Pilot had a runaway trim system.

Attributes
ACN	534759
Time
Date	200201
Day	Wed
Local Time Of Day	1801 To 2400
Place
Locale Reference	navaid : pxn.vortac
State Reference	CA
Altitude	msl bound lower : 9000 msl bound upper : 9500
Environment
Flight Conditions	IMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zoa.artcc tower : lft.tower
Operator	common carrier : air taxi
Make Model Name	Any Unknown or Unlisted Aircraft Manufacturer
Operating Under FAR Part	Part 135
Navigation In Use	other vortac
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : atp
Experience	flight time last 90 days : 170 flight time total : 3200 flight time type : 750
ASRS Report	534759
Person 2
Affiliation	government : faa
Function	controller : radar
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude other anomaly other spatial deviation
Independent Detector	other flight crewa
Resolutory Action	none taken : detected after the fact
Consequence	other
Supplementary
Problem Areas	Aircraft
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

I was at 9000 ft on autoplt, which suddenly disconnected and the electrical trim ran away. I did lose 600 ft and my heading by the time I disconnected the main trim, regain control and activate the auxiliary trim which is slower. I got 500 ft above 9000 ft and then regain control of the situation. This malfunction was reported to maintenance.

Original NASA ASRS Text

Title: PLT HAD A RUNAWAY TRIM SYS.

Narrative: I WAS AT 9000 FT ON AUTOPLT, WHICH SUDDENLY DISCONNECTED AND THE ELECTRICAL TRIM RAN AWAY. I DID LOSE 600 FT AND MY HDG BY THE TIME I DISCONNECTED THE MAIN TRIM, REGAIN CTL AND ACTIVATE THE AUX TRIM WHICH IS SLOWER. I GOT 500 FT ABOVE 9000 FT AND THEN REGAIN CTL OF THE SIT. THIS MALFUNCTION WAS RPTED TO MAINT.

Data retrieved from NASA's ASRS site as of July 2007 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.