ACFT EQUIP PROB. JAMMED ELEVATOR TRIM CAUSED BY BROKEN ELEVATOR TRIM CHAIN. ONLY 10-15 PERCENT OF NORMAL TRIM RANGE. FLC FOLLOWED QRH PROCS AND USED STANDBY ELEVATOR TRIM WHICH WAS ALSO FAULTY. RPTR CAPT STATES THAT THE QRH WAS NOT CLR ON APPROPRIATE FLAP SETTINGS. EMER WAS DECLARED AND FLC RETURNED TO DEP ARPT.

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AFTER DEP AS FLT FROM RAP TO DEN; I ENGAGED THE AUTOPLT OF ACFT (DEHAVILLAND DHC-8) ONCE THROUGH APPROX 10000 FT MSL. THE AUTOPLT WAS OPERATING FOR 5-10 MINS WHEN A 'PITCH MISTRIM; TRIM NOSE DOWN' ANNUNCIATION CAUGHT MY ATTN. I TRIED TO UTILIZE THE MANUAL ELEVATOR TRIM BUT IT WAS JAMMED SOMEHOW AND WOULD ALLOW ONLY LIMITED MOVEMENT AS SHOWN ON THE ELEVATOR TRIM INDICATOR. I THEN DISCONNECTED THE AUTOPLT TO SEE IF THIS WOULD FREE THE TRIM (THINKING IT MAY BE CONNECTED TO THE AUTOPLT TRIM SERVO SOMEHOW). THIS DID NOT HELP; SO THE FO AND I CAME TO THE CONCLUSION THAT WE HAD AN ELEVATOR JAM. THE MANUAL TRIM WHEEL WOULD MOVE; BUT THE RANGE WAS VERY LIMITED -- PROBABLY ONLY 10-15 PERCENT OF THE NORMAL RANGE (THIS WAS NOTED BY WATCHING THE TRIM INDICATOR). CTL OF THE ACFT WAS NOT HARD TO MAINTAIN; ALTHOUGH GREATER THAN NORMAL FORCE WAS NECESSARY FOR PITCH INPUTS. ONCE WE MADE SURE ACFT CTL WAS NOT A PROB AND ASCERTAINED THE NATURE OF THE MALFUNCTION; WE REFED THE QRH. ALL ITEMS IN THE QRH WERE FOLLOWED FOR AN 'ELEVATOR TRIM MALFUNCTION' (I'M NOT SURE OF THE EXACT TITLE) EXCEPT FOR USING THE STANDBY ELEVATOR TRIM. WHEN THE CHKLIST STATED TO USE THE STANDBY SYS; I ATTEMPTED TO DO SO A COUPLE OF TIMES; BUT IT SEEMED TO BE INEFFECTIVE. FROM THAT POINT WE ONLY USED ELEVATOR CTL; WITH THE ELEVATOR TRIM STATIONARY. WITH EVERY ITEM IN THE QRH DISCUSSED; FORESEEING NO PROB WITH ACFT CTL FOR LNDG; WE REQUESTED TO RETURN TO RAP WITH ATC AND DECLARED AN EMER (BECAUSE IT WAS A FLT CTL PROB). WE DECIDED THIS BECAUSE WE WERE ONLY ABOUT 50 MI FROM RAP; WHERE THE WX WAS CLR SKIES AND UNRESTR VISIBILITY. VFR CONDITIONS WERE REQUIRED FOR LNDG; ACCORDING TO THE QRH (DEN HAD LOW VISIBILITY IN SNOW; NOT VFR). ONCE CLRNC WAS RECEIVED; WE TURNED DIRECT RAP AND INFORMED DISPATCH OF OUR SIT ON SELCAL. THEY APPROVED US FOR LNDG AT RAP. A QUESTION AROSE BECAUSE THE QRH ONLY STATED A MAX FLAP SETTING OF 15 DEGS. IT DID NOT GIVE A MINIMUM. ALSO; THE QRH ONLY GAVE APCH SPDS FOR FLAPS 5 DEGS AND FLAPS 15 DEGS; NOT FOR FLAPS 0 DEGS. THROUGH DISPATCH; MAINT ADVISED THAT WE LAND WITH FLAPS 15 DEGS. NEXT WE ADVISED THE FLT ATTENDANT OF ALL PERTINENT INFO AND TOLD HER TO PREPARE FOR A NORMAL LNDG. WE SET THE ACFT UP ON A LONG FINAL TO MAKE SURE WE WERE STABILIZED AND ALSO TO VERIFY THAT WE HAD CONTINUOUS CTL OF THE ACFT. THE LNDG WENT WITHOUT A PROB. SOON AFTER LNDG; A MECH INSPECTED THE ACFT AND FOUND THAT A CHAIN (PART OF THE ELEVATOR TRIM CTL SYS) HAD BROKEN. THIS MUST HAVE HAPPENED DURING THE CLB. PRIOR TO ENG START THE ACCEPTANCE CHK HAD BEEN COMPLETED; WHICH INCLUDES OPERATING EACH TRIM SYS THROUGH THEIR RANGE OF MOTION.