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2. Auto flight

2.1. Overview
2.2. FCU
2.3. FMA
2.4. Flight director/ Autopilot
2.5. Autothrust
2.6. Flight management
2.7. Rules for use of autoflight systems
2.8. Guidance principals
2.9. Protections

2.1. Overview

Main processing is carried out by two identical Flight Management Guidance Computers (FMGC) which normally work in tandem. Together they are known as the flight management guidance system (FMGS). The pilots provide inputs to the FMGS using two Multipurpose Control and Display Units (MCDUs) on the center pedestal and a flight control unit (FCU) on the glare shield.

The flight management part of the FMGS controls navigation, flight planning, performance optimization, predictions and display management. The flight guidance part provides flight director, autopilot and auto-thrust commands. The flight augmentation part provides flight envelope computation, maneuvring speed computation, windshear detection, α-floor protection and various yaw functions.

The FMGCs normally receive information from "on-side" sensors, and communicate between themselves to validate data. The FCU feeds both FMGCs.

The master FMGC is determined by autopilot and/or flight director engagement. FMGC1 is master if AP 1 is on, both autopilots are on or both flight directors are on. The autothrust is driven by the master FMGC. The flight directors are always driven from their on side FMGC.

If the cross talk between the two FMGCs is lost, the FMGCs can operate independently. This mode of operation is indicated by an amber IND light at the top of each MCDU. In independent mode, the same information must be entered into both MCDUs to receive the same guidance.

In the event of loss of an FMGC, the remaining FMGC continues to operate normally. If the lost FMGC was master, the A/P and A/THR will disengage. The AP of the healthy FMGC can be engaged, and subsequently the A/THR can be engaged. This situation, known as single mode, is indicated by an amber FM1 light at the top of the MCDU on the failed side. The MCDU will now be copy of the MCDU driven by the healthy FMGC. The FMA will annunciate e.g. 2FD2 to indicate that both flight directors are being driven from one FMGC. The healthy FMGC also tunes the offside navaids. To restore the ND on the failed side, the range and mode must be set identically to that set on the healthy side.

If an FMGC has a software problem, the FMGS will temporarily revert to single mode while the affected FMGC auto resets. This may result in autopilot and autothrottle disconnect and reversion to selected modes. MAP NOT AVAIL is shown on the ND of the affected side and the status page with a "PLEASE WAIT" message appears on both MCDUs. Use of the MCDUs should be avoided during the reset as it will increase the reset time. Reset usually takes a few seconds.

The FCU display is driven by two redundant FCU controllers. A single failure will have very little effect. A double FCU failure will lead to the loss of both autopilots, both flight directors and the autothrust. The autothrust will revert to thrust locked until the thrust levers are manually moved. All targets are lost on the PFD. The EFIS control panels are lost, leading to a reversion to STD, rose NAV 80nm on the NDs and a reversion needle selection of VOR1 and ADF2. Weather radar image is also lost.

A locked MCDU may be recovered by turning it off, then back on after five seconds. This failure is not automatically detected. An amber FAIL on the MCDU requires the same procedure.

2.2. FCU

{TODO: Add picture of FCU}

The FCU has two channels, each able to drive the entire FCU.

In general, turning a knob will select a guidance target and pulling it will then engage a mode to guide the aircraft to that target. Pushing a knob, on the other hand, engages a mode managed by the FMGS. When pushed, dashes and a white dot appear in the associated window.

Change over between speed and mach occurs automatically at approximately FL300, although this can be overridden with the SPD MACH button.

The HDG-VS/TRK-FPA button toggles the lateral mode between heading and track and toggles the vertical mode between vertical speed and flight path angle. It also toggles the display of the flight path vector (FPV) (or "bird") and Flight Path Director (FPD) on the PFDs. FMA lateral mode annunciation are HDG and TRACK respectively. The FCU displays HDG and V/S legends when in attitude mode and TRK and FPA legends when in flight path mode.

The altitude window contains the next applicable clearance altitude selected by the crew with the target altitude knob and is never dashed. Pushing the knob engages a managed mode which guides the aircraft to the target altitude while adhering to altitude constraints set in the FMGS. Pulling the knob gives a selected mode that disregards these FMGS altitude constraints.

The VS/FPA knob differs from the other three in that there is no associated managed mode. Pushing this knob commands an immediate level off. Negative indications on the FPA/VS display on the FCU indicate descent.

The "EXPED" button temporarily sets speed to best climb or best descent speed in order to expedite towards the target altitude.

