Medium large transport climbed above assigned cruise altitude when the autoplt altitude hold did not hold the altitude at which the aircraft had leveled off to after climb.

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Attributes
ACN	335540
Time
Date	199605
Day	Sat
Local Time Of Day	1801 To 2400
Place
Locale Reference	atc facility : sgf
State Reference	MO
Altitude	msl bound lower : 31000 msl bound upper : 31300
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zkc
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	climbout : intermediate altitude cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	other other : other pilot : instrument pilot : flight engineer pilot : commercial pilot : atp
Experience	flight time last 90 days : 250 flight time total : 16000 flight time type : 250
ASRS Report	335540
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance 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 flight crew : overcame equipment problem
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

We were leveling off from climb at FL310. The autoplt was engaged and the aircraft leveled off at FL310 and then started to climb with the autoplt on altitude hold. We spotted the climb at about FL310 and rolled the autoplt pitch (vertical speed) control to descend. I was about to disengage the autoplt when it finally responded to inputs and leveled off and started back down to FL310 maximum. Altitude was FL313. The autoplt did the same thing again when I leveled off at FL310, but this time I disengaged the autoplt and trimmed aircraft by hand. Autoplt worked normally after re- engagement.

Original NASA ASRS Text

Title: MLG CLBED ABOVE ASSIGNED CRUISE ALT WHEN THE AUTOPLT ALT HOLD DID NOT HOLD THE ALT AT WHICH THE ACFT HAD LEVELED OFF TO AFTER CLB.

Narrative: WE WERE LEVELING OFF FROM CLB AT FL310. THE AUTOPLT WAS ENGAGED AND THE ACFT LEVELED OFF AT FL310 AND THEN STARTED TO CLB WITH THE AUTOPLT ON ALT HOLD. WE SPOTTED THE CLB AT ABOUT FL310 AND ROLLED THE AUTOPLT PITCH (VERT SPD) CTL TO DSND. I WAS ABOUT TO DISENGAGE THE AUTOPLT WHEN IT FINALLY RESPONDED TO INPUTS AND LEVELED OFF AND STARTED BACK DOWN TO FL310 MAX. ALT WAS FL313. THE AUTOPLT DID THE SAME THING AGAIN WHEN I LEVELED OFF AT FL310, BUT THIS TIME I DISENGAGED THE AUTOPLT AND TRIMMED ACFT BY HAND. AUTOPLT WORKED NORMALLY AFTER RE- ENGAGEMENT.

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.