PA-31 Navajo/Chieftan/Mojave/T1040
Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP); Flight Crew Flight Instructor
Flight Crew Last 90 Days 150; Flight Crew Total 12500; Flight Crew Type 150
Situational Awareness
Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Wake Vortex Encounter
Flight Crew Returned To Clearance
Aircraft; Environment - Non Weather Related
During level off at FL250; wake turbulence was encountered at 24;900 feet. The autopilot was engaged and commanded to level off at 25;000 MSL.  At 24;900 feet the aircraft encountered its only turbulence for the entire flight and seemed similar to other wake turbulence encounters.  During the turbulence encounter it seemed the autopilot was not going to achieve FL250 and it seemed prudent to disconnect the autopilot.  It became evident that during the wake turbulence event the autopilot trimmed the aircraft in a nose up condition because when the autopilot was disengaged the aircraft pitched up; and nose down trim was initiated; by the time the aircraft was trimmed; the aircraft ascended 250 feet above FL250.  It seemed as though the aircraft was in a nose up trim condition but the servo was holding the nose down until the autopilot was disconnected.
PA-31 Instructor Pilot reported an altitude deviation occurred following a wake turbulence encounter at FL250.