A DHC-8 IN CLB FROM 8000 FT TO FL220 NEEDED MAX R RUDDER TRIM. CENTERED TRIM AND NOTED REDUCED R RUDDER MOVEMENT. DIVERTED TO DEP ARPT.

Narrative

DEPARTING DENVER RWY 25 ENRTE TO ZZZ. AS PER ACFT CHKLIST; ALL FLT CTLS ARE CHKED PRIOR TO EVERY FLT AND PASSED PRIOR TO TAKEOFF. FO'S LEG WITH NORMAL DEP IN CALM WINDS. DURING CLB OUT THE FO NOTICED HE WAS NEEDING EXCESSIVE R RUDDER TRIM TO MAINTAIN COORDINATED FLT. THE TRIM WAS AT 10 DEGS TO R (MAXIMUM). I TOOK CTLS AND VERIFIED THAT THE PEDALS WERE TRIMMED AND UN-JAMMED. THE AUTOPLT WAS VERIFIED DISENGAGED. WE HAD COMPLETE CTL OF THE ACFT. WE SET TRIM TO CTR -- NEUTRAL INDICATION AND VERIFIED L RUDDER MOVEMENT WITH INDICATOR ON PANEL -- THEN SLOWLY APPLIED R PEDAL AND NOTED REDUCED MOVEMENT OF THE RUDDER INDICATOR. YAW FORCES WERE OBVIOUSLY APPROPRIATE WITH L AND REDUCED ON R. I DISCUSSED OPTIONS WITH FO AND WE ELECTED TO RETURN TO DEN. TAKING INTO ACCOUNT THE ROCKY MOUNTAIN ENVIRONMENT AND OUR DEST OF ZZZ OUR CONCERN WAS IF WE HAVE AN ENGINE PROB OR FAILURE (ESPECIALLY LEFT) WE COULD NOT BE CERTAIN WE WOULD HAVE FULL CTL OF THE ACFT. RE-TRIMMED FOR COORD FLT. RETURNED TO DEN AND ASSIGNED RWY 16R IN VFR CONDITIONS AND CALM TO LIGHT WINDS FROM S. ACFT HANDLED SAFELY THROUGHOUT LNDG PHASE. AFTER LNDG; TESTED THE RUDDER MOVEMENT ON THE GND AND FOUND PEDAL TRAVEL NORMAL AND RUDDER INDICATING FULL RANGE L AND R. TRIM RANGE L AND R WAS ALSO OPERATING NORMALLY. I NOTIFIED OUR COMPANY MAINT STAFF AND ENTERED DISCREPANCY INTO ACFT LOGBOOKS. MECHS ARE INVESTIGATING AT THIS TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE FLT CTLS CHKED NORMAL PRIOR TO TAKEOFF AND THAT WHILE CLBING OUT FROM 8000 FT TO FL220; THE FO HAD TO USE MAXIMUM RUDDER TRIM TO MAINTAIN CTL. THE RPTR INDICATED WITH THE RUDDER TRIM KNOB CENTERED; THERE WAS NORMAL MOVEMENT ON L RUDDER BUT SUBSTANTIALLY REDUCED MOVEMENT ON R RUDDER. RPTR AND FO CONSIDERED THE MOUNTAINOUS TERRAIN AND POSSIBLE ENGINE LOSS AND REQUIRED CTL FUNCTIONS AND DECIDED TO DIVERT BACK TO THE DEP ARPT. REPORTEDLY; ON THE GND; THE RUDDER TESTED NORMAL AND R RUDDER MOVEMENT WAS NOT RESTRICTED. MAINT REPORTEDLY REPLACED THE RUDDER PRESSURE REDUCER VALVE AND THE AIRPLANE WAS TEST FLOWN AND FAILED THE FLT TEST BECAUSE OF REDUCED R RUDDER MOVEMENT. THE RPTR STATED THAT MAINT THEN REPLACED BOTH RUDDER POWER UNITS AND THIS CORRECTED THE LIMITED RUDDER MOVEMENT PROB.