Narrative:

While level FL290, autoplt off in turbulence on top of clouds. Aircraft started to descend. Tried to trim back with yoke trim but no response. Tried to stop with yoke but could not asked for and received 26000 ft. Used alternate trim to regain control of elevator and altitude. Descended approximately 600 ft before elevator would respond. Asked for 24000 ft which center gave us then asked reason for which we responded we had a trim problem. Airspeed increased to .80 mach to .81 mach before I could get aircraft to respond to yoke. Had mechanic check out system after landing. Found only that both trim yoke switches were very sensitive. Complete check of system by crew and mechanic found nothing wrong. Later both captain and first officer yoke switches replaced. Was able to get control of yoke and elevator by using forward yoke, alternate trim motor to bring trim aft. Callback conversation with reporter revealed the following information: reporter states he was flying at 29000 ft with autoplt disengaged. Aircraft started a slight dive and he countered with the yoke being pulled aft. As airspeed increased the yoke was pulled all the way aft. He attempted to use the trim switches on the control wheel but there was no response, so he released one hand from the yoke and reached for the alternate trim switches. As alternate trim was used the elevator became more efficient. His total loss of altitude was 600 ft, but continued descent as he was close to his planned descent point. Upon landing, aircraft was written up for this condition. Mechanic found the trim switches in the control wheel to be very loose. These were changed and there has been no other incident like this in that aircraft during the past 4 weeks. An aci was with the flight crew when this happened and seemed to approve of the way the problem was handled. He also stated he had never encountered a similar situation. Aircraft was dc-9-51.

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Original NASA ASRS Text

Title: ACFT LOSS OF CTL AT HIGH ALT.

Narrative: WHILE LEVEL FL290, AUTOPLT OFF IN TURB ON TOP OF CLOUDS. ACFT STARTED TO DSND. TRIED TO TRIM BACK WITH YOKE TRIM BUT NO RESPONSE. TRIED TO STOP WITH YOKE BUT COULD NOT ASKED FOR AND RECEIVED 26000 FT. USED ALTERNATE TRIM TO REGAIN CTL OF ELEVATOR AND ALT. DSNDED APPROX 600 FT BEFORE ELEVATOR WOULD RESPOND. ASKED FOR 24000 FT WHICH CTR GAVE US THEN ASKED REASON FOR WHICH WE RESPONDED WE HAD A TRIM PROB. AIRSPD INCREASED TO .80 MACH TO .81 MACH BEFORE I COULD GET ACFT TO RESPOND TO YOKE. HAD MECH CHK OUT SYS AFTER LNDG. FOUND ONLY THAT BOTH TRIM YOKE SWITCHES WERE VERY SENSITIVE. COMPLETE CHK OF SYS BY CREW AND MECH FOUND NOTHING WRONG. LATER BOTH CAPT AND FO YOKE SWITCHES REPLACED. WAS ABLE TO GET CTL OF YOKE AND ELEVATOR BY USING FORWARD YOKE, ALTERNATE TRIM MOTOR TO BRING TRIM AFT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATES HE WAS FLYING AT 29000 FT WITH AUTOPLT DISENGAGED. ACFT STARTED A SLIGHT DIVE AND HE COUNTERED WITH THE YOKE BEING PULLED AFT. AS AIRSPD INCREASED THE YOKE WAS PULLED ALL THE WAY AFT. HE ATTEMPTED TO USE THE TRIM SWITCHES ON THE CTL WHEEL BUT THERE WAS NO RESPONSE, SO HE RELEASED ONE HAND FROM THE YOKE AND REACHED FOR THE ALTERNATE TRIM SWITCHES. AS ALTERNATE TRIM WAS USED THE ELEVATOR BECAME MORE EFFICIENT. HIS TOTAL LOSS OF ALT WAS 600 FT, BUT CONTINUED DSCNT AS HE WAS CLOSE TO HIS PLANNED DSCNT POINT. UPON LNDG, ACFT WAS WRITTEN UP FOR THIS CONDITION. MECH FOUND THE TRIM SWITCHES IN THE CTL WHEEL TO BE VERY LOOSE. THESE WERE CHANGED AND THERE HAS BEEN NO OTHER INCIDENT LIKE THIS IN THAT ACFT DURING THE PAST 4 WKS. AN ACI WAS WITH THE FLC WHEN THIS HAPPENED AND SEEMED TO APPROVE OF THE WAY THE PROB WAS HANDLED. HE ALSO STATED HE HAD NEVER ENCOUNTERED A SIMILAR SIT. ACFT WAS DC-9-51.

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.