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I recently watched Air Crash Investigation, Air France Flight 447.

The autopilot handed back control to the flight deck after the pitot tubes temporarily froze. The autopilot knew it was getting naff information and said: "We can't deal with faulty information so we are handing control back to the flight crew".

This got me thinking. What other instances, beside instrument error, if any, will the autopilot trip out, and hand control back to the crew?

An initial Google search was not helpful I am afraid. A pdf of the Airbus systems (350 pages long) was available, and a news source on everything the autopilot does, but nothing mentioning examples where all of a sudden the autopilot may switch off, even temporarily.

The reason I ask this is that if this is the only reason the autopilot may trip out, surely the crew of Flight 447 would have been aware that it was due to information that was not correct, and therefore to just remain as they were until the sensors came back to life again.

That is what I can not understand.

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    $\begingroup$ There are many such situations. Any erroneous flight data, failing self-test, any parameter going out of its allowed range, and of course several possible flight crew actions. A full list might be too much for an answer. $\endgroup$ – Dan Hulme Feb 23 '18 at 13:14
  • $\begingroup$ @DanHulme will it not inform you of why it has tripped out? $\endgroup$ – cmp Feb 23 '18 at 13:16
  • $\begingroup$ Part of the Air France 447 crash wasn't just that the autopilot disengaged, but that it switched to alternate law. Normally, the A330 won't let you put it into a stall, but alternate law will. It's assumed that this is why the crew ignored all the stall warnings. $\endgroup$ – Machavity Feb 23 '18 at 18:06
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What other circumstances may cause the autopilot to trip out?

There are numerous circumstances, a few at random

  • pilot presses the autopilot disconnect pushbutton.
  • applying force to control-stick that contradicts autopilot.
  • entering a stall.
  • activation of high AoA protection.
  • descending below MDA on a non-ILS approach.
  • inconsistent data.

will it not inform you of why it has tripped out?

Pilots check the ECAM for messages and suggested actions.

The AF447 accident report contains ECAM messages, note the ones at 02:12:44:

enter image description here

The Pilot in charge should assign which pilot handles ECAM messages, they should then have applied the ADR Check Procedure

1.16.8.2 Case of speed display anomalies

The philosophy for processing the anomaly is described in the “UNRELIABLE AIRSPEED INDICATION / ADR CHECK PROCEDURE” that is in Flight Manual (AFM) and transcribed in the Airbus FCOM and the Air France Operations Manual.

Airbus

The FCOM procedure indicates that:

  • The crew identifies the loss of consistency in indicated airspeeds;
  • And if the safety of the flight is affected by the indicated speed anomaly, and until the aeroplane reaches the safety altitude or the aerodrome circuit altitude, the crew first performs the Memory items indicated in the “UNRELIABLE AIRSPEED INDICATION / ADR CHECK PROCEDURE” inset. The objective of these memory items is to maintain the aeroplane within a safe flight envelope and to stabilize the flight path to allow time to find, in the QRH, the tables giving the more precise pitch attitude and thrust values to be used for the flight:

enter image description here

Notice that the first two items are for the pilots to make sure the autopilot (AP) and autothrottle (A/THR) are OFF.

The pilots did not systematically work through ECAM messages and did not apply the ADR check procedure (one they would have memorised in training). They were confused and acted erratically. They stalled the aircraft, ignored the stall warnings and held the aircraft in the stall from above it's operational ceiling right down to sea level.

The sad truth is that the only action really necessary was to keep the wings roughly level for a few minutes. It is possible that if both pilots had folded their arms, tucked their legs back and taken a 5-minute nap, everyone on that plane would still be alive.


Glossary

- ADR = Air Data Reference
- AFM = Flight Manual
- ALT = Altitude
- AOA = Angle of Attack
- AP/FD = Autopilot / Flight Director
- A/THR = Autothrottle
- CLB = Climb (throttle setting)
- ECAM = Electronic Centralised Aircraft Monitor
- FCOM = Flight Crew Operating Manual
- ILS = Instrument Landing System
- L/G = Landing Gear
- MDA = Minimum Descent Altitude
- MSA = Minimum Safe Altitude
- NAV = Navigation
- PF = Pilot flying
- PIC = Pilot in command
- PROC = Procedure
- RED = Reduction (thrust reduction altitude - during climb after take off)
- SPD = Airspeed indicators
- TOGA = Take-Off / Go-Around (throttle setting)
- QRH = Quick Reference Handbook

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  • $\begingroup$ Many thanks for this answer. It is helpful. Would you mind explaining the error messages you have highlighted? $\endgroup$ – cmp Feb 23 '18 at 15:01
  • $\begingroup$ @cmp I suspect AIR SPD X CHECK is "air speed cross-check" (check whether the various air speed instruments show the same value). Rather surprising the computers can't perform this check automatically, though. AOA is almost certainly Angle of Attack. I don't know off hand what ADR would be. $\endgroup$ – a CVn Feb 23 '18 at 15:10
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    $\begingroup$ Pardon me for being a bit thick, did these error messages simply not help the flight crew of AF447? $\endgroup$ – cmp Feb 23 '18 at 15:11
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    $\begingroup$ @cmp: The AF447 pilots failed to coordinate their actions, allocate work, communicate effectively or to work systematically through the ECAM messages. The pilot flying pulled the control stick hard back and kept it there for over four minutes from initial upset to impact, ignoring the stall warnings completely and releasing pressure only for a few seconds. We do not know what was going through his mind. It seems he was completely confused and at a loss and his physical actions are inexplicable and completely counter to what is expected, he disregarded altitude and warnings. $\endgroup$ – RedGrittyBrick Feb 23 '18 at 15:28
  • $\begingroup$ You give a good answer. Thanks. $\endgroup$ – cmp Feb 23 '18 at 16:10
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There could be lot of contributing factors:

  1. Environmental Factors: Freezing conditions causes pitot tubes to block, painting in and around pitot tubes without protecting pitot tubes
  2. Insects in pitot tubes or any blockage
  3. Technical Malfunction: Hampered Power supply to autopilot or any instruments operating
  4. Human errors: A sudden Elbow press to turn Autopilot off

But in all these circumstances, alarm rings and alerts are shown. These are known conditions from various air crash Investigations.

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