C421 captain reported a runaway pitch trim system during cruise. Captain disconnected the autopilot;...

Attributes
ACN	1746512
Time
Date	202006
Place
Locale Reference	ZZZ.ARTCC
State Reference	US
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Make Model Name	Golden Eagle 421
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Horizontal Stabilizer Control
Person 1
Function	Captain Pilot Flying
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Instrument Flight Crew Multiengine
Experience	Flight Crew Total 6500 Flight Crew Type 140
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Excursion From Assigned Altitude Deviation - Procedural Clearance Deviation - Procedural Published Material / Policy

Narrative:

Level in cruise flight the aircraft started to oscillate. I decide to disconnect the autopilot and hand fly. After disconnecting the autopilot at some point notice the pitch trim running away nose up. I was fumbling looking for the pitch trim circuit breaker and trying to fly the airplane. This was the first time I had flown this particular aircraft and the autopilot configuration was different from other 421's I currently fly. Eventually the control forces became too strong to maintain good control of the aircraft. I had entered a descent with the spoilers extended to maintain control and killed all avionics power in an effort to take power from pitch trim actuator. I had another pilot with me which was helping; at some point we got the aircraft trimmed up manually. Pulled the circuit breaker and then turned on avionics bus back on and established communications with ATC. Decided not to declare an emergency only because he had gained control and were in normal operations after that point. We were a few minutes away from ZZZ and had regained control of the aircraft. Made and uneventful landing ZZZ to ops-check the trim and autopilot system.

Original NASA ASRS Text

Title: C421 Captain reported a runaway pitch trim system during cruise. Captain disconnected the autopilot; regained control of the aircraft and landed at the nearest suitable airport.

Narrative: Level in cruise flight the aircraft started to oscillate. I decide to disconnect the autopilot and hand fly. After disconnecting the autopilot at some point notice the pitch trim running away nose up. I was fumbling looking for the pitch trim circuit breaker and trying to fly the airplane. This was the first time I had flown this particular aircraft and the autopilot configuration was different from other 421's I currently fly. Eventually the control forces became too strong to maintain good control of the aircraft. I had entered a descent with the spoilers extended to maintain control and killed all avionics power in an effort to take power from pitch trim actuator. I had another pilot with me which was helping; at some point we got the aircraft trimmed up manually. Pulled the circuit breaker and then turned on avionics bus back on and established communications with ATC. Decided not to declare an emergency only because he had gained control and were in normal operations after that point. We were a few minutes away from ZZZ and had regained control of the aircraft. Made and uneventful landing ZZZ to ops-check the trim and autopilot system.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.

C421 captain reported a runaway pitch trim system during cruise. Captain disconnected the autopilot; regained control of the aircraft and landed at the nearest suitable airport.