshear Architecture

NOTE: When one FAC is incorrect, the two PFDs use data from the remaining FAC.
The FAC which does the detection function gives the signals necessary to send the windshear warning.
This warning is activated when:

The windshear function is activated with pin programming.
A windshear condition is detected.
The aircraft is in the takeoff or landing phase (altitude and slats/flaps configuration conditions).
Specified monitoring functions do not detect windshear function defects.

The flight crew knows about the windshear warning with:

The red "WINDSHEAR" message which comes into view on the PFDs above the horizon line (sky area).
The "windshear" aural warning which is heard three times.
When the parameters which cause this warning are not correct, the windshear warning is prevented in the related FACs.
When the two FACs cannot give this warning (two inhibit), the FWC gives the related amber level 2 warning message on the slat extension and is shown on the upper ECAM Display Unit (DU).

(3) Calculation
It makes a comparison between the instantaneous energy situation of the aircraft or its short predictable situation, and the minimum energy situation for aircraft security.
To do so, it is necessary:

To detect longitudinal winds (head wind or tail wind) mixed or not with a down draft.
To give a warning, independently of the alpha floor, in relation to the existing structure of the same alpha floor which shows increments of the AOA value of the aircraft with equivalent AOA values given by wind detections. This lets some anticipation in relation to a normal alpha floor law related to the aircraft AOA.
To show this warning and possibly to show that it is not available.
With the Flight Direction (FD) or Autopilot (AP) takeoff or GA (SRS law) modes, to follow a safe path if there is a windshear detection.

(4) Test
This test lets the operator do a check that the system transmits and shows:

Visual and aural indications of the "WINDSHEAR" warning
Messages to show the loss of the function.

It is done with the Centralized Fault Display System (CFDS) with the means of the MCDU (aircraft on the ground with engines stopped).
It is activated when the operator selects these functions on the MCDU:

CFDS
SYSTEM REPORT/TEST
Automatic Flight System (AFS).

Then, it is necessary to scroll the different MCDU pages.

MCDU - AFS/Windshear Test

For "AFS/WINDSHEAR TEST-3 & -6" pages, when the predictive windshear function is active:

On the Engine/Warning Display (EWD), make sure that the "REAC W/S DET FAULT" message (and not the "WINDSHEAR DET FAULT" message) comes into view.
On the "STATUS" page, make sure that the "REAC W/S DET" message (and not the "WINDSHEAR DET FAULT" message) comes into view.

G. Low energy warning

(1) General
The low energy function is used to prevent the aircraft from a low energy situation because it gives the pilot the audio warning: "SPEED...SPEED...SPEED".
The pilot must increase thrust and the low energy warning goes out of out view as soon as:

The thrust level is high.
The alpha-floor protection is activated, or
The pitch GA mode is activated.

The low energy warning is available in configuration 2, 3 and FULL and between 100 ft and 2000 ft Radio Altimeter (RA).

Low Energy Warning Principle

(2) Warning

Low Energy Warning Architecture

A combination of AOA, FPA and deceleration is calculated and compared to a threshold, in relation to the slat/flap configuration, usually 0.4 degrees to 1 degrees which is less than the alpha floor threshold.
When the result of the combination is more than this threshold for more than 0.5 seconds, a low energy condition is shown for a minimum of three seconds and sent to the FWC that sends the audio warning.

(3) System properties
The low energy detection is not duplicated.

H. Runway Overrun Prevention System (ROPS)/ Runway and Overrun Warning (ROW) Warnings

(1) General
This function is a safety enhancement for all the braking modes (manual and automatic) at landing that prevents overrun risk.
It helps the flight crew see an overrun risk during the landing phase. For this, it calculates the real-time braking distances and compares them with the landing distances available.

(2) Warning

ROPS Architecture ** ON A/C NOT FOR ALL

If there is a runway-overrun situation detected, for example, if this(these) calculated landing distance(s) is(are) more than the runway length, the ROPS function is used as follows:

The ROW sends alerts (PFD messages and aural alert) to help the flight crew to make a decision for a GA manoeuvre if necessary.
During landing roll:
. The ROP sends alerts (red PFD messages and aural alerts) which include full brake with pedals.
. The ROP sends alerts (red PFD messages and aural alerts) which include to select or keep maximum reverse thrust without delay.

These alerts go off if the runway overrun risk is not detected at this time.

(3) Calculation
The FAC calculates the minimum landing distances on the dry and wet runways. It detects the risk of runway overrun and sends the ROW alert requests.
It calculates an estimate of the distance to stop, sends ROP alert requests and monitors the availability of the ROPS function.

(4) General
This function is a safety enhancement for all the braking modes (manual and automatic) at landing that prevents overrun risk.
It helps the flight crew see an overrun risk during the landing phase. For this, it calculates the real-time braking distances and compares them with the landing distances available.

(5) Warning

ROPS Architecture ** ON A/C NOT FOR ALL

If there is a runway-overrun situation detected, for example, if this(these) calculated landing distance(s) is(are) more than the runway length, the ROPS function is used as follows:

The ROW sends alerts (PFD messages and aural alert) to help the flight crew to make a decision for a GA manoeuvre if necessary.
During landing roll:
. The ROP sends alerts (red PFD messages and aural alerts) which include full brake with pedals.
. The ROP sends alerts (red PFD messages and aural alerts) which include to select or keep maximum reverse thrust without delay.

These alerts go off if the runway overrun risk is not detected at this time.

(6) Calculation
The FAC calculates the minimum landing distances on the dry and wet runways. It detects the risk of runway overrun and sends the ROW alert requests.
It calculates an estimate of the distance to stop, sends ROP alert requests and monitors the availability of the ROPS function.

(7) WET/DRY condition selection

ROPS Rotary Selector-Switch ** ON A/C NOT FOR ALL

The ROW/ROP selector switch lets the flight crew manually set the wet or dry runway condition.

[Rev.10 from 2021] 2026.03.31 23:10:27 UTC