Date: 201710
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: PA-34-200T Turbo Seneca II
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZZZ
ATC / advisory: TRACON ZZZ
Aircraft component: Autopilot
Component problem: Malfunctioning
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Multiengine
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem; Flight Crew Returned To Clearance
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: After contacting approach and receiving and updating the altimeter with the altimeter setting; I notice the aircraft altimeter was about 120 feet high. I turned off the altitude hold function and commanded the autopilot with the autopilot trim knob to slowly descend the aircraft from 7100 MSL to 7000 MSL. After a few seconds; I noticed the aircraft was slowly climbing despite the previous descent command input. I once again rotated the pitch trim knob forward to command the autopilot to trim the aircraft for a descent with no apparent effect. I disconnected the auto pilot and attempted to use the electric trim switch on the yoke to trim the aircraft for a descent for the slight climb of the aircraft. The electric trim switch did not have an effect for down trim input from the electric yoke trim switch; nor did work for nose up trim input from the switch. I then tried manually trimming the aircraft using the manual trim wheel. The manual trim wheel was difficult to move in either direction down or up and it seemed have more resistance than normal. Having tried the aforementioned procedures; there was no effect was noticed on the slow climb of the aircraft. By this time the aircraft had climbed to approximately to 7350 MSL. I disconnected the electric trim by pushing the electric trim power button to the OFF position and then reattempting to use the manual trim wheel. This seemed to free up the manual trim wheel and I was able to adjust the aircraft trim manually for a slight descent back to the correct altitude of 7000 MSL. After retrimming the aircraft manually and re-establishing the correct altitude; I turn ON the power button for the electric trim. I used the pilot's electric trim switch on the yoke to activate the electric trim function which reacted normally to the electric trim inputs from the pilot's yoke switch. I then engaged the autopilot heading and altitude hold functions to see if the autopilot would operate normally. I operationally checked and monitored the autopilot and trim functions by engaging and commanding altitude hold and altitude trim for climb and descent changes for the rest of the flight with no anomalies noticed. It seemed that for the one time where the autopilot did not follow the command to descend and that the autopilot trim function did not operate normally and turned out to be a one time event. I suspect for that one time occurrence; the trim motor might have not been disengaged when the autopilot was commanded to descend or when the autopilot was disengaged and the trim motor might have locked up with power on the trim system until I turned OFF the electric trim system by pushing electric trim power button to OFF.




Synopsis: PA34-200T pilot reported an altitude excursion in cruise when the electric trim malfunctioned.