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.