I was flying in the 737-800 yesterday, started the flight normally, vnav, lnav and CMD A engaged, partway through the cruise phase, CMD A disengaged and would not rearm.  FO CMD B was able to engage and I used that for the remainder of the flight.

 

The initial cause was I depressed my yoke with my chair while wrapping Christmas presents during flight!  Then dropped from FL330 to about 16,000 before I noticed!  Yes I know that's not acceptable cockpit protocol.  Upon regaining control and altitude, I could not reengage CMD A.  There were not any messages on the FMC.

 

I don't have any of the random malfunctions selected.  Are there any thoughts as to what was happening?

 

Thanks and Merry Christmas

 

Jeff      

 

 

Are there any thoughts as to what was happening?

 

If your hardware sends a large enough "spike" (an errant signal - go to a calibration screen and see if the cursor there is shaking...that shaking is noise, and that noise can occasionally get pretty large and cause a "spike" or large deflection), then it will kick off the autopilot as if you were the one moving your hardware like that. The computer doesn't know the difference between noise caused by the hardware, or actions you're causing personally, so that large deflection ends up breaking the shear bolt in the sim and not allowing you to use CMD A any more. This is mentioned in the intro manual (shear bolt breakage, and how it's simulated).

 

The solution would be to calibrate your hardware.

Interesting, Thank you.

 

Jeff Callender

 

 

Interesting, Thank you.

 

Welcome! Let us know if that doesn't work and we can look into it a bit further.