SMA MAKES EXTREME TURN WHEN AUTOPLT ENGAGED. DISCONNECT; THEN HSI FAILS.

Narrative

AFTER TKOF IN HVY RAIN ON DEC/XX/91 AT ABOUT XA00 AM LCL; WE WERE AT APPROX 1500 FT MSL WHEN I ENGAGED THE AUTOPLT DURING CLBOUT. WHEN THE AUTOPLT ENGAGED THE PLANE WENT INTO A 45 DEG AND L HAND BANK. I IMMEDIATELY DISENGAGED THE AUTOPLT. AT THAT TIME WE LOST OUR HSI. AT THAT POINT WE CONTINUED TO CLB BUT HAD ONLY THE ATTITUDE INDICATOR AND TURN AND BANK. THE TWR SUGGESTED WE LEVEL AT 300 MSL WHICH WE DID AND WE CONTINUED TO FLY WITH A COMPASS. AFTER THE TWR GAVE US A TIMED TURN DURING AUTO I CHKED ALL THE BREAKERS AND ALTHOUGH IT DID NOT SEEM LIKE WE HAD A PROBLEM THERE THE HSI RETURNED TO OP. AT THAT POINT WE RETURNED TO COURSE AND CONTINUED THE FLT. THE ONLY RECURRENCE WAS AN INCORRECT READING ON THE OBS PER BOSTON CENTER HOWEVER AFTER SWITCHING TO ALBANY IT APPEARED THAT OUR OBS WAS FUNCTIONING PROPERLY. ALTHOUGH WE DID NOT USE THE AUTOPLT ANYMORE THAT DAY; I DID TRY IT SEVERAL DAYS LATER ON THE RETURN FLT. IT SEEMED TO WORK OK. OUR OBSERVATION WAS THAT IT MIGHT HAVE BEEN A WATER PROBLEM AS BEFORE TKOF SEVERAL DAYS LATER ALL TRIM CTLS WERE FROZEN SOLID WITH WATER AND REQUIRED EXTENSIVE PREHEAT TO MAKE THE PLANE FLYABLE.