Narrative:

On departure at about AM00 the aft cargo door light illuminated. Departure control was advised that we had a problem and that we would level off and work on the problem and probably return to land. The checklist was accomplished, fuel dumped to landing weight and we were given vectors to runway 9 at mem. While being vectored to the approach course, with the autoplt engaged and altitude hold selected, the 'stabilizer out of trim light' illuminated, and the aircraft began to descend unnoticed. The autoplt had functioned normally up to this point (approximately 10 mins). On the previous flight a different crew had entered a maintenance write-up of the autoplt having pitch problems. The pitch controller had been swapped prior to our departure. The descent was unnoticed due to the crew being distracted by the stabilizer out of trim light, discussing the problem, communications with ATC and preparation for the INS approach. When the descent was observed the aircraft was 500' off the assigned altitude. The autoplt was disconnected, altitude was regained, and a hand-flown approach was accomplished. There were no comments from the approach controller about our altitude deviation. At the gate the autoplt cruise trim circuit breaker was reset and the system appeared to operate normally. The pitch computer was replaced one more time. On the subsequent leg to den, the autoplt operated normally until maneuvering while on vector for the approach. When the altitude hold function was selected the aircraft abruptly pitched up, autoplt was disconnected and then reconnected in each mode ('a' and 'B') with the same result. The problem was noted in the aircraft maintenance log.

Google
 

Original NASA ASRS Text

Title: ACR FLT CREW REPORTS OF ALT DEVIATION CREATED BY AUTOPLT EQUIPMENT PROBLEMS AND OTHER FACTORS.

Narrative: ON DEP AT ABOUT AM00 THE AFT CARGO DOOR LIGHT ILLUMINATED. DEP CTL WAS ADVISED THAT WE HAD A PROB AND THAT WE WOULD LEVEL OFF AND WORK ON THE PROB AND PROBABLY RETURN TO LAND. THE CHKLIST WAS ACCOMPLISHED, FUEL DUMPED TO LNDG WT AND WE WERE GIVEN VECTORS TO RWY 9 AT MEM. WHILE BEING VECTORED TO THE APCH COURSE, WITH THE AUTOPLT ENGAGED AND ALT HOLD SELECTED, THE 'STAB OUT OF TRIM LIGHT' ILLUMINATED, AND THE ACFT BEGAN TO DSND UNNOTICED. THE AUTOPLT HAD FUNCTIONED NORMALLY UP TO THIS POINT (APPROX 10 MINS). ON THE PREVIOUS FLT A DIFFERENT CREW HAD ENTERED A MAINT WRITE-UP OF THE AUTOPLT HAVING PITCH PROBS. THE PITCH CTLR HAD BEEN SWAPPED PRIOR TO OUR DEP. THE DSCNT WAS UNNOTICED DUE TO THE CREW BEING DISTRACTED BY THE STABILIZER OUT OF TRIM LIGHT, DISCUSSING THE PROB, COMS WITH ATC AND PREPARATION FOR THE INS APCH. WHEN THE DSCNT WAS OBSERVED THE ACFT WAS 500' OFF THE ASSIGNED ALT. THE AUTOPLT WAS DISCONNECTED, ALT WAS REGAINED, AND A HAND-FLOWN APCH WAS ACCOMPLISHED. THERE WERE NO COMMENTS FROM THE APCH CTLR ABOUT OUR ALT DEVIATION. AT THE GATE THE AUTOPLT CRUISE TRIM CB WAS RESET AND THE SYS APPEARED TO OPERATE NORMALLY. THE PITCH COMPUTER WAS REPLACED ONE MORE TIME. ON THE SUBSEQUENT LEG TO DEN, THE AUTOPLT OPERATED NORMALLY UNTIL MANEUVERING WHILE ON VECTOR FOR THE APCH. WHEN THE ALT HOLD FUNCTION WAS SELECTED THE ACFT ABRUPTLY PITCHED UP, AUTOPLT WAS DISCONNECTED AND THEN RECONNECTED IN EACH MODE ('A' AND 'B') WITH THE SAME RESULT. THE PROB WAS NOTED IN THE ACFT MAINT LOG.

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.