Medium large transport experiences altitude deviation while on autoplt.

Attributes
ACN	235834
Time
Date	199303
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : fam
State Reference	MO
Altitude	msl bound lower : 35000 msl bound upper : 35200
Aircraft 1
Controlling Facilities	artcc : zkc
Operator	common carrier : air carrier
Make Model Name	Medium Large Transport, Low Wing, 2 Turbojet Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 200 flight time total : 6000 flight time type : 2800
ASRS Report	235834
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : instrument pilot : atp pilot : commercial
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	other flight crewa
Resolutory Action	flight crew : returned to intended course or assigned course other
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

At cruise FL350 with autoplt engaged, airplane started to climb with no other cockpit warning. At FL352 I disengaged autoplt and flew aircraft back to assigned altitude with no comment from ATC. Twice more autoplt would not hold altitude after engaging. All other autoplt/automatic throttle functions were normal. Had not both pilots had full attention on flight instruments at the time of autoplt malfunction, a serious altitude deviation might have occurred. This is the first autoplt altitude malfunction I've had in over 2800 hours in medium large transport's. It renews my thoughts of close attention even when the autoplt is on!

Original NASA ASRS Text

Title: MLG EXPERIENCES ALTDEV WHILE ON AUTOPLT.

Narrative: AT CRUISE FL350 WITH AUTOPLT ENGAGED, AIRPLANE STARTED TO CLB WITH NO OTHER COCKPIT WARNING. AT FL352 I DISENGAGED AUTOPLT AND FLEW ACFT BACK TO ASSIGNED ALT WITH NO COMMENT FROM ATC. TWICE MORE AUTOPLT WOULD NOT HOLD ALT AFTER ENGAGING. ALL OTHER AUTOPLT/AUTO THROTTLE FUNCTIONS WERE NORMAL. HAD NOT BOTH PLTS HAD FULL ATTN ON FLT INSTS AT THE TIME OF AUTOPLT MALFUNCTION, A SERIOUS ALTDEV MIGHT HAVE OCCURRED. THIS IS THE FIRST AUTOPLT ALT MALFUNCTION I'VE HAD IN OVER 2800 HRS IN MLG'S. IT RENEWS MY THOUGHTS OF CLOSE ATTN EVEN WHEN THE AUTOPLT IS ON!

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.