Narrative:

This flight was a ferry flight from dallas to waco, tx, for maintenance. The day prior to the flight, I had the autoplt servos removed for overhaul: a) elevator servo, B) aileron servo, C) rudder servo. Level flight at 2000 ft, I engaged the autoplt (forgot about the servos being removed). But the elevator electric trim started trimming the aircraft nose-down, but the autoplt did not engage. No other electric trim motors on this aircraft, a cessna 421 C. Being, also an aircraft and pwrplant mechanic, I punched the autoplt engage switch 'on' again and held the switch down, and here is when it occurred to me why the autoplt would not engage, the servos were removed, and at this point I set up a climb attitude, from 2000-4000 ft. During this climb attitude, I pushed the autoplt switch to the 'engage' position and held the switch down, the elevator trim motor was trimming the nose down and I tried to override the electric trim motor and could not override the trim motor. That concerned me. Not a good feeling. Upon releasing the autoplt 'engage' switch, the electric trim motor stopped trimming nose-down. I have previously, on numerous occasions, have over-ridden an autoplt, on several different aircraft. I know the difference between an autoplt and electric trim override. The question in my mind is, why did anything happen? Especially why did the electric trim motor get engaged! That is a no no. That should not be allowed to happen. The reason for this report: several mu 2's, and now the ATR 72, have bought the farm in a nose-down attitude. Ice is being blamed. It may not be ice. It could be an electronic error not yet known to the aviators. I cannot believe that neither pilot on any aircraft did not try to override the system. I think the pilots could not override the system, being the autoplt or electric trim. As pilots, we are trained, in a run-a-way trim situation, pull the circuit breaker, but!!! Hopefully NASA will investigate the autoplt/electric trim relationship. I have been flying for more than 25 yrs and numerous different makes and models of aircraft and many times these 'electronic gremlins' are where they are not supposed to be and doing things they are not supposed to be doing. Hope this may shed some light on 'gear-down' lndgs rather than 'nose-down' lndgs. Callback conversation with reporter revealed the following information: reporter states he believes as a mechanic and a&P that electronic gremlins do occur as in his case with the C421. He has no interest in any investigation of that incident, but feels incidents where aircraft go straight down could be involved in some such situation. He has been actively involved in simulating an MU2 accident to determine the cause. Reporter feels strongly that some follow up should be done to see if electric trim problems could be involved in the ATR and MU2 crashes. When reporter discussed with avionics people they wanted $100 an hour to investigate. He spoke with an FAA rep who has been a friend for 15 yrs and was advised to contact oklahoma city. Feeling he would get nowhere he did not. With the recent accidents he was again reminded of his experience and feels he would like to help in any investigation that may occur. His incident did indeed happen in 1991.

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Original NASA ASRS Text

Title: CORP SMT HAS RUNAWAY TRIM AT CRUISE.

