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.

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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.