A320 Auto Flight System FAQ

The A320 Auto Flight system is built from four component types: two 𝗙𝗠𝗚𝗖𝘀 (Flight Management and Guidance Computers), one 𝗙𝗖𝗨 (Flight Control Unit), two 𝗠𝗖𝗗𝗨𝘀 (Multipurpose Control and Display Units), and two 𝗙𝗔𝗖𝘀 (Flight Augmentation Computers).

Together they form the 𝗙𝗠𝗚𝗦 - the Flight Management and Guidance System. One cohesive system, deliberately distributed.

Reference: WZZ FCOM DSC-22_10-10

𝗙𝗠𝗚𝗦 stands for Flight Management and Guidance System. It is the integrated system that handles long-term flight planning and predictions (FMGCs), tactical target selection (FCU), pilot data entry (MCDUs), and envelope protection (FACs). The architecture distributes computation deliberately so that when one component goes silent, the others carry the load.

Reference: WZZ FCOM DSC-22_10-10

Two FMGCs - 𝗙𝗠𝗚𝗖𝟭 and 𝗙𝗠𝗚𝗖𝟮. In normal operation they cross-talk over a dedicated bus to keep both pilot interfaces synchronized. Master assignment between them follows an autopilot-then-flight-director ladder. The slave FMGC continuously synchronizes data with the master.

Reference: WZZ FCOM DSC-22_10-30

The 𝗙𝗠𝗚𝗖𝘀 are the long-term computational engine - they hold the flight plan, perform predictions, and manage performance computation. The 𝗙𝗖𝗨 is the short-term tactical interface where the pilot selects targets and engages modes. The 𝗠𝗖𝗗𝗨𝘀 are the data interface (keyboards and screens) for the FMGCs. The 𝗙𝗔𝗖𝘀 handle flight envelope and dynamic protection functions. Each component owns a specific job; the roles do not overlap by accident.

Reference: WZZ FCOM DSC-22_10-10

The 𝗙𝗠𝗔 (Flight Mode Annunciator) is located at the top of the PFD and is the authoritative display of what the Auto Flight system is actually doing. The FMA shows active and armed modes in five columns. The FCU shows your intent; the FMA shows the truth. Pilots should always confirm mode changes from the FMA, never from the FCU.

Reference: WZZ FCOM DSC-22_30-100

Master assignment follows an 𝗔𝗣-𝘁𝗵𝗲𝗻-𝗙𝗗 ladder.

Reference: WZZ FCOM DSC-22_10-30

𝗜𝗻𝗱𝗲𝗽𝗲𝗻𝗱𝗲𝗻𝘁 𝗺𝗼𝗱𝗲 is a degraded FMGS condition triggered by abnormal conditions such as different navigation database validity between the two FMGCs. In independent mode, the two FMGCs work separately and each MCDU only affects its onside peripherals. In flight, the crew must make every entry on both MCDUs to maintain synchronization. Independent mode is rare and signals that the system can no longer vouch for cross-channel coherence.

Reference: WZZ FCOM DSC-22_10-30

No. Independent mode is triggered by 𝗮𝗯𝗻𝗼𝗿𝗺𝗮𝗹 𝘀𝘆𝘀𝘁𝗲𝗺 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀 such as database validity mismatch between the two FMGCs - not by routine data-entry mismatches. Treating an ordinary entry mismatch as a trigger for independent mode misreads the architecture.

Reference: WZZ FCOM DSC-22_10-30

The FMGC prediction engine continuously computes:

It uses the active F-PLN, the FMGC databases (including the onboard Aircraft Performance database), real-time ground tracking, and wind data entered on the MCDU performance pages.

Reference: WZZ FCOM DSC-22_10-10

𝗘𝗢 𝗖𝗟𝗥 is a soft-key prompt on the MCDU PERF page that appears when the FMGC detects an engine-out condition. It exists as a recovery path for cases where the FMGC 𝗳𝗮𝗹𝘀𝗲𝗹𝘆 detected an engine-out and you need to return the system to two-engine computation. The prompt is there for false-detection recovery, not for use during real engine failures.