The AP buttons engage one or both autopilots. The A/THR button engages or arms the autothrust. Disengagement of autopilot or autothrottle may be acheived by pressing a lit button, but this is not a recommended procedure.

2.3. FMA

The Flight Mode Annunciator (FMA) is displayed at the top of the PFD. It is split into 5 columns:

  • Thrust

  • Vertical

  • Lateral

  • Approach capability and DA/MDA

  • FMGS engagement status

In certain modes the vertical and lateral columns combine to display "common modes" where the lateral and vertical modes are closely linked.

Each column has three lines. In the first three columns, the first line shows engaged mode, the second line shows armed modes and the third line shows reminders or messages.

When a mode changes, the mode is boxed on the FMA. A star next to a mode indicates a capture phase.

2.4. Flight director/ Autopilot

The flight directors come on automatically when the aircraft is first powered.

The AP can be used from just after lift off until the end of the landing roll-out. In most cases only one autopilot may be engaged at a time. The exception if on a coupled ILS approach, where the second autopilot may be engaged after arming approach.

Autopilots are disconnected by pushing the red pushbutton on either sidestick. Disconnection triggers a single aural alert and a temporary master warning with AP OFF message on the E/WD. A second push on the button cancels these warnings. Autopilots may also be disconnected by significant movement of the sidestick or rudder pedals, or by pushing a lit AP button on the FCU. This is treated as an involuntary disconnection and leads to a repeating aural warning and permanent master warning and message.

When flying manually with reference to the FD, the symbols for each PFD are driven by their on-side FGMC.

When taking off or landing on an ILS equipped runway, the flight director bars are replaced by a vertical green yaw bar to assist centerline tracking in LVOs.

When the bird is displayed, FD indications change to give FPA commands. The objective becomes to centre and align the bird with the green triangles at the wing tips. If TRK/VPA is selected when the basic modes are in force (HDG/VS), these modes automatically change to TRK and FPA.

2.5. Autothrust

Autothrust modes are automatically linked to AP/FD vertical modes. Autothrust speed mode, where the autothrust adjusts thrust to maintain a speed, is linked to trajectory type AP/FD vertical modes. Autothrust fixed mode is linked to AP/FD vertical modes where the speed is controlled by adjusting the aircraft attitude.

There are four detents on the throttle quadrant:



Gives max climb thrust


Gives FLEX thrust for takeoff or max continuous thrust


Give max takeoff or go around thrust.

The thrust levers do not move when autothrust adjustments are made. Instead, the thrust lever position controls the maximum thrust available to the autothrust system. It therefore does not operate when the thrust levers are at IDLE or in the reverse range.

The autothrust automatically arms when TOGA or FLEX/MCT is set for takeoff. At this point the thrust is still under manual control, indicated my a MAN indication in the FMA thrust segment. The autothrottle engages when the thrust levers are set to CL. Engagement is indicated by THR CLB appearing in green in the FMA thrust segment and A/THR appearing in the FMA engagement segment. In normal ops, the thrust levers are left at CL until retarded in the flare. Increased thrust may, however, be manually selected at any time by advancing the thrust levers beyond the CL detent. If an engine failure occurs, the autothrottle range is automatically extended to include the range between CL and MCT.

If autothrust disconnection is desired in flight, retard the thrust levers until the thrust lever position symbol roughly matches the present N1, then push the disconnect button on the side of the thrust lever. Autothrust may be completely inhibited for the remainder of the flight by holding one of these buttons down for more than 15 seconds.

2.6. Flight management

Each FMGC independantly calculates the aircraft position based on data from the ADIRS, the radio navigation aids and the GPS receivers. In normal ops, each FMGC uses the average position of the three IRSs. This is called Mix IRS. If an IRS fails, each FMGC uses a single IRS, either on-side or IRS3 as available. Each IRS also calculates a GPIRS position based on its own position and the average position indicated by the two GPS receivers. The FMGC uses only one of the three GPIRS positions which is automatically selected according to merit. The GPIRS data is subjected to integrity criteria, and if it fails, the GPS mode is rejected and the system falls back on radio position updating based on on-side auto-tuned radio navigation aids (DME,VOR and ILS). If this occurs, an amber GPS PRIMARY LOST message appears in the ND and the MCDU scratchpad.

The FMGC continually calculates a vector, known as BIAS, which represents the offset of the GPIRS or radio position (as available) from Mix IRS. The last known value of this vector is used to calculate the aircraft position if GPS and radio data become unavailable.

