MD80 CREW HAD THE AUTOPLT AUTOTRIM SYS FAIL. AN ALT OVERSHOOT WAS THE RESULT.

Narrative

AUTOPLT MALFUNCTION ALTDEV. WHILE APCHING ASSIGNED ALT OF 13000 FT WITH AUTOPLT ENGAGED IN PMS CLB MODE WE WERE SIMULTANEOUSLY GIVEN A HEADING CHANGE; CLRNC DIRECT TO ELX; AND A FREQ CHANGE TO ZAU. AFTER OBSERVING RATE OF CLB DECREASE AND AUTOTHROTTLES RETARD TO BEGIN LEVELOFF; BOTH PLTS WERE MOMENTARILY DIVERTED FROM MONITORING LEVELOFF AS CAPT CHANGED HEADING SELECT KNOB AND FMS AND FO CHANGED RADIO FREQ. CAPT OBSERVED ACFT CLBING THROUGH 13100 FT WITH AUTOPLT TRIM LIGHT ILLUMINATED. 13000 FT WAS PROPERLY SET AND ARMED. AUTOPLT WAS DISENGAGED AND PWR REDUCED BY 13200 FT. ACFT WAS TRIMMED NOSE UP AND ACFT PITCHED UP NOTICEABLY BEFORE FORWARD PRESSURE ON YOKE AND RETRIMMING ARRESTED CLB AT APPROX 13450 FT. AS ACFT DSNDED THROUGH 13200 FT ATC CLRED US TO CLB TO FL230. NO COMMENT MADE ABOUT ALTDEV. SEATBELT SIGN WAS ILLUMINATED BUT SOME PAX WERE PROBABLY STARTLED BY THRUMP OF AUTOPLT DISCONNECT; ABRUPT MANEUVER; AND A LITTLE LIGHTNESS IN SEAT. WE DETERMINED THAT AUTOPLT/ALTERNATE TRIM WAS INOP. SUBSEQUENTLY RETRIMMED ACFT MANUALLY WITH PRIMARY TRIM WHENEVER AIRSPD OR CONFIGN CHANGES WERE MADE. MALFUNCTION ENTERED IN ACFT LOGBOOK.