Narrative: THIS FLT WAS A FERRY FLT FROM DALLAS TO WACO, TX, FOR MAINT. THE DAY PRIOR TO THE FLT, I HAD THE AUTOPLT SERVOS REMOVED FOR OVERHAUL: A) ELEVATOR SERVO, B) AILERON SERVO, C) RUDDER SERVO. LEVEL FLT AT 2000 FT, I ENGAGED THE AUTOPLT (FORGOT ABOUT THE SERVOS BEING REMOVED). BUT THE ELEVATOR ELECTRIC TRIM STARTED TRIMMING THE ACFT NOSE-DOWN, BUT THE AUTOPLT DID NOT ENGAGE. NO OTHER ELECTRIC TRIM MOTORS ON THIS ACFT, A CESSNA 421 C. BEING, ALSO AN ACFT AND PWRPLANT MECH, I PUNCHED THE AUTOPLT ENGAGE SWITCH 'ON' AGAIN AND HELD THE SWITCH DOWN, AND HERE IS WHEN IT OCCURRED TO ME WHY THE AUTOPLT WOULD NOT ENGAGE, THE SERVOS WERE REMOVED, AND AT THIS POINT I SET UP A CLB ATTITUDE, FROM 2000-4000 FT. DURING THIS CLB ATTITUDE, I PUSHED THE AUTOPLT SWITCH TO THE 'ENGAGE' POS AND HELD THE SWITCH DOWN, THE ELEVATOR TRIM MOTOR WAS TRIMMING THE NOSE DOWN AND I TRIED TO OVERRIDE THE ELECTRIC TRIM MOTOR AND COULD NOT OVERRIDE THE TRIM MOTOR. THAT CONCERNED ME. NOT A GOOD FEELING. UPON RELEASING THE AUTOPLT 'ENGAGE' SWITCH, THE ELECTRIC TRIM MOTOR STOPPED TRIMMING NOSE-DOWN. I HAVE PREVIOUSLY, ON NUMEROUS OCCASIONS, HAVE OVER-RIDDEN AN AUTOPLT, ON SEVERAL DIFFERENT ACFT. I KNOW THE DIFFERENCE BTWN AN AUTOPLT AND ELECTRIC TRIM OVERRIDE. THE QUESTION IN MY MIND IS, WHY DID ANYTHING HAPPEN? ESPECIALLY WHY DID THE ELECTRIC TRIM MOTOR GET ENGAGED! THAT IS A NO NO. THAT SHOULD NOT BE ALLOWED TO HAPPEN. THE REASON FOR THIS RPT: SEVERAL MU 2'S, AND NOW THE ATR 72, HAVE BOUGHT THE FARM IN A NOSE-DOWN ATTITUDE. ICE IS BEING BLAMED. IT MAY NOT BE ICE. IT COULD BE AN ELECTRONIC ERROR NOT YET KNOWN TO THE AVIATORS. I CANNOT BELIEVE THAT NEITHER PLT ON ANY ACFT DID NOT TRY TO OVERRIDE THE SYS. I THINK THE PLTS COULD NOT OVERRIDE THE SYS, BEING THE AUTOPLT OR ELECTRIC TRIM. AS PLTS, WE ARE TRAINED, IN A RUN-A-WAY TRIM SIT, PULL THE CIRCUIT BREAKER, BUT!!! HOPEFULLY NASA WILL INVESTIGATE THE AUTOPLT/ELECTRIC TRIM RELATIONSHIP. I HAVE BEEN FLYING FOR MORE THAN 25 YRS AND NUMEROUS DIFFERENT MAKES AND MODELS OF ACFT AND MANY TIMES THESE 'ELECTRONIC GREMLINS' ARE WHERE THEY ARE NOT SUPPOSED TO BE AND DOING THINGS THEY ARE NOT SUPPOSED TO BE DOING. HOPE THIS MAY SHED SOME LIGHT ON 'GEAR-DOWN' LNDGS RATHER THAN 'NOSE-DOWN' LNDGS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE BELIEVES AS A MECH AND A&P THAT ELECTRONIC GREMLINS DO OCCUR AS IN HIS CASE WITH THE C421. HE HAS NO INTEREST IN ANY INVESTIGATION OF THAT INCIDENT, BUT FEELS INCIDENTS WHERE ACFT GO STRAIGHT DOWN COULD BE INVOLVED IN SOME SUCH SIT. HE HAS BEEN ACTIVELY INVOLVED IN SIMULATING AN MU2 ACCIDENT TO DETERMINE THE CAUSE. RPTR FEELS STRONGLY THAT SOME FOLLOW UP SHOULD BE DONE TO SEE IF ELECTRIC TRIM PROBS COULD BE INVOLVED IN THE ATR AND MU2 CRASHES. WHEN RPTR DISCUSSED WITH AVIONICS PEOPLE THEY WANTED $100 AN HR TO INVESTIGATE. HE SPOKE WITH AN FAA REP WHO HAS BEEN A FRIEND FOR 15 YRS AND WAS ADVISED TO CONTACT OKLAHOMA CITY. FEELING HE WOULD GET NOWHERE HE DID NOT. WITH THE RECENT ACCIDENTS HE WAS AGAIN REMINDED OF HIS EXPERIENCE AND FEELS HE WOULD LIKE TO HELP IN ANY INVESTIGATION THAT MAY OCCUR. HIS INCIDENT DID INDEED HAPPEN IN 1991.

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.