Autoplt causes an uncommanded altitude deviation.

Attributes
ACN	325510
Time
Date	199601
Day	Mon
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : lnk
State Reference	NE
Altitude	msl bound lower : 31000 msl bound upper : 31350
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zmp
Operator	common carrier : air carrier
Make Model Name	Commercial Fixed Wing
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : flight engineer pilot : instrument pilot : cfi
ASRS Report	325510
Person 2
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : commercial pilot : instrument pilot : flight engineer pilot : atp
ASRS Report	325508
Events
Anomaly	altitude deviation : excursion from assigned altitude non adherence : far non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Uncommanded climb on autoplt to 31350 ft before climb stopped and return to FL310. Supplemental information from acn 325508: we also experienced the autoplt leaving altitude when in VNAV and at other flight levels.

Original NASA ASRS Text

Title: AUTOPLT CAUSES AN UNCOMMANDED ALTDEV.

Narrative: UNCOMMANDED CLB ON AUTOPLT TO 31350 FT BEFORE CLB STOPPED AND RETURN TO FL310. SUPPLEMENTAL INFO FROM ACN 325508: WE ALSO EXPERIENCED THE AUTOPLT LEAVING ALT WHEN IN VNAV AND AT OTHER FLT LEVELS.

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.