King air C90 pilot experiences runaway trim during descent to 4;000 feet; resulting in momentary los...

Attributes
ACN	1147534
Time
Date	201402
Local Time Of Day	0601-1200
Place
Locale Reference	ZZZ.Airport
State Reference	US
Environment
Flight Conditions	IMC
Light	Daylight
Aircraft 1
Make Model Name	King Air C90 E90
Operating Under FAR Part	Part 91
Flight Phase	Initial Approach
Route In Use	Vectors
Flight Plan	IFR
Component
Aircraft Component	Elevator Trim System
Person 1
Function	Captain Pilot Flying Single Pilot
Qualification	Flight Crew Instrument Flight Crew Flight Instructor Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine
Experience	Flight Crew Last 90 Days 200 Flight Crew Total 8400 Flight Crew Type 60
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Altitude Overshoot Inflight Event / Encounter Loss Of Aircraft Control

Narrative:

After an uneventful departure; cruise and descent phase of flight while receiving radar vectors to the RNAV/GPS RW27 final approach course and descending through 4;500 feet altitude to the assigned 4;000 feet altitude we experienced a runaway pitch trim in the nose down direction. The aircraft immediately entered a greater than 3;500 FPM descent. I brought the throttles to flight idle while disconnecting the autopilot/trim disconnect switch to minimize the descent rate and manually returned the aircraft to the assigned altitude. We descended below 3;000 feet during the event. We notified ATC of our situation and intentions during the recovery. The flight continued to the destination without further incident under manual control. ATC handled our dire situation in a positive way providing assistance as requested. Upon arrival we had the aircraft inspected and found autopilot/electric trim operational abnormality.

Original NASA ASRS Text

Title: King air C90 pilot experiences runaway trim during descent to 4;000 feet; resulting in momentary loss of control and descent below 3;000 feet. The autopilot/trim disconnect switch is used successfully to regain manual control.

Narrative: After an uneventful departure; cruise and descent phase of flight while receiving radar vectors to the RNAV/GPS RW27 final approach course and descending through 4;500 feet altitude to the assigned 4;000 feet altitude we experienced a runaway pitch trim in the nose down direction. The aircraft immediately entered a greater than 3;500 FPM descent. I brought the throttles to flight idle while disconnecting the autopilot/trim disconnect switch to minimize the descent rate and manually returned the aircraft to the assigned altitude. We descended below 3;000 feet during the event. We notified ATC of our situation and intentions during the recovery. The flight continued to the destination without further incident under manual control. ATC handled our dire situation in a positive way providing assistance as requested. Upon arrival we had the aircraft inspected and found autopilot/electric trim operational abnormality.

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.