2010-03 · NASA ASRS report 880587
CL65 flight crew experiences autopilot failure at 11;000 feet during arrival in moderate turbulence; causing 200 feet of altitude loss. Aircraft is then hand flown to a normal landing.
The First Officer was the Flying Pilot. On downwind we were on radar vectors due to weather and level at 11;000ft. The flight conditions were moderate turbulence and IMC. The aircraft's autopilot disengaged on its own and [we had an] uncommanded downward pitch with an excessive downward force. This along with a STAB TRIM and MACH TRIM being disengaged with no pilot input; caused the STAB TRIM and MACH trim amber caution messages; along with the AP PITCH ND message; [to appear]. The aircraft lost approx 200ft of altitude and the aircraft was now being hand flown by the First Officer. He regained altitude almost immediately and leveled off back at 11;000ft. Approach Control shortly thereafter gave us a descent to 7;000ft. The First Officer then hand flew the rest of the approach and landing and the STAB & MACH TRIM were able to be reset. We attempted to reset the autopilot but were unsuccessful. A normal landing was performed and a discrepancy was logged and reported to our Maintenance.
We were at 11;000ft. Flight conditions were poor with moderate turbulence and IMC. The aircraft's autopilot disengaged on its own; along with Master Caution messages STAB TRIM; MACH TRIM and AP TRIM ND. The aircraft lost approximately 200 feet. I quickly recovered back to the assigned altitude. The stab and mach trims were re-engaged; but the AP would not stay engaged. I hand flew the rest of the arrival and approach to a normal landing. Maintenance was notified of the issues when we parked at the gate.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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