PPRuNe Forums - 737-800 A/P disconnection

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737-800 A/P disconnection

I have searched, maybe not long enough, but I'll still ask anyway. What are the events that would trigger an automatic A/P disconnection in flight, ie. one NOT commanded by either pilot?

Thanks in advance.

PM

I'm not familiar with the 737 but in general there are a host of things which will cause auto disonnect, here's some from the top of my head:

AP reaching it's authority limit
Computer failure
System failure (feedback from servo's etc)
Hydraulic system failure
Manual input to stick (possible involuntary input by leaning on it)
Bus failure
Transient gremlins!

I had a strange incident a few months back in the -800. We were in the descent somewhere in the FL200's with the speed brake fully extended at flight detent. As I slowly retracted the speed brake while we were in a shallow enough turn the autopilot disconnected, then the aircraft banked to 40 degrees and we got the 'Bank Angle' warning. I would guess that with the speed brake not fully retracted it caused the high roll rate, just as the book says however it was still a bit frightening as the aircraft really snaps into the roll. Another theory is that it was a strange aircraft as we had a generator failure on the next flight:confused:

Quote:

The A/P automatically disengages when any of the following occurs:
• pushing either A/P disengage switch
• pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
engaged in CWS or CMD;
• below 2000 feet RA or,
• with flaps not up or,
• G/S engaged
• pushing either Takeoff/Go-around (TO/GA) switch with a single A/P
engaged in CWS or CMD above 2000 feet RA with flaps not up or G/S
engaged.
• pushing either TO/GA switch after touchdown with both A/Ps engaged in
CMD
• pushing an illuminated A/P ENGAGE switch
• pushing the A/P DISENGAGE bar down
• activating either pilot’s control wheel trim switch
• moving the STAB TRIM AUTOPILOT cutout switch to CUTOUT
• either left or right IRS system failure or FAULT light illuminated
• loss of electrical power or a sensor input which prevents proper operation
of the engaged A/P and mode
• loss of respective hydraulic system pressure.
Note: Loss of the system A engine-driven hydraulic pump, and a heavy demand
on system A, may cause A/P A to disengage.

From FCOM Chapter 4