The FMGC also calculates an estimated position error (EPE) for RNP navigation purposes. EPE and FMGS database derived RNP are displayed on the MDCU. An ACCUR value is also provided. This is usually HIGH, but indicates LOW if EPE>RNP. If ACCUR is LOW, a RNPA NAV ACCUR DOWNGRAD warning is shown on the ND. If this occurs, or if GPS PRIMARY is lost, FM position should be manually cross checked with raw data. If the position is within 3nm, the FM position may continue to be used, but hourly raw data checks should be made.

The FMGS allows both a primary and a secondary flight plan to be entered. The secondary flight plan can be quickly activated when required.

When flying in NAV mode the aircraft is guided on a leg defined by a from and to waypoint. The to waypoint is shown in white on the MCDU and in the top right corner of the ND.

The flight plan is entered using the INIT page on the MCDU. It may be entered using a company route, a departure detination pair and manual entry of route or by sending a request for an active F-PLN initialization.

2.7. Rules for use of autoflight systems

Rules regarding FM navigation and flight planning

  1. Crosscheck FM navigation accuracy periodically if GPS is not primary and whenever GPS PRIMARY LOST or NAV ACCUR DOWNGRADE messages occur. This is done by comparing the FMS bearing and distance to a beacon against raw data.

  2. Ensure proper waypoint sequencing by monitoring the TO waypoint. If in heading mode with a large cross track error, waypoints will not be sequenced and will therefore require clearing manually.

  3. Keep a flight plan discontinuity only when desired.

  4. Anticipate your actions on the MCDU.

Rules regarding predictions

  1. Predictions are based on the assumption that the F-PLN route is flown in managed modes. If the aircraft is off flight plan, a realistic trajectory for recapture is assumed.

  2. Vertical deviation is shown on the altimeter as a round symbol (yoyo) in descent and against a scale as a brick in approach. In the latter case, 1 dot indicates 100ft deviation.

Rules regarding guidance

  1. Engagement of the managed vertical modes (CLB and DES) is not possible in non managed lateral modes (HDG or TRK). If the lateral mode is changed to a non managed mode, the vertical mode will revert to a non managed mode (OP CLIMB in climb, V/S or FPA in descent).

  2. NAV mode may be armed when HDG or TRK mode is used for F-PLN interception if the track crosses the active leg before the TO waypoint.

Rules regarding the displays

  1. If GPS is not primary and the FM accuracy check is failed, raw data must be displayed on the ND.

  2. Monitor FMS in managed modes. When NAV mode is used, monitor status on the FMA and adherence to required trajectory on ND. When CLB or DES modes are used, monitor altitude targets, speed targets and VDEV indications on PFD and pseudo waypoints on ND.

2.8. Guidance principals

A star after a mode on the FMA indicates a transitive capture mode, e.g. LOC* is displayed during the localiser capture maneuvre.

CLB is always associated to ALT mode. ALT will appear in the armed line in magenta if climb restrictions are associated with waypoints in the flight plan, otherwise it will appear in blue.

In DES mode, if the aircraft becomes high, the FMS prediction for regaining the profile assumes that half speed brakes will be extended.

When the aircraft reaches the cruise altitude set in the MCDU, ALT CRZ is displayed in the FMA. If the aircraft levels off prior to reaching this level, ALT is displayed.

RWY lateral mode provides lateral guidance from the start of the takeoff roll up to 30ft if a suitable LOC signal is available.

SRS (speed reference system) vertical mode provides vertical guidance up to acceleration altitude. As long as slats are extended and V2 is available to the FMS, SRS engages automatically when power is applied for takeoff. It commands a speed of V2+10 during normal operations. If an engine fails, V2 is commanded. SRS also maintains a minimum rate of climb without regard to speed control to provide windshear protection. At acceleration altitude SRS is automatically replaced by CLB mode, which accelerates the aircraft to initial climb speed.

NAV mode engages automatically at 30ft provided a RWY and SID have been inserted in the flight plan.

Thrust reduction is indicated by a flashing LVR CLB message in the message area of the FMA thrust segment. Thrust levers must be manually moved to the CLB detent, wherupon the autothrottle will engage.

The localiser may be intercepted in NAV mode providing that accuracy is HIGH (i.e. epe>rnp) or GPS is primary.

In order to arm approach, the ILS and RA must be serviceable, both ILS receivers must be tuned to the same frequency and have the same course and the aircraft must be above 400ft. G/S* will generally not engage unless LOC* or LOC modes are active.

