View Full Version : 737-800 FMC / Autopilot Question

4th Mar 2009, 13:14
If the Right FMC fails:
A. Autopilot Disengage if Autopilot A is in use.
B. Autopilot Disengage if Autopilot B is in use.
C. Autopilot Disengage if either Autopilot is in use.
D. Autopilot doesn't disengage regardless which is in use.

This is a question from my annual renewal. The Instructor seemed to think it is B, but I think the answer is D, however I can not find a reference to prove it.

Anyone know the answer with a reference to the 737 technical manual.

Cheers to all :ok:

4th Mar 2009, 14:19
You lose your Right Nav display, LNAV and VNAV from autopilot B. You don't want to continue using it, so engage A(OM 11.32.5). From the QRH 11.4, you are attempting to configure for single FMC operation, so FMC Source Swith, both on L.
Answer B

4th Mar 2009, 14:36
thanks for the quick reply, if anyone disagrees let us know.

4th Mar 2009, 14:37
Tricky one! I'm not sure. You have lost your weight input for g/a flap retraction speeds, but not the FCC nor ILS signals if on the approach. LNAV/VNAV goes too. The Q actually asks about automatic disengagement I think? I'm not sure it will. A job for the sim!

Where are you. CaptainS&L?

Funnel Cloud
4th Mar 2009, 20:05
Having had some FMC failures on -800s with only single-fmc installed there was never an autopilot disconnect at all. All that happens is LNAV and VNAV disconnect. If this happens during the cruise you will get CWR-roll and ALT HOLD. The Autopilot remains engaged.

The Autothrottle remains engaged but gets no further inputs from the FMC. I believe it will work in CRZ, descent and V/S mode but not in climb. Somehow it doesn't calculate the max N1 values. I'm sorry that I can't back this up with references at the moment.

Just select some basic autopilot/flight director modes like HDG SELECT and you will be perfectly able to fly on autopilot.

Hope this helps,


4th Mar 2009, 20:13
Correct answer D

The AP does not dis-engage but degrades to CWS R and/or CWS P if AP B in use and FMC autopilot modes are in use. ie LNAV/VNAV not available. If the AP is in a basic mode such as LVL CHG, VOR LOC, etc then nothing else happens.

4th Mar 2009, 23:18
Did I read it wrong? I took the meaning of 'Autopilot disengage if Autopilot B in use' to mean 'you should disengage autopilot B if engaged', which, logically, you should do. If it means 'the autopilot will disengage itself if autopilot B in use', then no, that is not correct as it will indeed stay engaged, and I did say 'You don't want to continue using it, so engage A' meaning autopilot B will still be engaged, but you don't really want to use it when you have A available with a full Nav picture. Sorry if I caused confusion!

5th Mar 2009, 02:07
Thanks again guys. Yes i believe the question asks if the autopilot drops out automatically. So answer D is correct; but yes in this case you would not want to continue using autopilot B.

Good replies.

5th Mar 2009, 05:18
You can continue with autopilot B but only in basic direct modes VOR LOC, HDG SEL, LVL CHG, V/S etc until you complete the QRH action items.

If you want to keep the VNAV/LNAV functions urgently you can just change to autopilot A and reselect VNAV/LNAV as required. Then carry out the QRH actions.

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