Narrative:

Numerous times during the climb and descent the out of trim light would come on. This means the stabilizer trim had moved from a neutral position with the autoplt on to an out of trim state for no reason. We were flying in a stabilized condition with a constant airspeed during each event. When we disconnected the autoplt the airplane did jump in pitch proving that it was out of trim. In order to prevent the stabilizer trim motor from overheating and becoming inoperative, I decided to declare an emergency, disconnect the autoplt, and have the copilot hand fly the aircraft for the approach and landing. We landed without incident. I decided on this conservative course of action to prevent matters from deteriorating any further. Callback conversation with reporter revealed the following information: the reporter stated the stabilizer was being trimmed when trimming was not required. The reporter said when the autoplt was disconnected the airplane jumped due to being out of trim. The reporter stated the fix for the problem is unknown as no report was received from maintenance.

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Original NASA ASRS Text

Title: A DC9-50 IN CLB AT 12000 FT DECLARED AN EMER AND DIVERTED DUE TO ERRATIC STABILIZER TRIM OP WITH AUTOPLT ENGAGED.

Narrative: NUMEROUS TIMES DURING THE CLB AND DSCNT THE OUT OF TRIM LIGHT WOULD COME ON. THIS MEANS THE STABILIZER TRIM HAD MOVED FROM A NEUTRAL POS WITH THE AUTOPLT ON TO AN OUT OF TRIM STATE FOR NO REASON. WE WERE FLYING IN A STABILIZED CONDITION WITH A CONSTANT AIRSPD DURING EACH EVENT. WHEN WE DISCONNECTED THE AUTOPLT THE AIRPLANE DID JUMP IN PITCH PROVING THAT IT WAS OUT OF TRIM. IN ORDER TO PREVENT THE STABILIZER TRIM MOTOR FROM OVERHEATING AND BECOMING INOP, I DECIDED TO DECLARE AN EMER, DISCONNECT THE AUTOPLT, AND HAVE THE COPLT HAND FLY THE ACFT FOR THE APCH AND LNDG. WE LANDED WITHOUT INCIDENT. I DECIDED ON THIS CONSERVATIVE COURSE OF ACTION TO PREVENT MATTERS FROM DETERIORATING ANY FURTHER. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE STABILIZER WAS BEING TRIMMED WHEN TRIMMING WAS NOT REQUIRED. THE RPTR SAID WHEN THE AUTOPLT WAS DISCONNECTED THE AIRPLANE JUMPED DUE TO BEING OUT OF TRIM. THE RPTR STATED THE FIX FOR THE PROB IS UNKNOWN AS NO RPT WAS RECEIVED FROM MAINT.

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.