Narrative:

Trim failure/altitude deviation. While leveling at FL290 and accelerating to cruise speed with the autoplt engaged, the altitude trim motor failed, causing the aircraft to climb even though altitude hold was annunciated on the FMA. The autoplt was disconnected and the aircraft retrimmed using the primary trim motor. Cruise altitude was exceeded by 500 ft. After several mins the altitude trim started functioning again and the flight was continued to destination without further incident.

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Original NASA ASRS Text

Title: MD80 CREW HAD ALTDEV.

Narrative: TRIM FAILURE/ALTDEV. WHILE LEVELING AT FL290 AND ACCELERATING TO CRUISE SPD WITH THE AUTOPLT ENGAGED, THE ALT TRIM MOTOR FAILED, CAUSING THE ACFT TO CLB EVEN THOUGH ALT HOLD WAS ANNUNCIATED ON THE FMA. THE AUTOPLT WAS DISCONNECTED AND THE ACFT RETRIMMED USING THE PRIMARY TRIM MOTOR. CRUISE ALT WAS EXCEEDED BY 500 FT. AFTER SEVERAL MINS THE ALT TRIM STARTED FUNCTIONING AGAIN AND THE FLT WAS CONTINUED TO DEST WITHOUT FURTHER INCIDENT.

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.