Autoflight (A320)

These notes cover the autoflight system of the Airbus A320: the flight management and guidance system (FMGS), the autopilot, flight directors and autothrust. They were written as questions and answers for crews. If anything here conflicts with your FCOM, the FCOM wins.

What the FMGS does

The FMGS has three jobs. Flight guidance gives the autopilot (AP), flight director (FD) and autothrust (A/THR) commands. Flight management handles navigation and navigation radios, flight planning, performance prediction and optimisation, and display management. Flight augmentation, done by the flight augmentation computers (FACs), covers rudder control, rudder trim, yaw damping, flight envelope and speed data, and the low energy and windshear warnings.

Modes and failures of the FMGC

In independent mode, both MCDU scratchpads show INDEPENDENT OPERATION and the IND light on top of the MCDU is lit. The position monitor and GPS monitor pages do not show the position from the other FMGC, and navaids tuned on the other MCDU are blank on the RAD NAV page. In single mode, the MCDU on the side of the failed FMGC returns to the MCDU MENU page with a prompt to connect to the other FMGC. The navigation display on that side shows MAP NOT AVAIL and SET OFFSIDE RNG/MODE if the displays are in different modes or ranges, or an amber OFFSIDE FM CONTROL message if they match. Both position monitor pages then show the same position.

Master FMGC logic: FMGC 1 is master with only FD1 on, with both FDs on, with both FDs and AP1, and with both FDs and both APs. FMGC 2 is master with only FD2, or with both FDs and AP2. If no AP or FD is engaged, FMGC 1 controls the A/THR.

MCDU NAV B/UP links an MCDU to its own IRS so the crew can monitor navigation and use basic flight planning if both FMs fail. It was not on every IndiGo aircraft. It gives aircraft position from the onside IRS or IRS 3 or from GPIRS, the flight plan as stored in the MCDU, flight plan display and automatic sequencing, limited lateral revisions, bearing and distance to the TO waypoint, true track between waypoints, time estimates from the onside IRS ground speed, and total time and distance to destination. Even with both FMGCs failed, the crew can still navigate and finish the flight.

MCDU lights: FAIL means the MCDU has failed. MCDU MENU tells the crew to call up a peripheral such as ACARS, ATSU or CFDS. FM lights while a peripheral is displayed, to say the FMGC has an important message. On top, FM1 or FM2 mean that FM has failed, IND means independent operation while both are healthy, and RDY means the MCDU passed its power up test after being switched off with the DIM key. A blank MCDU means the pilot cannot enter data or call up pages, though all FMGS functions remain. A star next to a line key prompt means pressing it changes parameters affecting the active situation. Amber boxes show mandatory entries. A temporary flight plan appears whenever a revision is made on the F-PLN A or B pages, the LAT REV page or the VERT REV page.

Autothrust and pitch modes

Speed is controlled either by the AP/FD pitch channel or by the A/THR, depending on the vertical mode shown on the FMA.

• A/THR holds SPEED or MACH in V/S-FPA, DES, ALT, ALT CST, ALT CRZ, G/S and FINAL modes.
• A/THR holds THR CLB in CLB, OP CLB, EXP CLB and SRS.
• A/THR holds THR IDLE in DES, OP DES and EXP DES.
• With no AP/FD pitch mode engaged, A/THR reverts to SPEED/MACH.

A/THR is active when it controls thrust or speed. Its range is IDLE to the CL detent with both engines, and IDLE to MCT with one engine out. The thrust lever position sets the maximum thrust it can command, except in alpha floor. When active, the FCU button lights, the FMA shows the mode in green in the first column and A/THR in white in the fifth. Above the CL detent with both engines, A/THR becomes armed, A/THR turns blue and MAN THR shows in white. Below the CL detent, an A/THR LIMITED caution repeats every 5 seconds and LVR CLB flashes. LVR ASYM shows when one lever is in the detent and the other is not. Pulling the levers to idle disconnects A/THR. Takeoff without a FLEX temperature entered is still possible.

If A/THR disconnects by itself with the levers in the CLB gate, thrust lock activates: THR LK flashes amber, ENG THRUST LOCKED flashes every 5 seconds, THR LEVERS MOVE shows, and a single chime and master caution repeat every 5 seconds. Using the instinctive disconnect button gives a master caution for up to 3 seconds, A/THR OFF on ECAM for up to 9 seconds and one chime. Holding the button for more than 15 seconds disconnects A/THR for the rest of the flight, alpha floor included, until the next FMGC power up on the ground. Below 3,200 feet radio altitude with at least CONF 1, approach autothrust makes the logic respond faster to speed changes.

