Narrative:

While at FL250 in VMC, smooth air conditions, there occurred a violent pitch up motion of the aircraft causing the passenger and other unsecured objects to be thrown about the cabin. Immediately, fearing an imminent stall, I reduced power to an idle and applied a great amount of forward pressure on the yoke along with the aid of the copilot. I then pulled the circuit breakers for the ap/FD and the 3 associated with the sas system. I then manually trimmed the nose down to release the required forward pressure of the yoke being applied by the copilot. At this point, approximately 20 seconds, after the start of the problem, I looked at the vsi and saw a descent of 2000 FPM and an altimeter reading of less than 19000. We had now regained control of the aircraft and attempted to contact ZJX. However, during the chaos, I had also pulled the circuit breakers for the radios. My concern in stalling probably caused me to be over zealous in my need to keep up my speed (at FL250 the IAS was only 131 KTS) and therefore I probably overcompensated by establishing the 200 FPM. I reset the circuit breakers and called ZJX. Of course, they wanted to know what was up and I told them that we had an unexplainable pitch up motion of the plane but that we had the situation under control and wanted to maintain FL180. They advised that we would need to maintain FL190 for our direction (approximately 350 degrees). That was the extent of our conversation about the situation with ZJX, no other questions were asked by them. I then called our avionics technician (by flight telephone) and discussed the problem with him. He advised that he had seen this problem with the autoplt and that it was not the sas system. He said that so long as we flew without the 'altitude hold' engaged that all would be ok for the rest of the trip. He said that the autoplt computer received a bad signal from some source and caused aircraft to trim itself to its 'full up' trim position. Leaving the 'altitude hold' off would not allow the trim to be artificially 'masked' by the altitude hold so that when the autoplt was disengaged or a servo clutch slipping to allow a pitch up or down. Aircraft is now in service to locate the problem.

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Original NASA ASRS Text

Title: AN SMT PLT HAD AN AUTOPLT PROBLEM THAT CAUSED A PITCH UP. HE LOST 6000 FT RECOVERING. THERE WAS NO DAMAGE OR INJURIES RPTED.

Narrative: WHILE AT FL250 IN VMC, SMOOTH AIR CONDITIONS, THERE OCCURRED A VIOLENT PITCH UP MOTION OF THE ACFT CAUSING THE PAX AND OTHER UNSECURED OBJECTS TO BE THROWN ABOUT THE CABIN. IMMEDIATELY, FEARING AN IMMINENT STALL, I REDUCED PWR TO AN IDLE AND APPLIED A GREAT AMOUNT OF FORWARD PRESSURE ON THE YOKE ALONG WITH THE AID OF THE COPLT. I THEN PULLED THE CIRCUIT BREAKERS FOR THE AP/FD AND THE 3 ASSOCIATED WITH THE SAS SYS. I THEN MANUALLY TRIMMED THE NOSE DOWN TO RELEASE THE REQUIRED FORWARD PRESSURE OF THE YOKE BEING APPLIED BY THE COPLT. AT THIS POINT, APPROX 20 SECONDS, AFTER THE START OF THE PROBLEM, I LOOKED AT THE VSI AND SAW A DSCNT OF 2000 FPM AND AN ALTIMETER READING OF LESS THAN 19000. WE HAD NOW REGAINED CTL OF THE ACFT AND ATTEMPTED TO CONTACT ZJX. HOWEVER, DURING THE CHAOS, I HAD ALSO PULLED THE CIRCUIT BREAKERS FOR THE RADIOS. MY CONCERN IN STALLING PROBABLY CAUSED ME TO BE OVER ZEALOUS IN MY NEED TO KEEP UP MY SPD (AT FL250 THE IAS WAS ONLY 131 KTS) AND THEREFORE I PROBABLY OVERCOMPENSATED BY ESTABLISHING THE 200 FPM. I RESET THE CIRCUIT BREAKERS AND CALLED ZJX. OF COURSE, THEY WANTED TO KNOW WHAT WAS UP AND I TOLD THEM THAT WE HAD AN UNEXPLAINABLE PITCH UP MOTION OF THE PLANE BUT THAT WE HAD THE SITUATION UNDER CTL AND WANTED TO MAINTAIN FL180. THEY ADVISED THAT WE WOULD NEED TO MAINTAIN FL190 FOR OUR DIRECTION (APPROX 350 DEGS). THAT WAS THE EXTENT OF OUR CONVERSATION ABOUT THE SITUATION WITH ZJX, NO OTHER QUESTIONS WERE ASKED BY THEM. I THEN CALLED OUR AVIONICS TECHNICIAN (BY FLT TELEPHONE) AND DISCUSSED THE PROBLEM WITH HIM. HE ADVISED THAT HE HAD SEEN THIS PROBLEM WITH THE AUTOPLT AND THAT IT WAS NOT THE SAS SYS. HE SAID THAT SO LONG AS WE FLEW WITHOUT THE 'ALT HOLD' ENGAGED THAT ALL WOULD BE OK FOR THE REST OF THE TRIP. HE SAID THAT THE AUTOPLT COMPUTER RECEIVED A BAD SIGNAL FROM SOME SOURCE AND CAUSED ACFT TO TRIM ITSELF TO ITS 'FULL UP' TRIM POS. LEAVING THE 'ALT HOLD' OFF WOULD NOT ALLOW THE TRIM TO BE ARTIFICIALLY 'MASKED' BY THE ALT HOLD SO THAT WHEN THE AUTOPLT WAS DISENGAGED OR A SERVO CLUTCH SLIPPING TO ALLOW A PITCH UP OR DOWN. ACFT IS NOW IN SVC TO LOCATE THE PROBLEM.

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.