GA small aircraft experienced runaway stabilizer trim and incurred altitude excursion.

Attributes
ACN	93040
Time
Date	198808
Day	Tue
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : bdl
State Reference	CT
Altitude	msl bound lower : 7400 msl bound upper : 8000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Operator	general aviation : personal
Make Model Name	Small Aircraft, Low Wing, 1 Eng, Retractable Gear
Flight Phase	cruise other
Route In Use	enroute airway : v405
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : private
Experience	flight time last 90 days : 19 flight time total : 595
ASRS Report	93040
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

I was assigned 8000' by ATC. I experienced a rapid descent (greater than 1500 FPM). ATC was informed immediately and corrective action was taken. The problem was a runaway trim switch which trimmed the plane to full nose down in a few seconds overriding the altitude hold and causing the condition. The plane nosed over and about 600' (at least, I was too busy to notice) of altitude was lost before the plane was retrimmed and proceeded to climb back to 8000' (assigned). ATC said 'no problem' and asked to be informed when 8000' was reached. During those few moments (20 seconds approximately) I disconnected the altitude system and scanned for the problem. With full down trim it was impossible to stop the descent by pulling back on the yoke. Upon spotting the position on the trim indicator I tried using the electric trim up, which was now operational, successfully. The flight proceeded normally after this event and the trim was operating properly. The trim indicator is below knee level and is not usually part of the regular scan of the instruments, making this condition hard to spot initially, even though I saw the plane descending and tried to pull up.

Original NASA ASRS Text

Title: GA SMA EXPERIENCED RUNAWAY STABILIZER TRIM AND INCURRED ALT EXCURSION.

Narrative: I WAS ASSIGNED 8000' BY ATC. I EXPERIENCED A RAPID DSCNT (GREATER THAN 1500 FPM). ATC WAS INFORMED IMMEDIATELY AND CORRECTIVE ACTION WAS TAKEN. THE PROB WAS A RUNAWAY TRIM SWITCH WHICH TRIMMED THE PLANE TO FULL NOSE DOWN IN A FEW SECS OVERRIDING THE ALT HOLD AND CAUSING THE CONDITION. THE PLANE NOSED OVER AND ABOUT 600' (AT LEAST, I WAS TOO BUSY TO NOTICE) OF ALT WAS LOST BEFORE THE PLANE WAS RETRIMMED AND PROCEEDED TO CLB BACK TO 8000' (ASSIGNED). ATC SAID 'NO PROB' AND ASKED TO BE INFORMED WHEN 8000' WAS REACHED. DURING THOSE FEW MOMENTS (20 SECS APPROX) I DISCONNECTED THE ALT SYS AND SCANNED FOR THE PROB. WITH FULL DOWN TRIM IT WAS IMPOSSIBLE TO STOP THE DSCNT BY PULLING BACK ON THE YOKE. UPON SPOTTING THE POS ON THE TRIM INDICATOR I TRIED USING THE ELECTRIC TRIM UP, WHICH WAS NOW OPERATIONAL, SUCCESSFULLY. THE FLT PROCEEDED NORMALLY AFTER THIS EVENT AND THE TRIM WAS OPERATING PROPERLY. THE TRIM INDICATOR IS BELOW KNEE LEVEL AND IS NOT USUALLY PART OF THE REGULAR SCAN OF THE INSTRUMENTS, MAKING THIS CONDITION HARD TO SPOT INITIALLY, EVEN THOUGH I SAW THE PLANE DSNDING AND TRIED TO PULL UP.

Data retrieved from NASA's ASRS site as of August 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.