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Under the investigation section of the First Air Flight 6560 incident, it describes the catalyst for the accident:

The approach was entirely flown on autopilot, which was correctly set to capture the localizer signal and track along the runway centreline (VOR/LOC capture mode). However, an inadvertent movement of the control column by the captain during the turn onto the final approach track caused the autopilot to disengage from VOR/LOC mode and revert to maintaining the current heading, resulting in the aircraft rolling out to the right (east) of the runway centreline.

My question: Is there any obvious warning (on common commercial aircraft, i.e. 737, A320) when auto-pilot is disengaged or when the mode changes?

I would have expected there to be an audible notification, some flashing light or something to make it as apparent as possible that something has changed, if it's as easy to change as 'nudging the stick'.

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    $\begingroup$ Related: Why is an auditory alarm necessary when the autopilot disengages? $\endgroup$ – ymb1 Nov 28 '19 at 16:07
  • $\begingroup$ @ymb1 Thanks, I did have a look at that question. But I would like to know specifically what the warnings are on a 737, A320 and why said warnings didn't prevent the above incident. If you think my question could be worded a bit better, please feel free to edit. $\endgroup$ – Cloud Nov 28 '19 at 16:10
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    $\begingroup$ @JohnK please avoid answers in comments $\endgroup$ – Jamiec Nov 29 '19 at 8:39
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    $\begingroup$ It looks like the wording in the Wikipedia article is somewhat unfortunate, since (at least to me), the terms "disengage" and "mode change" are different, yet the Wikipedia article uses the term "disengage from VOR/LOC mode" to talk about a mode change. In particular, a disengagement will issue a warning, since it is not commanded by the pilot, but a mode change is commanded by the pilot, and it is usually not needed to warn the pilot about something they did themselves. The problem in this case, of course, is that the pilot did not notice that he commanded a mode change. $\endgroup$ – Jörg W Mittag Nov 30 '19 at 1:51
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You asked about commercial aircraft in general, so I will give an answer from that point of view.

Is there any obvious warning when auto-pilot is disengaged?

Yes, both visually as flashing lights, and aurally. Furthermore, the lights and tone does not go away until a second confirmation is received from the pilot. For example, pushing the button on the yoke / stick once will disengage the autopilot but trigger the warning. A second click is needed to silence it.

Is there any obvious warning when mode changes?

It is not a warning, as mode changes are explicitly made by the pilot. I'd still consider it "obvious" because the active mode is shown right on top of the Primary Flight Display (the instrument you should spend most of the time on).

enter image description here

Picture: A/P disengage button on yoke, A/P disengage warning light and A/P mode indicator circled in red.

Image source https://www.team-bhp.com/forum/attachments/commercial-vehicles/1506426d1463061690-boeing-777-pilots-review-boeing777cockpit.jpg


So what happened on First Air Flight 6560? In this particular instance though, there are a few factors (in my opinion) that contributed to the crew not noticing the autopilot state:

  1. The 737-200 is a rather old design. Compared to a modern cockpit where things are seen "at a glance", it requires more concentration to fly.
  2. The 737 series have a rather unpopular feature called Control Wheel Steering (CWS). It allows the pilot to partially disengage one axis (pitch / roll) of the autopilot by moving the yoke in that axis only. On newer models, this feature is removed since very few pilots actually use it. On the Boeing 777 for example, such yoke movement would result in A/P disengagement and the trigger of the warnings I described.

Note that this incident is quite similar to the crash of Eastern Air Lines Flight 401, where the captain accidentally bumped the yoke, causing the autopilot to switch from altitude hold mode to CWS mode in pitch. The pilots failed to notice the change in time to avoid the crash.

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The Boeing 737 allows a mode called control wheel steering (CWS). The A320 doesn't. For what is CWS, see: Is it possible to disengage only one axis of a two-axis autopilot?

The wording on Wikipedia doesn't emphasize this point; however, the final report does.

The pilots may also manually control the aircraft in a normal manner with the control wheel and column (control wheel steering [CWS]) without disengaging the pitch or roll axes of the autopilot system. Pilots can then assist the autopilot system in flying to a selected heading or course. Use of CWS does not disengage either channel of the autopilot system. The autopilot system was modified from the original design to allow for the use of GPS guidance for the course signals to the autopilot (section 1.6.10).

Note that Wikipedia says "(...) disengage from VOR/LOC mode (...)".


(All bold emphasis mine.)

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Most autopilots feature both an aural warning siren or similar aural cue as well as visual cues on both autopilot units as well as cockpit displays, so there is a means to alert a flight crew of an autopilot disengagement.

As to changing autopilot modes, there a visual cues for this on autopilot units, flight displays and separate autopilot mode displays to allow a flight crew to visually verify when an autopilot is engaged.

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