Flight directors and the FMA

The pitch bar shows when a vertical mode is engaged, and the roll bar when a lateral mode is engaged. Below 30 feet, with a localizer available, a yaw bar replaces the roll bar. In the fifth FMA column, both FDs on reads 1FD2, FD1 off reads -FD2, FD2 off reads 1FD-, and FD1 failed reads 2FD2. The pitch bar flashes if glide data is lost in G/S, G/S* or LAND above 100 feet radio altitude. The roll bar flashes if localizer data is lost in LOC, LOC* or LAND above 15 feet.

During taxi out, the FMA shows CLB in blue in the second column, NAV in blue in the third, and 1FD2 in white in the fifth. If NAV is disarmed with a heading preset, only CLB and 1FD2 show, and OP CLB engages at the acceleration altitude. With the levers at TOGA, the FMA shows MAN TOGA in white, SRS in green over CLB in blue, RWY in green over NAV in blue, and 1FD2 in white with A/THR in blue. If both FDs fail on takeoff, the FMA shows MAN TOGA and A/THR in blue, A/THR stays armed, LVR CLB still flashes, A/THR engages in SPEED mode when levers go to CLB, and the target speed does not change at acceleration altitude.

SRS, RWY and NAV

With both engines, SRS targets V2 plus 10, limited to V2 plus 15. With one engine out, the target is the higher of V2 or current speed, limited to V2 plus 15. SRS limits nose up attitude to 18 degrees (22.5 in windshear), keeps a minimum vertical speed of 120 ft/min, and caps the target at V2 plus 15. With both engines it drops out at acceleration altitude, or when ALT* or ALT CST* engage above 400 feet radio altitude. With one engine out it does not drop out by itself. At acceleration altitude, CLB replaces SRS. For go around, SRS holds the speed at engagement or VAPP, whichever is higher, limited to VLS plus 25 with both engines and VLS plus 15 with one. The go around FMA shows MAN TOGA, SRS over CLB, GA TRK in green and A/THR in blue. Go around engages when a lever goes to TOGA with flaps at 1 or more, in flight or on the ground for less than 30 seconds.

RWY mode needs V2 in the PERF TAKEOFF page, slats extended, at least 30 seconds on the ground, a localizer signal within half a dot, heading within 20 degrees of the ILS course, and an ILS course matching the runway in the flight plan. RWY TRK engages at 30 feet if NAV does not. A heading preset is cancelled by DIR TO, by pushing the HDG/TRK knob, or by disengaging the AP/FD.

NAV engages at 30 feet after takeoff if armed, on DIR TO (not below 700 feet in LOC mode), when the HDG/TRK knob is pushed within about 1 nm of the active leg, or in flight when armed and the capture zone is reached. It disengages when another lateral mode engages or when the flight plan is lost or has a discontinuity. It disarms if the HDG/TRK knob is pulled, a preset is chosen, LOC is armed, GA mode is chosen, LAND engages, or APPR is pressed with an ILS selected. Pressing LOC while NAV is engaged leaves NAV engaged.

The speed profile without NAV is V2, speed limit, economy climb, economy cruise, economy or preset descent, speed limit, VAPP. With NAV it also includes speed constraints and holding speed where they apply.

Climb, descent and expedite

Pulling the ALT knob in managed climb sends the system to open climb. In managed descent above profile, A/THR commands idle and the pitch channel controls speed at the top of the managed range. Below profile, A/THR commands speed and pitch controls vertical speed. VDEV on the PROGRESS page shows the vertical deviation from the computed profile and where the crew can rejoin it.

EXPEDITE in climb moves to EXP CLB, with a green dot target speed held by pitch and climb thrust. In descent it moves to EXP DES, with a target of 340 knots or Mach 0.8 held by pitch and idle thrust. Expedite ends if the crew pull V/S or FPA, pull ALT or SPD/MACH to get OP CLB or OP DES, push ALT for CLB or DES, or pick a higher or lower altitude in EXP DES or EXP CLB without a follow up action within 5 seconds (a triple click sounds and a white box flashes for 10 seconds), or if a reversion to V/S protects against VLS or VMAX. Selecting a lower speed in EXP DES changes to OP DES with THR IDLE. OP DES ignores altitude constraints but not speed constraints. It is not for final approach, so that A/THR stays in SPEED mode and not idle.

Protections and reversions

In open descent with FDs only, if the crew do not follow the bars and speed reaches VLS minus 2, both FDs disengage and A/THR reverts to SPEED, with a triple click. In open climb the same happens at VMAX plus 4. In V/S mode, the FMGS adjusts the pitch bar to hold VLS or VMAX.

A mode reversion is an automatic mode change when the autoflight cannot hold the setting. It is normal and helps keep the aircraft in the envelope. For example, setting the FCU altitude below the aircraft in climb, or above it in descent, gives V/S or FPA on the present rate. Selecting a heading in managed climb changes CLB to OPEN CLB. In managed descent it moves A/THR to SPEED and pitch to V/S.