Reference: WZZ FCOM DSC-22_20-60-40; FCTM AO-020

Pressing 𝗘𝗢 𝗖𝗟𝗥 during an actual engine failure is an 𝗶𝗿𝗿𝗲𝘃𝗲𝗿𝘀𝗶𝗯𝗹𝗲 action that deletes single-engine predictive computation and returns the FMGC to a two-engine profiling template. Single-engine performance predictions cannot be recovered for the ongoing failure unless the system detects a new engine-out condition. The FCTM specifically warns against pressing EO CLR during a real engine-out.

Reference: WZZ FCOM DSC-22_20-60-40; FCTM AO-020

During an engine failure, the 𝗣𝗥𝗢𝗚 (Progress) page displays the 𝗥𝗘𝗖 𝗠𝗔𝗫 𝗘𝗢 altitude - the recommended maximum cruise altitude with one engine inoperative. The altitude is computed by the FMGC based on current gross weight and atmospheric conditions. This altitude becomes the primary cockpit baseline for drift-down strategy execution per QRH guidance. Flying above the REC MAX EO compromises the single-engine climb gradient.

Reference: WZZ FCOM PRO-ABN-ENG

The 𝗙𝗖𝗨 (Flight Control Unit) is the glareshield panel where the pilot selects tactical targets (speed, heading, altitude, vertical speed) and engages AP, FD, and A/THR modes. The FCU does not compute trajectories, run performance math, or store the flight plan. It selects short-term targets that the FMGC uses to steer the airplane.

Reference: WZZ FCOM DSC-22_10-40

𝗣𝘂𝘀𝗵𝗶𝗻𝗴 an FCU knob engages 𝗺𝗮𝗻𝗮𝗴𝗲𝗱 𝗺𝗼𝗱𝗲, which delegates the target to the FMGC database. The system uses speeds, altitudes, or tracks computed from the F-PLN. 𝗣𝘂𝗹𝗹𝗶𝗻𝗴 an FCU knob engages 𝘀𝗲𝗹𝗲𝗰𝘁𝗲𝗱 𝗺𝗼𝗱𝗲, which uses the specific value the pilot manually dialed into the FCU window.

Reference: WZZ FCTM OP-030

𝗣𝘂𝗹𝗹𝗶𝗻𝗴 is the only way to instantly override the FMGC programming without amending the flight plan. When ATC issues an immediate vector, speed change, or altitude restriction, the pilot pulls the relevant knob to assert the new target. Pulling is the architectural escape path the system was designed to accommodate, separate from the slower process of MCDU re-programming.

The 𝗙𝗖𝗨 panel windows display 𝘀𝗲𝗹𝗲𝗰𝘁𝗲𝗱 𝘁𝗮𝗿𝗴𝗲𝘁𝘀 - what the pilot has asked for. The 𝗙𝗠𝗔 displays 𝗮𝗰𝘁𝗶𝘃𝗲 𝗺𝗼𝗱𝗲𝘀 - what the system is actually doing. Modes can shift on their own through reversions, system priority transitions, and autopilot priority handovers. A pilot who confirms modes from the FCU panel will be wrong precisely when the cockpit demands precision. Always call mode changes from the FMA.

Reference: WZZ FCOM DSC-22_10-40

The primary operational MCDU pages are:

These pages are 𝗶𝗻𝗽𝘂𝘁𝘀, not displays - data entry on them alters the underlying flight path model that the FMGC uses to steer the airplane.

Reference: WZZ FCOM DSC-22_10-40

There is no spell-checker on the MCDU. The FMGC trusts whatever is typed in. A typographical error in weights or runway designators instantly skews the auto flight system's thrust limits, climb predictions, and characteristic speeds. The FMGC will produce precise math from inaccurate inputs - which means the airplane will fly with confidence to the wrong answer. Crew cross-verification of MCDU entries is critical.

