Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 54; Flight Crew Total 20000; Flight Crew Type 550
Situational Awareness
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; Flight Crew Became Reoriented; Flight Crew Returned To Clearance
Aircraft; Human Factors
This 300 foot altitude loss occurred while on a GPS direct clearance just passing PXT. Since I was in IMC and night conditions I was tuning the radar to plan any action necessary to deviate from the planned routing due to a cold front crossing ahead. I noticed that the altitude had descended from cruise. The autopilot altitude hold light was still illuminated but not correcting. I disengaged the autopilot and had to fight a full down trim situation. The electric trim did not work and the manual trim reverted quickly to a continuous nose down trim electric input. The aircraft was promptly returned to the assigned altitude and reported to ATC. While holding against the full nose down trim I located and pulled the electric trim circuit breaker which then allowed for manual trim to stabilize. ATC said they were wondering why we were down there and did not indicate if there was any traffic conflict. The remaining hour and twenty minutes of the flight was hand flown and conducted without the autopilot. Since it was at night and in the turbulent weather it was full of tasks that I was glad to have already prepared for. The runaway electric trim down input exceeded the ability and limit of the autopilot altitude hold feature to maintain altitude. This aircraft does not have any trim movement tones or altitude select or altitude alert features. The autopilot stayed engaged and did not give a disconnect tone but tried to maintain altitude and couldn't since it reached its limit. When tasks other than basic flight are added to the operation a constant recheck of the basics should be checked and rechecked. Working with the radar and comparing to the NexRad displays consumed time that added to this altitude loss not being seen earlier. Helpful hints would to be very familiar with your circuit breaker panels especially in the dark. Hand fly practice while communicating and navigating for cruise to approach situations. Whenever there is a lull in cruise activity have approach charts and navigation systems at the ready and preset as much as possible well ahead of time.
C-310 pilot reported an altitude excursion when the autopilot electric trim ran away in the nose down direction.