AN MD80 FLT CREW NOTICED THEIR RUDDER TRIM WAS BINDING. MAINT WORKED ON IT BUT THE FLT CREW FOUND THE PROB HAD RETURNED ON THE NEXT LEG.

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UPON PREFLIGHTING AIRCRAFT WE DISCOVERED THERE WAS A PROBLEM WITH THE RUDDER TRIM. WHEN I FIRST STARTED MOVING THE TRIM KNOB; THERE WAS A 'POP' AND THEN I NOTICED THE KNOB WAS DIFFICULT TO MOVE; ESPECIALLY TO THE L. MAINT WAS SUMMONED AND THEY SPENT OVER TWO HOURS WORKING THE PROBLEM. IT APPEARED TO ME THAT THE MECHANIC ON DUTY DID EVERYTHING IN HIS POWER TO CORRECT THE PROBLEM AND ENSURE THE AIRPLANE WAS AIRWORTHY FOR FLT. I PERSONALLY WATCHED HIM DISASSEMBLE THE HOUSING AROUND THE RUDDER TRIM KNOB; CLEAN THE AREA UNDER THE HOUSING WITH A CAN OF COMPRESSED AIR; AND LASTLY HE REMOVED A STRAY PIECE OF PLASTIC THAT WAS TRAPPED IN THE AILERON TRIM HOUSING; WHICH WE SPECULATE FELL INTO THE AILERON HOUSING FROM THE RUDDER TRIM HOUSING. HE ALSO PLACED A TORQUE METER ON THE TRIM AND VERIFIED IT TO BE WITHIN ACCEPTABLE LIMITS. AFTER EVERYTHING WAS RE-ASSEMBLED WE TOOK THE AIRPLANE. ENROUTE I NOTICED THE TRIM SEEMED VERY STIFF IN OPERATION. AFTER LANDING WE AGAIN CHECKED THE TRIM AND THIS TIME I NOTICED THE TRIM AS EVEN STIFFER THAN IT WAS EARLIER IN THE DAY. ALSO; THE STIFFNESS SEEMED TO GET NOTICEABLY WORSE AFTER PASSING FIVE UNITS OF RUDDER TRIM TO THE L; BUT IT WAS CONSISTENT IN ALL OTHER OPERATIONS UNTIL PASSING FIVE UNITS TO THE L WHERE IT GOT VERY STIFF. THIS WAS ENTERED INTO THE LOGBOOK AND THE PLANE WAS TOWED TO THE HANGAR WHERE A CABLE PROBLEM WAS FINALLY CORRECTED. THIS PROBLEM WAS WRITTEN UP A TOTAL OF THREE TIMES (TWICE BY ME AND ONCE BY ANOTHER CREW) BEFORE IT WAS FINALLY FIXED. IT DISTURBS ME THAT SOMETHING AS SERIOUS AS FLIGHT CONTROLS HAS TO BE WRITTEN UP THREE TIMES BEFORE IT'S SUCCESSFULLY REPAIRED.