Loss of FMGC synchronization is the trigger for 𝗶𝗻𝗱𝗲𝗽𝗲𝗻𝗱𝗲𝗻𝘁 𝗺𝗼𝗱𝗲. In independent mode, each MCDU affects only its onside peripherals. The crew must duplicate every data entry on both MCDUs to maintain coherent navigation. This is a degraded condition - not a routine condition - and signals that the system can no longer vouch for cross-channel coherence.

Reference: WZZ FCOM DSC-22_10-30

The 𝗙𝗔𝗖𝘀 (Flight Augmentation Computers) handle flight envelope and dynamic protection functions.

The same FACs handle the flight envelope work and the rudder augmentation work in flight controls. One pair of computers, two distinct jobs.

Reference: WZZ FCOM DSC-22_40-10, DSC-22_40-30, DSC-22_40-40

𝗔𝗹𝗽𝗵𝗮-𝗳𝗹𝗼𝗼𝗿 is the FAC-triggered protection that commands TOGA thrust when the angle of attack crosses a configuration-dependent threshold. The FAC sends the signal to the FMGC, which bypasses the thrust lever and drives the engines to maximum thrust. Alpha-floor is active between lift-off and 100 ft RA on approach - it is inhibited below 100 ft on final, where a TOGA command would be more dangerous than letting the AoA situation resolve through normal flare.

Reference: WZZ FCOM DSC-22_40-30

Yes - the FAC commands alpha-floor 𝗿𝗲𝗴𝗮𝗿𝗱𝗹𝗲𝘀𝘀 of whether A/THR is engaged or disengaged. The FAC forcibly activates the autothrust circuitry to drive thrust to maximum, even if the A/THR pushbutton on the FCU is off. However, alpha-floor is lost if 𝗯𝗼𝘁𝗵 𝗙𝗠𝗚𝗖𝘀 𝗳𝗮𝗶𝗹, because the command signal path runs through the FMGCs. The FAC pulls the trigger, A/THR fires the gun, and the FMGCs are the wire between them.

Reference: WZZ FCOM DSC-22_40-30, DSC-22_30-90

The FACs compute the full PFD characteristic speed set:

These speeds are normally computed using FMS gross weight, with an aerodynamic-data backup path if the FMS is lost. The PFD speed tape margins come from the FACs - not from the FMGCs or the ADIRUs alone.

Reference: WZZ FCOM DSC-22_40-10, DSC-22_10-50-20

The 𝗙𝗔𝗖 speed tape margins are real-time aerodynamic protection limits computed by the FAC and displayed continuously on the PFD speed tape. The 𝗙𝗠𝗚𝗖 predictions are trajectory-based future-state estimates (fuel, time, altitude crossings) computed from F-PLN and performance data. The FAC values are immediate protection; the FMGC values are projected operations. Both serve the pilot, but they answer different questions.

Dual FAC failure triggers 𝗙/𝗖𝗧𝗟 𝗔𝗟𝗧𝗡 𝗟𝗔𝗪 (𝗣𝗥𝗢𝗧 𝗟𝗢𝗦𝗧) with a comprehensive loss set:

Reference: WZZ FCOM PRO-ABN-AUTO_FLT

Yes. In alternate or direct law, alpha-floor is lost and the low-energy aural alert is inhibited. Both functions are tied to the protection logic that those degraded laws step away from. Reactive windshear detection follows its own failure logic and is 𝗻𝗼𝘁 automatically lost simply due to law degradation - it depends on the specific failure conditions.

Reference: WZZ FCOM DSC-22_40-30, DSC-22_40-40

The 𝗙𝗠𝗔 has five columns, left to right:

The three left columns carry the active and armed guidance modes; the fourth column carries approach capability and minima; the fifth column carries engagement status.