If the ILS ground transmitter becomes unserviceable during an approach, the deviation bars are removed, and the FD bars flash. The AP does, however, remain engaged in G/S and LOC modes.

If the aircraft suffers a dual ILS receiver failure, the deviation bars are replaced by failure flags, the guidance reverts to basic modes and the autopilot trips out.

LAND mode engages below 400ft when LOC and G/S modes are engaged. When LAND mode is engaged, inputs on the FCU are disregarded. It can only be disengaged by a go-around.

FLARE mode engages at 40ft. The FD bars are replaced by the yaw bar and the AP/FD commands a suitable pitch angle for the flare. If A/THR is active, an automatic RETARD call is made at 10ft.

Below 200ft, a red AUTOLAND warning will be triggered if:

  • both autopilots trip off

  • there is a loss of or excessive deviations in LOC (inhibited below 15ft)

  • there is a loss of or excessive deviations in G/S (inhibited below 100ft)

  • there is a disagreement in RA indications

The FMGS allows non precision approaches to be flown in managed mode as long as the approach is available in the nav database and the required aids and courses are manually set in the MCDU RADIO NAV page. Unless GPS PRIMARY is displayed, accuracy must be checked against raw data. Also, selected modes must be used if accuracy is LOW. The managed mode approach is armed with the APPR pb. If the lateral mode at this time is NAV, APP NAV will immediately engage. Otherwise it will arm, and will engage following the standard rules of NAV engagement. The vertical mode will arm FINAL and the FINAL APPR combined mode will engage when the preplanned decent path is intercepted. A V/DEV scale to the right of the attitude indicator then shows deviations from this path. The FINAL APPR mode disengages at MDA-50ft or 400ft AGL if MDA was not entered.

2.9. Protections

Engine failure compensation

If an engine fails with the AP on, the FMGC provides automatic yaw compensation. This is acheived using the yaw damper during take-off and go-around and the automatic rudder trim in all modes.

SRS pitch mode automatically adjusts the target speed if an engine fails during take-off or go-around.

Low speed protection

The A/THR will not fly speeds below Vls, even if selected by the pilot.

When between 100ft and 2000ft and in configuration 2,3 or FULL, a drop in speed that is significantly below Vls, taking into account deceleration rate and flight path angle, will trigger a repetetive "SPEED, SPEED, SPEED aural warning.

If angle of attack increases above a given threshold known as the alpha floor and the aircraft is above 100ft, the A/THR engages in A FLOOR mode and commands TOGA thrust. This occurs even if the A/THR is turned off. Alpha floor protection is usually available from lift off until 100ft RA on approach, but may be lost in cases of multiple failures and when an engine-out occurs above CONF 1. When A FLOOR mode exits, TOGA LK mode engages which locks the thrust at TOGA without regard to thrust lever position. To regain control of the thrust, the auto-throttle must be disengaged.


Reactive windshear warnings are available from lift off to 1300ft at take off and from 1300ft down to 50ft on landing, provided at least CONF 1 is selected. It triggers an aural "WINDSHEAR, WINDSHEAR, WINDSHEAR" alert and a red WINDSHEAR message is displayed on the attitude indicator.

When SRS mode is engaged a minimum rate of climb is commanded regardless of speed. If the angle of attack gets too great, the autopilot disengages, but pilot assistance is still provided by the fly by wire maximum angle of attack protection.

In approach, the GS mini function adjusts speed with wind variation, ensuring that ground speed does not drop below a minimum value.

Mode reversions

When an altitude that is lower than the current altitude is selected during climb or an altitude higher than the current altitude is selected during descent, the vertical mode reverts to VS at the current rate of climb or descent.

If the lateral mode is changed from a managed to an unmanaged mode when the vertical mode is CLB, it will revert to OP CLB. The aircraft attitude will not change, but altitude constraints will be lost. Similarly a change to unmanaged lateral mode in descent leads to DES changing to VS with the current rate of descent. Again, aircraft attitude is unchanged.

With FDs engaged and AP disengaged, if failure to follow flight director commands with the A/THR in THR IDLE mode results in speed dropping to Vls-2kt (Vls-17kt with speedbrakes extended), the flight director bars are removed. This causes the A/THR mode to revert to SPEED and the target speed to be recaptured. Similarly, if the aircraft speed reaches Vmax+4kt with the A/THR in THR CLB mode, the A/THR will revert to SPEED to recapture the target speed.