Flight crew of an medium large transport climbed above assigned altitude when asked by ATC to increase their speed due to an altitude autoplt problem.

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Attributes
ACN	301860
Time
Date	199504
Day	Mon
Local Time Of Day	0601 To 1200
Place
Locale Reference	atc facility : pgs
State Reference	AZ
Altitude	msl bound lower : 31000 msl bound upper : 31500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zla tower : dtw
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 : captain oversight : pic
Qualification	pilot : flight engineer pilot : atp
Experience	flight time last 90 days : 200 flight time total : 17000 flight time type : 294
ASRS Report	301860
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

During cruise, center requested maximum forward speed for arrival sequencing at lax. During acceleration, the autoplt stayed engaged but aircraft began climbing. Got 31500 ft before we could correct back. We informed ZLA of deviation. (At first we thought the climb was due to mountain wave). Aircraft did this again later, but we immediately caught it. We wrote up malfunction at lax.

Original NASA ASRS Text

Title: FLC OF AN MLG CLBED ABOVE ASSIGNED ALT WHEN ASKED BY ATC TO INCREASE THEIR SPD DUE TO AN ALT AUTOPLT PROB.

Narrative: DURING CRUISE, CTR REQUESTED MAX FORWARD SPD FOR ARR SEQUENCING AT LAX. DURING ACCELERATION, THE AUTOPLT STAYED ENGAGED BUT ACFT BEGAN CLBING. GOT 31500 FT BEFORE WE COULD CORRECT BACK. WE INFORMED ZLA OF DEV. (AT FIRST WE THOUGHT THE CLB WAS DUE TO MOUNTAIN WAVE). ACFT DID THIS AGAIN LATER, BUT WE IMMEDIATELY CAUGHT IT. WE WROTE UP MALFUNCTION AT LAX.

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.