Narrative:

This situation occurred while at a cruise altitude of FL260 and on the scurry arrival from shv into dfw. We were flying at .76 mach in smooth air and in VMC conditions. I was flying the medium large transport with the #2 autoplt engaged. Altitude hold was annunciated on both flight mode annunciators. While approximately 80 mi east of scurry, we received a clearance to FL240 at pilot's discretion. FL240 was dialed into the altitude reminder and altitude arm was annunciated on both FMA's. While looking over the charts in preparation for the arrival, the aircraft began a slow climb (approximately 300 FPM) west/O any pilot input. I first noticed the deviation when the altimeter indicated 600' high. I immediately disconnected the autoplt and returned the aircraft to FL260. While leveling at FL260, ft worth asked us to confirm our altitude. We did so and offered a brief explanation of what had happened. The autoplt was re-engaged and we began our descent. While on the arrival at 11000', again with altitude hold annunciated, the autoplt stated a slow uncommanded descent of approximately 200 FPM. This time the autoplt corrected itself before going more than 100' low. The aircraft had been written up the previous day for the same type of malfunction, and maintenance action had been taken. We had also flown the aircraft on 5 previous legs this same day with no other autoplt problems. Upon arrival at dfw, we entered a logbook write up and briefed the outgoing crew.

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Original NASA ASRS Text

Title: ACFT CLIMBED 600' ABOVE ASSIGNED ALT.

Narrative: THIS SITUATION OCCURRED WHILE AT A CRUISE ALT OF FL260 AND ON THE SCURRY ARR FROM SHV INTO DFW. WE WERE FLYING AT .76 MACH IN SMOOTH AIR AND IN VMC CONDITIONS. I WAS FLYING THE MLG WITH THE #2 AUTOPLT ENGAGED. ALT HOLD WAS ANNUNCIATED ON BOTH FLT MODE ANNUNCIATORS. WHILE APPROX 80 MI E OF SCURRY, WE RECEIVED A CLRNC TO FL240 AT PLT'S DISCRETION. FL240 WAS DIALED INTO THE ALT REMINDER AND ALT ARM WAS ANNUNCIATED ON BOTH FMA'S. WHILE LOOKING OVER THE CHARTS IN PREPARATION FOR THE ARR, THE ACFT BEGAN A SLOW CLB (APPROX 300 FPM) W/O ANY PLT INPUT. I FIRST NOTICED THE DEVIATION WHEN THE ALTIMETER INDICATED 600' HIGH. I IMMEDIATELY DISCONNECTED THE AUTOPLT AND RETURNED THE ACFT TO FL260. WHILE LEVELING AT FL260, FT WORTH ASKED US TO CONFIRM OUR ALT. WE DID SO AND OFFERED A BRIEF EXPLANATION OF WHAT HAD HAPPENED. THE AUTOPLT WAS RE-ENGAGED AND WE BEGAN OUR DSCNT. WHILE ON THE ARR AT 11000', AGAIN WITH ALT HOLD ANNUNCIATED, THE AUTOPLT STATED A SLOW UNCOMMANDED DSCNT OF APPROX 200 FPM. THIS TIME THE AUTOPLT CORRECTED ITSELF BEFORE GOING MORE THAN 100' LOW. THE ACFT HAD BEEN WRITTEN UP THE PREVIOUS DAY FOR THE SAME TYPE OF MALFUNCTION, AND MAINT ACTION HAD BEEN TAKEN. WE HAD ALSO FLOWN THE ACFT ON 5 PREVIOUS LEGS THIS SAME DAY WITH NO OTHER AUTOPLT PROBS. UPON ARR AT DFW, WE ENTERED A LOGBOOK WRITE UP AND BRIEFED THE OUTGOING CREW.

Data retrieved from NASA's ASRS site as of August 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.