PAY2 AUTOPLT MALFUNCTION RESULTS IN ALT OVERSHOOT. PLT PULLS AUTOPLT CIRCUIT BREAKER TO STOP CLB.

Narrative

THE PROB AROSE WHEN LEVELING AT 12000 FT MSL. I SELECTED THE ALT HOLD ON THE AUTOPLT. THE ACFT KEPT CLBING AND THE AUTOPLT WOULD NOT DISENGAGE. PRESSURE WAS APPLIED TO THE YOKE WHICH CAUSED THE AUTOPLT TO INCREASE THE UP TRIM RESULTING IN A CLB TO 13000 FT MSL BEFORE THE AUTOPLT CIRCUIT BREAKER WAS PULLED. WHILE NOTIFYING CTR OF THE ALTDEV; I DSNDED TO THE ASSIGNED ALT. IN RETROSPECT; I BELIEVE TRYING TO DISENGAGE THE FAILED AUTOPLT SWITCH; ADJUST THE AUTOPLT TRIM SWITCH; AND THE MANUAL CTL TRIM; THE AUTOPLT CIRCUIT BREAKER SHOULD HAVE BEEN DISCONNECTED SOONER. THE EXCESSIVE PRESSURE OF PITCH UP TRIM WITH THE DISTR CAUSED THE ACFT TO CLB. MY INITIAL AND RECURRENT TRAINING HAS FOCUSED ON ELECTRIC RUN-AWAY TRIM (PLUS WARNING LIGHTS IN THE PANEL); BUT VERY LITTLE ON AUTOPLT MALFUNCTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE WAS BEING LAZY AND ENGAGED THE AUTOPLT ALT HOLD AS HE WAS APCHING THE LEVELOFF ALT; BUT WHILE THE ACFT WAS STILL CLBING. RPTR ADVISED THAT THE ACFT THEN BALLOONED PAST THE ASSIGNED ALT AND WHEN HE APPLIED DOWNWARD PRESSURE ON THE YOKE; THE AUTOPLT COUNTERED WITH INCREASED UP TRIM. RPTR STATED THAT THE AUTOPLT DISCONNECT SWITCH DID NOT FUNCTION PROPERLY BECAUSE OF A LOOSE WIRE IN THE YOKE WHICH WAS DISCOVERED AFTER THE ACFT WAS SENT TO AN AVIONICS SHOP. RPTR STATES THAT HE HAS NOT HEARD OF ANY RPTS OF SIMILAR PROBS WITH PAY2 AUTOPLT WIRING. RPTR ADVISED THAT THE ACFT IS A 1978 MODEL AND THE AUTOPLT IS ORIGINAL FACTORY EQUIP.