Medium large transport has autoplt problem, altitude deviation.

Attributes
ACN	290645
Time
Date	199412
Day	Sun
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : tol
State Reference	OH
Altitude	msl bound lower : 24000 msl bound upper : 25000
Aircraft 1
Controlling Facilities	artcc : zob
Operator	common carrier : air carrier
Make Model Name	DC-9 Undifferentiated or Other Model
Operating Under FAR Part	Part 121
Flight Phase	cruise other
Route In Use	enroute : direct
Flight Plan	IFR
Aircraft 2
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 : commercial pilot : atp
Experience	flight time last 90 days : 200 flight time total : 23500 flight time type : 700
ASRS Report	290645
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 : overshoot conflict : airborne less severe non adherence : clearance other anomaly other
Independent Detector	aircraft equipment other aircraft equipment : unspecified other flight crewa
Resolutory Action	other
Consequence	Other
Miss Distance	vertical : 800
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

The first officer was flying and using the autoplt. He was leveling off at FL240 and, using the autoplt thumb wheel, could not keep the aircraft from climbing. At 24250 ft I disconnected the autoplt and made a manual correction back to FL240. He put the airplane back on the autoplt but again the aircraft began to climb. At FL242 we got an RA on the TCASII. An aircraft was passing overhead at FL250. Again, the autoplt was disconnected and a manual correction back to FL240 was made, and the RA ceased. This was purely an autoplt malfunction and we never got any closer to the traffic above us than 800 ft.

Original NASA ASRS Text

Title: MLG HAS AUTOPLT PROB, ALTDEV.

Narrative: THE FO WAS FLYING AND USING THE AUTOPLT. HE WAS LEVELING OFF AT FL240 AND, USING THE AUTOPLT THUMB WHEEL, COULD NOT KEEP THE ACFT FROM CLBING. AT 24250 FT I DISCONNECTED THE AUTOPLT AND MADE A MANUAL CORRECTION BACK TO FL240. HE PUT THE AIRPLANE BACK ON THE AUTOPLT BUT AGAIN THE ACFT BEGAN TO CLB. AT FL242 WE GOT AN RA ON THE TCASII. AN ACFT WAS PASSING OVERHEAD AT FL250. AGAIN, THE AUTOPLT WAS DISCONNECTED AND A MANUAL CORRECTION BACK TO FL240 WAS MADE, AND THE RA CEASED. THIS WAS PURELY AN AUTOPLT MALFUNCTION AND WE NEVER GOT ANY CLOSER TO THE TFC ABOVE US THAN 800 FT.

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.