PLT OF AN SMT TWIN EXPERIENCES ERRATIC AUTOPLT WHEN TURNED ON AT UNANNOUNCED TIMES.

Narrative

ON JUN/TUE/95; THE ACFT DESCRIBED WAS SENT TO AN AVIONICS SHOP TO CLR SEVERAL DISCREPANCIES; BOTH INVOLVING BAD NAV/COMS AND AUTOPLT FAILURES. THIS AIRPLANE HAS OLDER STYLE EQUIP THAT REQUIRES REGULAR PART REPLACEMENT THAT SEEMS TO BE A NEVER ENDING PROB. ONE PROB THAT HAS PLAYED A ROLE IN THE PROB THAT OCCURRED THE OTHER DAY DEALS WITH AN AUTOPLT THAT SEEMINGLY WORKS GOOD SOME OF THE TIME; BUT DURING DIFFERENT PHASES OF FLT TENDS TO HAVE PITCH AND ROLL DEPS FROM NORMAL; AND AT NO GIVEN TIME INCREMENT OR DAY. IE; SPASTIC AUTOPLT SYNDROME. THIS PROB CAN NOT BE DUPLICATED ON THE GND OR ON A TEST BENCH; OR ANYTIME IMMEDIATELY AFTER THE FAILURE HAS OCCURRED WITH CONSISTENCY. THE AVIONICS SHOP SUGGESTED TRYING TO REMOVE ALL CONNECTORS AND REINSTALLING THEM; THAT A POSSIBLE LOOSE CONNECTED COULD BE AT FAULT. ON AUG/XX/95 I FOUND THIS NOT TO HAVE BEEN THE FIX FOR THE PROB. WHILE ON THE LOC 5 ILS APCH TO BHM AT 3000 FT; THE AUTOPLT ENTERED AN UNCOMMANDED ROLL TO THE R; I DISCONNECTED THE AUTOPLT MANUALLY AND RE-INTERCEPTED THE LOC. I DO NOT BELIEVE THAT I EXCEEDED THE MAX LOC TOLERANCE; AS I WAS REALIGNED ON THE LOC I RE-ENGAGED THE AUTOPLT TO SEE IF IT WAS ACTING AS IT HAD IN THE PAST. IT SEEMED TO BE FUNCTIONING NORMALLY AND SO I CONTINUED THE APCH. AS I INTERCEPTED THE GS AGAIN; THE AUTOPLT STARTED AN UNCOMMANDED ROLL AND PITCH DOWN; AGAIN I DISCONNECTED THE AUTOPLT; REMAINED ON THE LOC AND GS; DOWN TO A NORMAL LNDG. I DID NOT EXCEED THE LIMITS OF THE APCH HOWEVER; TO THE APCH CTLR IT MAY HAVE SEEMED THAT I WAS DOING AEROBATICS ON THE ILS. THE AUTOPLT HAS BEEN INSPECTED ONCE AGAIN; AND NO PROBS TO DATE HAVE BEEN FOUND.