Autopilot disconnects and ILS

An AP disconnect from a system failure gives a flashing red master warning, a red ECAM AUTO FLT AP OFF, a continuous cavalry charge of at least 1.5 seconds, and a lit CLR button. Disconnecting by the AP button on the FCU gives the same. Using the take over button gives a master warning for up to 3 seconds, the AP OFF message for up to 9 seconds, and a cavalry charge of 0.5 to 1.5 seconds.

Pressing APPR, G/S* cannot engage unless LOC* is engaged. The ILS can be dropped by pressing APPR above 400 feet. LAND engages automatically below 400 feet with LOC and G/S engaged, and ends on go around or when both AP/FDs are off. If LOC or G/S is lost for more than 7 seconds above 200 feet, the AP/FD disengage and the FDs return to basic modes. After a total ILS loss, red LOC and G/S flags show, the AP disengages and the FD reverts to HDG V/S or TRK FPA. A flashing red AUTOLAND light below 200 feet in LAND mode comes with both APs off, excess LOC deviation (a quarter dot above 15 feet) or glide deviation (one dot above 100 feet), loss of LOC above 15 feet or glide above 100 feet, or radio altimeters differing by more than 15 feet. The FMGC works out its landing capability (CAT 1, CAT 2, CAT 3 single or dual) and a downgrade gives a triple click. The CHECK APPR SELECTION message shows when a non ILS approach is in the flight plan but an ILS is tuned, as a reminder that APP NAV and FINAL are the guidance modes. In autoland rollout, the ELACs take a nose wheel steering command from the FMGCs and pass it to the BSCU.

Alpha floor, low energy and windshear

Alpha floor sets TOGA thrust whatever the lever position. It triggers when alpha exceeds 9.5 degrees in configuration 0, 15 in configurations 1 and 2, 14 in configuration 3 and 13 in FULL, or when the sidestick is more than 14 degrees nose up with pitch or angle of attack protection active. It works from lift off to 100 feet radio altitude before landing. TOGA LK in green with a flashing amber box shows when the aircraft leaves alpha floor conditions. It is cancelled only by disconnecting A/THR. Alpha floor is lost with SFCC1 and FAC2, SFCC2 and FAC1, both FCU channels, one EIU or both FMGCs failed, in alternate or direct law, with FADECs in N1 degraded mode, and with one engine out and slats or flaps extended.

The aural SPEED SPEED SPEED low energy warning repeats every 5 seconds in configurations 2, 3 and FULL. The FAC works it out from configuration, deceleration rate and flight path angle. The FACs give the windshear warning when predicted energy falls below a threshold. The function is available from 3 seconds after lift off to 1,300 feet, and from 1,300 feet to 50 feet on landing, with at least CONF 1. In windshear the crew set go around thrust at once, which also triggers SRS, and follow its pitch orders. There is no ECAM for windshear or low energy warnings, for degraded ILS capability, for mode reversion or for loss of GPS primary.

Other procedures

• The minimum thrust reduction altitude on the takeoff page is 400 feet.
• Position can be updated in flight on the PROG page through the UPDATE AT field. Losing GPS does not downgrade accuracy at once. The drift on the POSN MONITOR page is each IRS position's deviation in nautical miles from the onside FMGC position.
• A go around without TOGA means the FMS does not enter go around phase, and passing within 7 nm of the airport sequences the destination waypoint.
• Cost index can be changed on the descent page if descent is not active.
• A takeoff shift is the distance from the runway start to the takeoff position, entered for intersection takeoffs.
• CHECK GROSS WEIGHT appears if the FMC and FAC weights differ by more than 7 t. If the MCDU weight is right, disregard VLS, F, S and green dot on the PFD and use QRH speeds.
• For AUTO FLT AUTO THR OFF at preparation, arm A/THR with the FCU button, then disarm with the instinctive disconnect button.
• If the first officer's MCDU locks, pull its circuit breaker and push it back after 10 seconds. If it freezes after DIR TO with A/C POSITION INVALID showing, reset both FMGCs by the manual reset procedure.
• ALIGN IRS means someone must open the IRS INIT page. F-PLN FULL means delete a leg or the alternate flight plan.
• MAN PITCH TRIM ONLY in red shows with loss of both elevators. USE MAN PITCH TRIM in amber shows in direct law.
• A continuous green track line shows in ROSE NAV or ARC when HDG or TRK is selected. A flight path vector above the horizon means the aircraft is climbing. A vector right of centre means wind from the left.
• Ground speed mini uses aircraft inertia as wind changes on approach. The FMGS keeps ground speed at or above the minimum by setting an indicated speed target so energy stays above a level that protects the stall margin.

Part of the A320 systems series. See also Airbus Tech.