B737-500 RETURNED TO THE DEP FIELD DUE TO UNSCHEDULED NOSE DOWN TRIM FROM 200 FT AGL UP TO 12000 FT MSL. TRIM CORRECTED WITH YOKE TRIM SWITCHES.

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AT APPROX 200 FT AGL ON TKOF FROM RWY 34L WE EXPERIENCED UNSCHEDULED NOSE-DOWN ELEVATOR TRIM. IT WAS 'LOW SPD' AND WAS CANCELABLE BY USING THE ELECTRIC YOKE TRIM SWITCHES. IT WAS ALSO POSSIBLE TO RE-TRIM WITH THE ELECTRIC YOKE SWITCHES. THE RATE OF UNSCHEDULED TRIM SEEMED TO MATCH THAT OF THE NORMAL 'SPD TRIM' SYS; BUT WOULD RUN LONG ENOUGH TO PLACE THE ACFT OUT OF TRIM. THE ACFT REMAINED CTLABLE BY USING THE ELECTRIC TRIM TO COUNTERACT THE INTERMITTENTLY OUT-OF-TRIM CONDITION AS WE ENTERED THE CLOUDS AT ABOUT 1200 FT AGL. WE CONTINUED TO FLY THE DEP CLRNC AND CLBED THROUGH 12000 FT MSL. DURING THIS CLB; THE UNSCHEDULED LOW SPD TRIM OCCURRED 4 OR 5 TIMES. IN EACH OCCURRENCE THE NOSE-DOWN OUT-OF-TRIM CONDITION WAS CORRECTABLE WITH ELECTRIC TRIM. I ASKED FO TO ADVISE APCH CTL THAT WE WOULD BE WORKING A PROB AND WOULD SUBSEQUENTLY BE REQUESTING A RETURN TO DEN. WE ASKED; HE ADVISED ATC THAT OUR PROB WAS RELATED TO 'UNSCHEDULED ELEVATOR TRIM.' DURING ANOTHER UNSCHEDULED; LOW SPD; NOSE-DOWN TRIM EVENT; I REACHED DOWN AND SELECTED BOTH TRIM SWITCHES ON THE CTR CONSOLE TO 'CUT-OUT.' FO REVIEWED THE FLT MANUAL IN AN UNSUCCESSFUL EFFORT TO FIND AN IRREGULAR OPS CHKLIST THAT WOULD ADDRESS THE SITUATION. DURING THIS PROCESS; I RETURNED THE CONSOLE 'CUT-OUT' SWITCHES TO THE NORMAL POS. THE OUT-OF-TRIM CONDITION HAD NEVER BECOME EXTREME; REMAINED INTERMITTENT; AND SEEMED LIKE IT MIGHT HAVE BEEN RELATED TO ONE OF THE YOKE SWITCHES RATHER THAN A 'RUN-AWAY' CONDITION ELSEWHERE IN THE SYS. THIS RESULTED IN FURTHER OUT-OF-TIME EVENTS AS PREVIOUSLY DESCRIBED; SO I RETURNED THE SWITCHES TO CUT-OUT. AT NO TIME WAS THE OUT-OF-TRIM CONDITION SEVERE ENOUGH TO WARRANT CTL COLUMN MOVEMENTS THAT WERE LARGE ENOUGH TO ACTUATE THE UNDER-FLOOR CUT-OUT FEATURE. AT THIS TIME; THE TRIM WHEEL STARTED MOVING WITH THE SWITCHES IN 'CUT-OUT;' AND WE HAD TO EACH ALTERNATIVELY HOLD THE WHEEL TO KEEP IT FROM MOVING. FO WAS ATTEMPTING TO HOLD THE TRIM WHEEL WITH HIS HAND WHILE READING; AND THEN I TOOK MANUAL CTL OF THE TRIM WHEEL WHILE REMAINING THE PF. FO ADVISED THAT THE ONLY CHKLIST AVAILABLE WAS THE QRC LIST; SO WE RAN THAT LIST. AT THIS TIME I BELIEVE WE CYCLED THE 'CUT-OUT' SWITCHES TO ON; THEN OFF IN ORDER TO PROPERLY RUN THE QRC (THIS IS MY RECOLLECTION AND I AM NOT SURE IF BOTH THE 'MAIN' AND 'AUTOPLT' SWITCHES WERE RETURNED TO ON; THEN OFF; BUT BOTH SWITCHES WERE DEFINITELY LEFT IN THE OFF POS UPON COMPLETION OF THE QRC LIST). NO MORE WHEEL MOVEMENT OCCURRED; AND I WAS ABLE TO DEPLOY AND USE THE MANUAL CRANK HANDLE TO KEEP THE ELEVATOR IN TRIM THROUGHOUT THE FLAP AND LNDG GEAR EXTENSION. A NORMAL APCH AND LNDG WAS COMPLETED ON RWY 35R. I DID NOT DECLARE AN EMER. AT MY REQUEST; FO BRIEFED FLT ATTENDANT AND INSTRUCTED THAT NO 'CABIN ADVISORY' OR 'CABIN PREPARATION' WAS NECESSARY. FO MADE A PA ANNOUNCEMENT ADVISING THE PAX THAT A MECHANICAL CONDITION NECESSITATED A RETURN TO THE DENVER ARPT. ATC HANDLING: AFTER ADVISING DENVER APCH THAT WE WOULD BE RETURNING TO THE ARPT; WE WERE GIVEN A CLRNC TO MAINTAIN FL190. DURING THE CONTINUED CLB FROM 12000 FT TO FL190 WE WERE ALTERNATIVELY OCCUPIED WITH CHKLIST CONDUCT; FLT HANDLING; DIAGNOSIS; AND COM TASKS. BOTH FO AND I FAILED TO SELECT STANDARD ALTIMETER REF AT THE TRANSITION LEVEL OF FL180 AND; THEREFORE; I LEVELED THE ACFT AT FL186 RATHER THAN FL190 (LCL ALTIMETER SETTING WAS 30.40 INCHES). AT ONE POINT; ATC ASKED IF WE COULD MAINTAIN FL190; AND RPTED US AT FL186. WE RESET ALTIMETERS AND THEN CONTINUED A SLIGHT CLB TO FL190. THEY WERE AWARE THAT WE WERE WORKING THROUGH A CTL IRREGULARITY AND I DID NOT HAVE THE IMPRESSION THAT A DELAYED CLB TO FL190 RESULTED IN ANY LOSS OF SEPARATION. AT NO TIME WAS THE AUTOPLT SELECTED IN EITHER NORMAL OR 'CTL WHEEL STEERING' MODES. THE TKOF WAS MADE WITH FLAPS 1 DEG AND REDUCED N1. FLT DIRECTORS WERE USED THROUGHOUT THE FLT. I NEVER XFERRED CTL OF THE ACFT TO FO BECAUSE THE UNUSUAL TRIM CONDITIONS MADE THIS INADVISABLE; IN MY OPINION.