Reference: WZZ FCOM DSC-22_30-100

𝗘𝗻𝗴𝗮𝗴𝗲𝗱 modes are shown in 𝗴𝗿𝗲𝗲𝗻 on the first line of the three left columns. 𝗔𝗿𝗺𝗲𝗱 modes are shown on the second line in 𝗯𝗹𝘂𝗲 or 𝗺𝗮𝗴𝗲𝗻𝘁𝗮. 𝗪𝗵𝗶𝘁𝗲 is reserved for approach capability indications (column 4) and AP/FD/A/THR engagement status (column 5) - not for active guidance modes. If a candidate describes a mode as white, they are likely reading the engagement-status field rather than an active mode.

Reference: WZZ FCOM DSC-22_30-100

Both colors indicate armed modes on the second line of the FMA. 𝗠𝗮𝗴𝗲𝗻𝘁𝗮 carries an extra signal: the mode is armed or engaged because of a 𝗰𝗼𝗻𝘀𝘁𝗿𝗮𝗶𝗻𝘁 (such as an altitude or speed constraint in the F-PLN), not by free pilot selection. 𝗕𝗹𝘂𝗲 indicates armed modes from free selection. Treating blue and magenta interchangeably on an oral exam misreads the FMA's signaling, and examiners specifically test for this distinction.

Reference: WZZ FCOM DSC-22_30-100

The FMGS transition from APPROACH phase to GO-AROUND phase requires 𝘁𝗵𝗿𝗲𝗲 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻𝘀 to be true simultaneously:

Advancing thrust levers short of TOGA (to FLEX or MCT) fails to trigger the transition, keeping the system in approach evaluation logic.

Reference: WZZ FCOM DSC-22_30-60

If the thrust levers are not advanced to TOGA during a go-around, the FMGS does not activate the GO-AROUND phase. The primary flight plan collapses into a 𝗣𝗣𝗢𝗦-𝗗𝗜𝗦𝗖𝗢𝗡𝗧 (Present Position Disconnection) sequence, the approach guidance remains armed, and the flight director bars continue tracking the approach glide path while the aircraft accelerates. This is a classic examiner oral trap.

Reference: WZZ FCTM PIR-010

Candidates often assign characteristic speed computation to the FMGC or to the ADIRUs. The correct attribution: 𝗩𝗟𝗦, 𝗙-𝘀𝗽𝗲𝗲𝗱, 𝗦-𝘀𝗽𝗲𝗲𝗱, 𝗴𝗿𝗲𝗲𝗻 𝗱𝗼𝘁, 𝗩𝗠𝗢, 𝗩𝗠𝗔𝗫 - all 𝗙𝗔𝗖. The ADIRUs provide air data. The FACs normally use FMS gross weight in the computation, with aerodynamic-data backup if the FMS is gone. Examiners use this question to probe whether the candidate truly understands how envelope awareness is preserved when primary navigation computers fail.

Reference: WZZ FCOM DSC-22_40-10

No. Autopilot engagement is completely 𝗯𝗹𝗼𝗰𝗸𝗲𝗱 in direct law because the automated stabilization loops are bypassed. Hand-flying is mandatory for the remainder of the flight, including manual trim management. The autopilot will also automatically disconnect during the transition to direct law if it was engaged when the law degradation occurred.

Reference: WZZ FCOM DSC-27, FCTM AO-027

𝗧𝗢𝗚𝗔 𝗟𝗞 is the locked-out state that follows alpha-floor activation. As the aircraft climbs clear of the alpha-floor envelope, A.FLOOR transitions to TOGA LK on the FMA, locking the engines to maximum thrust regardless of physical thrust lever position.

Reference: WZZ FCOM DSC-22_30-90; FCTM OP-030

The autopilot is 𝗼𝗻𝗲 𝗲𝗻𝗴𝗮𝗴𝗲𝗺𝗲𝗻𝘁 𝗹𝗶𝗴𝗵𝘁 on the FCU. The system that engages it consists of seven physical boxes in four component types - 2 FMGCs, 1 FCU, 2 MCDUs, 2 FACs. Naming only the autopilot misses the architectural cast that does the actual work. Examiners use the question to test whether the candidate has memorized the engagement panel or understood the architecture.