Narrative:

On jun/tue/95, the aircraft described was sent to an avionics shop to clear several discrepancies, both involving bad navigation/communications and autoplt failures. This airplane has older style equipment that requires regular part replacement that seems to be a never ending problem. One problem that has played a role in the problem that occurred the other day deals with an autoplt that seemingly works good some of the time, but during different phases of flight tends to have pitch and roll departures from normal, and at no given time increment or day. Ie, spastic autoplt syndrome. This problem can not be duplicated on the ground 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 problem. While on the localizer 5 ILS approach to bhm at 3000 ft, the autoplt entered an uncommanded roll to the right, I disconnected the autoplt manually and re-intercepted the localizer. I do not believe that I exceeded the maximum localizer tolerance, as I was realigned on the localizer 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 approach. As I intercepted the GS again, the autoplt started an uncommanded roll and pitch down, again I disconnected the autoplt, remained on the localizer and GS, down to a normal landing. I did not exceed the limits of the approach however, to the approach controller it may have seemed that I was doing aerobatics on the ILS. The autoplt has been inspected once again, and no problems to date have been found.

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Original NASA ASRS Text

Title: 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.

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.