Sa-227 metro pilot reported an uncommanded pitch up with the autopilot engaged.

Narrative:

I was level at 10;000 with autopilot engaged; when I noticed an uncommanded pitch up and climb. The electric trim amber light was illuminated on autopilot annunciator panel. I disengaged the autopilot to return to level flight. It took considerable forward pressure to lower the nose. The pilot's trim switch made its normal trim in motion sound. The trim indicator showed pitch up to the upper green limit and it didn't move. I switched to the copilot's trim and used the yoke switch and the override switch with similar results. I continued to struggle to keep the nose level for several minutes. I started slowing and below about 180 knots; the trim started responding normally. I [advised ATC] and made an uneventful landing using the ILS. Only sometime later did I remember that I'd had a similar problem on a flight the previous evening. Again; the nose pitched up while on autopilot and I had to disengage the autopilot and use the trim switches to regain control. The earlier event only lasted a few seconds and was easily rectified at cruise altitude and airspeed.

Original NASA ASRS Text

Title: SA-227 Metro pilot reported an uncommanded pitch up with the autopilot engaged.

Narrative: I was level at 10;000 with autopilot engaged; when I noticed an uncommanded pitch up and climb. The electric trim amber light was illuminated on autopilot annunciator panel. I disengaged the autopilot to return to level flight. It took considerable forward pressure to lower the nose. The pilot's trim switch made its normal trim in motion sound. The trim indicator showed pitch up to the upper green limit and it didn't move. I switched to the copilot's trim and used the yoke switch and the override switch with similar results. I continued to struggle to keep the nose level for several minutes. I started slowing and below about 180 knots; the trim started responding normally. I [advised ATC] and made an uneventful landing using the ILS. Only sometime later did I remember that I'd had a similar problem on a flight the previous evening. Again; the nose pitched up while on autopilot and I had to disengage the autopilot and use the trim switches to regain control. The earlier event only lasted a few seconds and was easily rectified at cruise altitude and airspeed.

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.

Attributes
ACN	1463210
Time
Date	201707
Local Time Of Day	0001-0600
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Make Model Name	SA-227 AC Metro III
Operating Under FAR Part	Part 91
Flight Phase	Cruise
Flight Plan	IFR
Component
Aircraft Component	Horizontal Stabilizer Trim
Person 1
Function	Pilot Flying Single Pilot
Qualification	Flight Crew Air Transport Pilot (ATP)
Events
Anomaly	Aircraft Equipment Problem Critical Inflight Event / Encounter Loss Of Aircraft Control