Every day, I received phone calls and emails from operators with questions about their avionics units or systems. Keep reading to find the answers to a recent question.
I am having a problem with my APS-80 autopilot. It is disengaging during change of attitude i.e. during climbing or descent. We have checked the APC and it is okay. What else should we check?
There are several possibilities causing this failure.
1) The APA-80 has torque limiting monitor circuitry that can cause a disconnect if it is starting to get out of tolerance. This will typically fail when a significant pitch change is initiated. You can isolate this by swapping out or substituting another APA.
2) There are also two accelerometers NAC-80 that both feed the APA for hard over monitoring. If one of them gets out of tolerance to where it outputs a signal indicating a "hard over" condition and the other one is not, that will also disconnect the autopilot.
One thing to look for is if the #1 NAC is also feeding the pilot's Flight Director Computer for signal conditioning of the vertical signal in altitude hold mode. This will sometimes show up as erratic Vbar jitter or pitch command when in alt hold mode.
Likewise, if the system has dual Flight Directors installed, the # 2 NAC-80 feeds the co-pilot's Flight Director for the same purpose.
Hope this helps.
TDR 94/94D Common Error Codes
Aircraft Compass System Alignment Formulae