Narrative:

During cruise flight at FL200 in an small transport, the autoplt disengaged and the aircraft pitched up abruptly. I was not able to stop the pitch with control action. At 25-30 degrees of pitch and with the control yoke hard forward against the physical stop and pitch still increasing, I rolled left to 70-80 degrees bank angle to get control of the pitch altitude. The nose subsequently started falling through to the horizon and the continuous forward trim I had been applying allowed normal control of the aircraft shortly. The procedure resulted in a climb of over 1000' and a heading change of 140-160 degrees. The analysis is that the autoplt had a malfunction that caused horizontal stabilizer trimming only in the aft direction (nose up) while at the same time maintaining level flight by commanding the elevator to hold the nose down. This occurred apparently over a period of time, so it was not noticeable by trim audio alert. The elevator apparently reached its physical stop and caused autoplt to disengaged resulting in the pitch. I believe that the actual elevator deflection that the autoplt is able to command is greater than that possible by the pilot. Callback conversation with reporter revealed the following: the audio alert was functioning properly, but giving only short bursts that did not get the pilot's attention and he did not check the trim indicator to note that all the trim was being applied in the nose up direction. He was flying in a mild mountain wave that normally calls for both short duration up and down trim when on the autoplt so the trim sound had faded into the background. There is no manual trim on the aircraft, so no physical motion takes place and the pilot must look at the trim indicator to monitor trim action. After the incident, the autoplt was deactivated and maintenance has been trying to locate the problem.

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Original NASA ASRS Text

Title: PLT LOST CTL OF AN SMT AT FL200 WHEN AUTOPLT TRIPPED OFF WITH ACFT TRIM FULL NOSE UP.

Narrative: DURING CRUISE FLT AT FL200 IN AN SMT, THE AUTOPLT DISENGAGED AND THE ACFT PITCHED UP ABRUPTLY. I WAS NOT ABLE TO STOP THE PITCH WITH CTL ACTION. AT 25-30 DEGS OF PITCH AND WITH THE CTL YOKE HARD FORWARD AGAINST THE PHYSICAL STOP AND PITCH STILL INCREASING, I ROLLED LEFT TO 70-80 DEGS BANK ANGLE TO GET CTL OF THE PITCH ALT. THE NOSE SUBSEQUENTLY STARTED FALLING THROUGH TO THE HORIZON AND THE CONTINUOUS FORWARD TRIM I HAD BEEN APPLYING ALLOWED NORMAL CTL OF THE ACFT SHORTLY. THE PROC RESULTED IN A CLB OF OVER 1000' AND A HDG CHANGE OF 140-160 DEGS. THE ANALYSIS IS THAT THE AUTOPLT HAD A MALFUNCTION THAT CAUSED HORIZ STABILIZER TRIMMING ONLY IN THE AFT DIRECTION (NOSE UP) WHILE AT THE SAME TIME MAINTAINING LEVEL FLT BY COMMANDING THE ELEVATOR TO HOLD THE NOSE DOWN. THIS OCCURRED APPARENTLY OVER A PERIOD OF TIME, SO IT WAS NOT NOTICEABLE BY TRIM AUDIO ALERT. THE ELEVATOR APPARENTLY REACHED ITS PHYSICAL STOP AND CAUSED AUTOPLT TO DISENGAGED RESULTING IN THE PITCH. I BELIEVE THAT THE ACTUAL ELEVATOR DEFLECTION THAT THE AUTOPLT IS ABLE TO COMMAND IS GREATER THAN THAT POSSIBLE BY THE PLT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: THE AUDIO ALERT WAS FUNCTIONING PROPERLY, BUT GIVING ONLY SHORT BURSTS THAT DID NOT GET THE PLT'S ATTN AND HE DID NOT CHK THE TRIM INDICATOR TO NOTE THAT ALL THE TRIM WAS BEING APPLIED IN THE NOSE UP DIRECTION. HE WAS FLYING IN A MILD MOUNTAIN WAVE THAT NORMALLY CALLS FOR BOTH SHORT DURATION UP AND DOWN TRIM WHEN ON THE AUTOPLT SO THE TRIM SOUND HAD FADED INTO THE BACKGROUND. THERE IS NO MANUAL TRIM ON THE ACFT, SO NO PHYSICAL MOTION TAKES PLACE AND THE PLT MUST LOOK AT THE TRIM INDICATOR TO MONITOR TRIM ACTION. AFTER THE INCIDENT, THE AUTOPLT WAS DEACTIVATED AND MAINT HAS BEEN TRYING TO LOCATE THE PROB.

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.