A PIPER MALIBU ON APCH EXPERIENCED AN UNCONTROLLED TFC PATTERN DEVIATION CAUSED BY THE AUTOPLT FAILURE TO DISENGAGE.

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I TKOF IFR TO ZZZ INTERNATIONAL. APROX 20 MINS AFTER TKOF THE AUTOPLT WAS TURNED ON; AFTER A FEW MINS THE AUTOPLT DISENGAGED; I RECYCLED THE AUTO PLT AND IT SEEMED TO WORK FINE. AS I APCHED ZZZ I WAS TOLD TO FLY OVER THE NUMBERS FOR RWY 17 AND EXPECTED A VISUAL APCH TO RWY 30 WITH RIGHT TFC. AS I GOT CLOSE TO THE ARPT I WAS TOLD TO FLY A HEADING OF 140 DEGS. I WAS WATCHING A PLANE TKOF ON RWY 26. I EXPECTED TO PASS BEHIND HIM. AS I APCHED THE ARPT THERE WERE STRONG TURBULENCE AND WIND SHEAR AS I TURNED TO THE NEW HEADING THE AUTOPLT ALARM WAS GOING OFF AND THE AUTOPLT WAS SHOWING A PITCH TRIM FAILURE. THE PLANE WOULD NOT TURN WITH THE HEADING BUG. I PUSHED THE CTL WHEEL STEERING BUTTON; WHICH SHOULD OVER RIDE THE AUTOPLT. THIS DID NOT WORK. I THEN HIT THE RED AUTOPLT DISCONNECT BUTTON; WHICH ALSO DID NOT WORK; I SLOWED THE PLANE DOWN TO MAKE SURE THE PLANE WOULD PASS IN FRONT OF ME; AS I HIT THE AUTOPLT MASTER AFTER A COUPLE OF SECS; THE AUTOPLT DISCONNECTED. AS I BEGAN TO TURN THE PLANE I HAD GOTTEN TOO CLOSE TO RWY 30 AND THE TWR ASKED ME IF I WAS FLYING RIGHT OR LEFT TFC. I TOLD THE TWR I WOULD LIKE LEFT TFC. BECAUSE THE AUTOPLT FAILURE I WAS TOO CLOSE TO MAKE RIGHT TFC. THE TWR TOLD ME I SHOULD HAVE REQUESTED LEFT TFC BECAUSE THE ACR JET'S COLLISION ALARM WENT OFF. I HAD A VISUAL ON THE ACR JET THE ENTIRE TIME; BUT WAS UNABLE TO MAKE RIGHT TFC BECAUSE OF THE RUNAWAY AUTOPLT. I CALLED THE PIPER REP AND FOUND OUT THERE WAS A RECENTLY ISSUED AD ON THE AUTOPLT. CALLBACK CONVERSATION REVEALED THE FOLLOWING INFO: THE RPTR STATED THIS AUTOPLT HAD PREVIOUS PROBLEMS BUT NOTHING SERIOUS UNTIL THIS INCIDENT. THE RPTR SAID THE PITCH TRIM FAILURE SHOULD HAVE CAUSED A DISCONNECT OF THE AUTOPLT BUT IT DID NOT. THE RPTR SAID MANUAL TRIM AND CTL WHEEL DISCONNECT WAS ATTEMPTED BUT THE AUTOPLT DISCONNECT SWITCH AND NOTHING HAPPENED THEN THE AUTOPLT MASTER SWITCH FINALLY DISCONNECTED THE AUTOPLT. THE RPTR SAID THE AIRPLANE WAS PUT IN THE SHOP WHERE IT WAS DISCOVERED THE AUTO PLT WAS THE SUBJECT OF AN AIRWORTHINESS DIRECTIVE. THE RPTR SAID THE AUTOPLT IS NOW BEING MODIFIED.