quarta-feira, 13 de fevereiro de 2008

Automation in Aviation - PROs & CONs

As long as one Flight Augmentation Computer (FAC) is valid, it governs the flight envelope function, the rudder position display, and the rudder trim indication regardless of what the pilot does with the FAC pushbutton.


The FAC controls the speed scale on the PFD. (see illustration image above)

(Refer to 1.31 in FCOM)

When both FACs are operative, FAC1 supplies data to PFD1 and FAC2 supplies it to PFD2.

The FAC computes :

The minimum and maximum speeds :
- VSW (stall warning)
- VFE and VFE for the next configuration

The maneuvering speeds :
- Green Dot Speed
- S speed
- F speed
(Refer to 3.04.10 for speed definition in the FCOM).

The FAC also computes the speed trend and displays it as an arrow on the PFD speed scale.
The PFD displays these various speeds as appropriate, and they also go to the FMGC to be used as limits for various guidance modes.

Note : The principle of the speed computation is as follows :

First, the FAC computes VS1G (stall speed). From VS1G it computes the gross weight which is also sent to the Elevator Aileron computers :
- When the aircraft is below 14500 feet and 250 knots, it computes this from current angle of attack, speed/Mach, altitude, thrust, and CG.
- When the aircraft is above 14500 feet or 250 knots, it computes this out of the GW, which it has memorized and updated with a fuel consumption model set in the FAC.

Finally the FAC computes the various minimum and maneuvering speeds, V prot and Vsn.

The accuracies of the various minimum and maximum speeds are functions of the accuracy with which the FAC computes aircraft gross weight. Normal accuracy for VLS in CONFIG FULL is about ± 3 kt.


Alpha-floor protection automatically sets the thrust at TOGA thrust, when the aircraft reaches a very high angle of attack.
The Flight Augmentation Computer (FAC) generates the signal that triggers the alpha-floor mode. This, in turn, sets TOGA thrust on the engines, regardless of the thrust lever positions (Refer to 1.22.30 A/THR modes in the FCOM).

The FAC sends this signal when :

the angle of attack is above a predetermined threshold, which is a function of the configuration.

In CONF3 and CONF FULL, this threshold decreases as a function of the aircraft deceleration rate (down to - 3°).

Alpha-floor is available from lift-off until the aircraft reaches 100 feet RA in approach.

Note :

Alpha-FLOOR is lost, when one of the following combinations of failures occurs:
SFCC1 and FAC2, or
SFCC2 and FAC1, or
Both FCU channels, or
1 EIU, or

Both FMGCs

Alpha-floor is lost under alternate or direct flight control law.

Alpha-floor is lost when the FADECs are in N1 degraded mode.

Alpha-floor is lost in engine-out, when slats/flaps are extended.


An aural low-energy "SPEED SPEED SPEED" warning, repeated every five seconds, warns the pilot that the aircraft's energy level is going below a threshold under which he has to increase thrust, in order to regain a positive flight path angle through pitch control.

It is available in Configuration 2, 3, and FULL. The FAC computes the energy level with the following inputs :
- Aircraft configuration

- Horizontal deceleration rate

- Flight path angle

The warning is inhibited when :

- TOGA is selected, or

- Below 100 feet RA, or

- Above 2000 feet RA, or

- Alpha-floor, or the ground proximity warning system alert is triggered, or

- In alternate or direct law, or

- If both radio altimeters fail.

During deceleration, the low-energy warning is triggered before alpha floor (unless alpha floor is triggered by stick deflection). The amount of time between the two warnings depends on the deceleration rate.

(scrap from A320 FCOM)

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