PA31T with autoplt altitude preset to 16000 ft. The airplane leveled off and altitude hold engaged. 45-60 seconds later; the autoplt disengaged and the airplane pitched up to 16500 ft.

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Attributes
ACN	683704
Time
Date	200601
Local Time Of Day	1801 To 2400
Place
Locale Reference	airport : eko.airport
State Reference	NV
Altitude	msl single value : 16000
Environment
Flight Conditions	VMC
Light	Night
Aircraft 1
Controlling Facilities	artcc : zlc.artcc
Operator	common carrier : air taxi
Make Model Name	PA-31T-1 Cheyenne I
Operating Under FAR Part	Part 135
Flight Phase	cruise : level
Flight Plan	IFR
Person 1
Affiliation	company : air taxi
Function	flight crew : single pilot
Qualification	pilot : atp pilot : cfi pilot : multi engine
Experience	flight time last 90 days : 91 flight time total : 8963 flight time type : 227
ASRS Report	683704
Events
Anomaly	aircraft equipment problem : critical altitude deviation : excursion from assigned altitude
Independent Detector	aircraft equipment other aircraft equipment : altimeter other flight crewa
Resolutory Action	flight crew : overcame equipment problem flight crew : regained aircraft control
Consequence	other
Supplementary
Problem Areas	Aircraft Flight Crew Human Performance
Primary Problem	Aircraft

Narrative:

Climbing out on autoplt with altitude preselect set to 16000 ft MSL and armed. At 16000 ft MSL; autoplt leveled off and altitude hold engaged. Approximately 45-60 seconds later; autoplt disengaged and aircraft pitched up abruptly climbing to 16500 ft before I could grab the yoke and return to 16000 ft. As I rest my leg against the trim wheel and did not detect any rolling of the trim wheel; I had no indications that this was about to happen. I don't know how anyone could react any faster.

Original NASA ASRS Text

Title: PA31T WITH AUTOPLT ALT PRESET TO 16000 FT. THE AIRPLANE LEVELED OFF AND ALT HOLD ENGAGED. 45-60 SECONDS LATER; THE AUTOPLT DISENGAGED AND THE AIRPLANE PITCHED UP TO 16500 FT.

Narrative: CLBING OUT ON AUTOPLT WITH ALT PRESELECT SET TO 16000 FT MSL AND ARMED. AT 16000 FT MSL; AUTOPLT LEVELED OFF AND ALT HOLD ENGAGED. APPROX 45-60 SECONDS LATER; AUTOPLT DISENGAGED AND ACFT PITCHED UP ABRUPTLY CLBING TO 16500 FT BEFORE I COULD GRAB THE YOKE AND RETURN TO 16000 FT. AS I REST MY LEG AGAINST THE TRIM WHEEL AND DID NOT DETECT ANY ROLLING OF THE TRIM WHEEL; I HAD NO INDICATIONS THAT THIS WAS ABOUT TO HAPPEN. I DON'T KNOW HOW ANYONE COULD REACT ANY FASTER.

Data retrieved from NASA's ASRS site as of January 2009 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.