Narrative:

There we were, cruising at 16000 ft with our anti and deicing equipment selected 'on.' neither the captain nor myself knew in this specific aircraft, with anti icing selected, the trim 'whooler' would periodically sound. I deselected the autoplt and the whooler continued to sound without any trim inputs from me. We felt this was the early stages of a trim runaway. The captain gave me the checklist as I selected the autoplt on again. He suggested I locate the 'trim runaway' checklist and the trim motor circuit breaker. He took control of the aircraft with the autoplt engaged. As he took the aircraft, he noticed a strong shaking with the control wheel so he pressed the touch control steering (tcs) button which temporarily disconnects the autoplt. As soon as he did this, the aircraft rolled violently to the right to about 45 degrees and just as violently to the left to 45 degrees. The captain's control inputs were full opposite aileron and rudder. As he did this, he disconnected the autoplt. It made a couple of more violent banks each direction as we began a descent. We notified ATC of our problem, requested an immediate descent and the current WX in quincy. As the controller gave us the WX, the banks ended and we regained full control of the aircraft. We notified the flight attendant and told her to situation down. We then began to trouble-shoot the problem. Out first thought was some sort of icing but none was found and all automatic deicing equipment was operating normally. We continued the flight with no other problems. We spoke with many people familiar with the light transport and no definite solution has been found. There had been a service bulletin for the aircraft which was for autoplt servos or something. Callback conversation with reporter revealed the following: a test flight was flown on this aircraft and showed the same problem. Often times on this aircraft, something will happen in the air that cannot be duplicated on the ground or on a test flight, so the reporter was overjoyed when the test flight failed. Maintenance changed the autoplt and apparently the problem has been rectified. The reporter has not heard of this same problem happening to other crews. His pilot's union has sent out a bulletin warning all crews flying this aircraft of the problem. The 'tcs' button is 'touch control steering.' this momentarily disconnects the autoplt while the button is pushed. The autoplt reengages when the button is released.

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Original NASA ASRS Text

Title: AN ACR LTT CREW HAD A SEVERE AUTOPLT PROB WITH WILD BANK ANGLE CHANGES.

Narrative: THERE WE WERE, CRUISING AT 16000 FT WITH OUR ANTI AND DEICING EQUIP SELECTED 'ON.' NEITHER THE CAPT NOR MYSELF KNEW IN THIS SPECIFIC ACFT, WITH ANTI ICING SELECTED, THE TRIM 'WHOOLER' WOULD PERIODICALLY SOUND. I DESELECTED THE AUTOPLT AND THE WHOOLER CONTINUED TO SOUND WITHOUT ANY TRIM INPUTS FROM ME. WE FELT THIS WAS THE EARLY STAGES OF A TRIM RUNAWAY. THE CAPT GAVE ME THE CHKLIST AS I SELECTED THE AUTOPLT ON AGAIN. HE SUGGESTED I LOCATE THE 'TRIM RUNAWAY' CHKLIST AND THE TRIM MOTOR CIRCUIT BREAKER. HE TOOK CTL OF THE ACFT WITH THE AUTOPLT ENGAGED. AS HE TOOK THE ACFT, HE NOTICED A STRONG SHAKING WITH THE CTL WHEEL SO HE PRESSED THE TOUCH CTL STEERING (TCS) BUTTON WHICH TEMPORARILY DISCONNECTS THE AUTOPLT. AS SOON AS HE DID THIS, THE ACFT ROLLED VIOLENTLY TO THE R TO ABOUT 45 DEGS AND JUST AS VIOLENTLY TO THE L TO 45 DEGS. THE CAPT'S CTL INPUTS WERE FULL OPPOSITE AILERON AND RUDDER. AS HE DID THIS, HE DISCONNECTED THE AUTOPLT. IT MADE A COUPLE OF MORE VIOLENT BANKS EACH DIRECTION AS WE BEGAN A DSCNT. WE NOTIFIED ATC OF OUR PROB, REQUESTED AN IMMEDIATE DSCNT AND THE CURRENT WX IN QUINCY. AS THE CTLR GAVE US THE WX, THE BANKS ENDED AND WE REGAINED FULL CTL OF THE ACFT. WE NOTIFIED THE FLT ATTENDANT AND TOLD HER TO SIT DOWN. WE THEN BEGAN TO TROUBLE-SHOOT THE PROB. OUT FIRST THOUGHT WAS SOME SORT OF ICING BUT NONE WAS FOUND AND ALL AUTO DEICING EQUIP WAS OPERATING NORMALLY. WE CONTINUED THE FLT WITH NO OTHER PROBS. WE SPOKE WITH MANY PEOPLE FAMILIAR WITH THE LTT AND NO DEFINITE SOLUTION HAS BEEN FOUND. THERE HAD BEEN A SVC BULLETIN FOR THE ACFT WHICH WAS FOR AUTOPLT SERVOS OR SOMETHING. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: A TEST FLT WAS FLOWN ON THIS ACFT AND SHOWED THE SAME PROB. OFTEN TIMES ON THIS ACFT, SOMETHING WILL HAPPEN IN THE AIR THAT CANNOT BE DUPLICATED ON THE GND OR ON A TEST FLT, SO THE RPTR WAS OVERJOYED WHEN THE TEST FLT FAILED. MAINT CHANGED THE AUTOPLT AND APPARENTLY THE PROB HAS BEEN RECTIFIED. THE RPTR HAS NOT HEARD OF THIS SAME PROB HAPPENING TO OTHER CREWS. HIS PLT'S UNION HAS SENT OUT A BULLETIN WARNING ALL CREWS FLYING THIS ACFT OF THE PROB. THE 'TCS' BUTTON IS 'TOUCH CTL STEERING.' THIS MOMENTARILY DISCONNECTS THE AUTOPLT WHILE THE BUTTON IS PUSHED. THE AUTOPLT REENGAGES WHEN THE BUTTON IS RELEASED.

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.