A320 Auto Flight System Explained: The Cast of Seven Boxes That Fly the Airplane

What is the A320 Auto Flight system?

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). Seven physical boxes, four jobs, one cohesive system, together forming the FMGS — the Flight Management and Guidance System.

Reference: FCOM DSC-22_10-10.

How is the master FMGC assigned on the A320?

Master assignment follows an AP-then-FD ladder. This is the single most-missed fact about master logic on A320 orals — candidates who memorize only "AP1 means FMGC1 master, AP2 means FMGC2 master" miss the dual-AP case and the no-AP cases that examiners specifically test.

Reference: FCOM DSC-22_10-30.

What is the difference between managed mode and selected mode on the A320?

Pushing an FCU knob engages managed mode, delegating 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, asserting the specific value the pilot has dialed into the FCU window. Pushing is automation delegation; pulling is pilot assertion. The push-pull philosophy is the central architectural principle of the FMGS and shows up on every FMA cycle of every flight. Pulling is the structural escape path that lets a pilot instantly override the FMGC programming when ATC drops a heading or speed change with no time to amend the F-PLN.

Reference: FCTM OP-030.

Why should I never confirm modes from the FCU panel on the A320?

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. The standing rule in modern A320 training: never call out a mode change by looking at the FCU. Always call from the FMA. The FCU shows you what you wanted. The FMA shows you what the airplane gave you. When those two diverge, the FMA wins the argument on modes.

Reference: FCOM DSC-22_10-40, DSC-22_30-100.

What is the FMA column order on the A320 PFD?

The FMA has five columns, left to right: A/THR mode, vertical mode, lateral mode, approach capability, and AP/FD/A/THR engagement status. 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. Get the column order wrong on an oral and the examiner knows you have been reading the FMA without reading the FCOM.

Reference: FCOM DSC-22_30-100.

What color are engaged modes shown in on the A320 FMA?

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 and AP/FD/A/THR engagement status — not for active guidance modes. The blue/magenta distinction matters: magenta carries an extra signal that the mode is armed or engaged because of a constraint, not by free pilot selection. Treating blue and magenta interchangeably misreads the FMA's signaling, and examiners specifically test for it.

Reference: FCOM DSC-22_30-100.

What is FMGS independent mode on the A320?

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. Routine data-entry mismatches between the captain's MCDU and the F/O's MCDU do not trigger independent mode — candidates who treat ordinary entry mismatches as the trigger misread the architecture. Independent mode is the system saying it can no longer vouch for cross-channel coherence. That is not normal.

Reference: FCOM DSC-22_10-30.

What does the EO CLR button do on the A320 MCDU?

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 the crew needs to return the system to two-engine computation. Pressing EO CLR during an actual engine failure is irreversible: it deletes single-engine predictive computation and forces the system back 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. The button is helpful; the press is not.

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

What does the PROG page show during an engine failure on the A320?

During a real engine failure, the PROG (Progress) page becomes the primary cockpit reference for drift-down strategy. It displays the REC MAX EO altitude — the recommended maximum cruise altitude with one engine inoperative, calculated by the FMGC based on current gross weight and atmospheric conditions. This altitude is set on the FCU and pulled to begin the drift-down profile after the QRH calls for it. Flying above the REC MAX EO compromises the single-engine climb gradient and burns more fuel per nautical mile than the optimal drift-down path. The PROG page tells the pilot what altitude to aim at without having to compute it — provided EO CLR has not been pressed before reading it.

Reference: FCOM PRO-ABN-ENG.

What do the FACs do in the A320 Auto Flight system?

The FACs handle flight envelope and dynamic protection functions in the Auto Flight system. They compute characteristic speeds (VLS, F-speed, S-speed, green dot, VMO, VMAX), generate the alpha-floor command, sound the low-energy warning (SPEED SPEED SPEED), and detect reactive windshear. The same FACs also run yaw augmentation, rudder trim, and rudder travel limit in the flight controls system — the FAC double duty across both system books. FAC speeds are normally computed using FMS gross weight, with an aerodynamic-data backup path if the FMS is gone. The FAC envelope work is closely tied to the FMS in normal operation, not isolated from it.

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

What is alpha-floor protection on the A320 and is it independent of A/THR?

Alpha-floor is the FAC-triggered protection that commands TOGA thrust when 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 (inhibited below 100 ft on final). The protection is active regardless of whether A/THR is engaged or disengaged — the FAC forcibly activates autothrust circuitry to command TOGA, even if the A/THR pushbutton on the FCU is off. However, alpha-floor is lost if both FMGCs fail, because the command path runs through the FMGCs. The FAC pulls the trigger, A/THR fires the gun, and the FMGCs are the wire between them.

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

Where do A320 characteristic speeds come from?

The dynamic characteristic speeds on the PFD speed tape (VLS, F-speed, S-speed, green dot, VMO, VMAX) are computed by the FACs. The ADIRUs provide raw air data. The FACs normally use FMS gross weight in the speed computation, with an aerodynamic-data backup path if the FMS is gone. Both halves of that sentence matter on orals — candidates who name only the FAC or only the FMS gross weight have incomplete answers. The single most common Auto Flight oral exam failure point is misattributing the source of characteristic speeds to the FMGC or the ADIRUs alone. The right answer: VLS, F-speed, S-speed, green dot, VMO, VMAX all come from the FAC, computed from FMS gross weight (with aerodynamic backup), drawing on raw air data from the ADIRUs.

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

Is the autopilot available in direct law on the A320?

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. Candidates must expect automatic AP disconnect when law degradations affect flight control surfaces. The autopilot does not fight a degraded airframe — it steps aside and gives the pilot the airplane. Politely, sometimes. Suddenly, other times.

Reference: FCOM DSC-27; FCTM AO-027.

What is the TOGA detent requirement for an A320 go-around?

The FMGS transition from APPROACH phase to GO-AROUND phase requires three conditions to be true simultaneously: the aircraft must be in flight, the slats must be extended, and at least one thrust lever must be advanced to the TOGA detent. Advancing thrust levers short of TOGA (to FLEX or MCT) fails to trigger the transition. When thrust levers reach the TOGA detent, the FMA instantly displays SRS and GA TRK as the captured modes, the missed approach routing becomes the active F-PLN, and any deployed speed brakes retract automatically. Missing the TOGA detent during a go-around is a classic examiner oral trap — it causes the FMGS to keep the approach phase active and the primary flight plan collapses into a PPOS-DISCONT sequence.

Reference: FCOM DSC-22_30-60; FCTM PIR-010.

Why is "the autopilot is flying the airplane" an incomplete A320 oral answer?

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. The Foundation Series answer covers the four components in their assigned roles: FMGC1 or FMGC2 as master per the AP-then-FD ladder, the FCU showing the cleared altitude as managed, the MCDUs sitting on whatever page is active, and the FACs computing characteristic speeds from FMS gross weight while watching for any AoA excursion. That is the four-component cast cold. That is what separates a Foundation Series candidate from a rehearsed candidate.