Each of the A300/310’s two autopilots has two modes of operation:
- In CWS (Control-Wheel Steering) mode, the autopilot(s) attempt to hold the attitude that the aircraft had at the moment of autopilot engagement; if the pilot(s) change the aircraft’s attitude using manual flight control inputs, the autopilot(s) attempt to hold the attitude that the aircraft has at the moment the pilots release the controls.1
- In CMD (Command) mode, the autopilot flies the aircraft automatically, based on commands from the aircraft’s FMS.23
The FMS has several modes that it can function in when commanding an autopilot in CMD mode (BASIC, ALT, LVL/CH, PROFILE, HED/SEL, NAV, VOR, LOC, TAKE OFF, LAND, and GO AROUND modes).3 In most of these modes, applying a force greater than 150 newtons to the control column will automatically disengage any autopilot(s) that were engaged in CMD mode.
- During glideslope capture in LAND mode, the pilot(s) can use manual pitch inputs to assist the autopilot in capturing the glideslope, helping to reduce the amount and severity of porpoising as the aircraft locks onto the glideslope signal.
- In LAND and GO-AROUND modes, if the aircraft’s height above ground level (AGL) is less than 400 feet, manual pitch inputs will override the autopilot, but not disconnect it, and the autopilot will counteract the pilots’ pitch inputs using horizontal-stabiliser trim; if this situation continues for any significant length of time, it can leave the aircraft severely out of trim at a height low enough to leave very little margin for recovery.4
Originally, applying manual pitch inputs to the control column with one or both autopilots in CMD mode would not disconnect the autopilot(s), no matter what the aircraft’s height AGL was or in what mode the FMS was feeding commands to the autopilot(s).
- In March 1985, an incident occurred when the flightcrew of a descending A300-600, not realising that an autopilot was engaged in CMD mode, overrode it with the aircraft in ALT mode, leading to compensatory autopilot nose-up trim commands which left the aircraft severely out of trim, sending the aircraft into a dangerously nose-high attitude until the aircraft switched to another mode and started commanding nose-down trim inputs; as a result, the A300/310’s autopilot system was modified to disengage the autopilot in response to manual pitch inputs, except in LAND and GO-AROUND modes.5
- In January 1989, one of the go-around levers in the cockpit of an A300B4 was inadvertently moved during approach, sending the aircraft into GO-AROUND mode and causing an automatic pitch up and increase in thrust; the captain countered this by retarding the throttles and applying down elevator, causing the autopilot to apply nose-up pitch trim in response until it was (apparently inadvertently) disconnected. When the flightcrew abandoned the approach, reengaged the autopilot, and increased thrust for a go-around, the aircraft pitched up dramatically and nearly stalled before the pilots were able to regain control using manual pitch trim.5 Later, in February 1991, the captain of an A310, while performing a go-around with autopilot engaged, attempted to use elevator inputs to moderate the very high rate of climb resulting from his aircraft’s light weight. The resulting nose-up trim inputs from the autopilot, combined with the engines being at full power, caused the aircraft to climb steeply and pitch up almost to vertical, resulting in a stall and loss of control; the aircraft climbed and stalled twice more before the pilots regained control by trimming nose-down with the electric trim switches (the autopilot had disengaged during the first steep climb, but the horizontal stabiliser had already moved to nearly the full nose-up position by this point). Following these two incidents, Airbus recommended that A300/310 operators modify the aircraft’s autopilot so that manual pitch inputs would disengage it even in LAND and GO-AROUND modes, as long as the aircraft was at least 400 feet AGL.5
- On 26 April 1994, an A300-600 suffered an accidental go-around-lever activation during approach, similar to the January 1989 incident; unlike in the earlier incident, the autopilot, once engaged, was not disconnected until much later, and it sent the horizontal stabiliser to the full nose-up position, leading to a steep climb, stall, and uncontrolled descent terminated when the aircraft crashed on airport grounds, killing all but seven of the 271 on board. The Airbus-recommended modification to the aircraft’s autopilot system had not been embodied on this aircraft; said modification was subsequently made mandatory.5
Although the situation is now much better than it originally was, why does the A300/310’s autopilot system still remain active, and trim against the pilots, if manual pitch inputs are applied below 400 feet AGL in LAND or GO-AROUND mode with one or both autopilots engaged in CMD mode?
1: Basically, the A300/310’s autopilot in CWS mode acts like SAS in Kerbal Space Program.
2: Only one of the two autopilots can be engaged in CMD mode at a time, unless the FMS is in the LAND or GO-AROUND mode; if one autopilot is engaged in CMD mode with the FMS in a mode other than LAND or GO-AROUND, and the other autopilot is then engaged in CMD mode, the first autopilot will automatically disconnect.
3: The required-control-input data from the FMS can also be fed to the aircraft’s flight-director system, allowing the pilots to fly the aircraft manually under FMS guidance.
4: To make things even worse, the pilots cannot use their yoke-mounted electric pitch trim switches to alleviate the autopilot-induced mistrim, as these switches are deactivated when one or both autopilots are engaged; instead, they must take one hand off the control yoke (while still continuing to maintain the large amount of force needed to counter the autopilot’s trim inputs) to rotate the manual pitch trim wheel mounted on the aircraft’s center console (which is mechanically connected to the stabiliser jackscrew, turns it directly, and disconnects the autopilot when used).
5: Information on all four occurrences is from the AAR for the April 1994 accident.