B757 RUNAWAY RUDDER TRIM.

Narrative

ON TAXI OUT OF LAX; I TURNED THE RUDDER CTL KNOB MOMENTARILY TO CTR RUDDER TRIM. THE COPLT; A MIN LATER; NOTICED THE RUDDER TRIM KNOB HAD NOT RECTRED AND IT HAD STUCK IN THE 'NOSE R POS.' THE RUDDER TRIM HAD GONE FULL R TRIM; 17 DEGS. WE RE-ZEROED THE TRIM AND BOTH OF US MADE A MENTAL NOTE TO BE CAREFUL ON THE NEXT FLT. ALL WENT WELL UNTIL 1000 FT ALT ON FINAL AT SLC. THE COPLT NEEDED SOME SLIGHT R TRIM; THE KNOB STUCK IN THE FULL R POS AND BEGAN TO DRIVE THE RUDDER. HE FELT THE RUDDER PEDALS MOVE AND QUICKLY LOOKED AT THE TRIM AND TRIM KNOB. IT WAS GOING THROUGH 7 DEGS OF R TRIM. HE RE-CTRED THE KNOB AND THEN ZEROED THE TRIM. HAD WE NOT CAUGHT THE TRIM PROB ON TKOF AT LAX; THE TKOF WOULD HAVE BEE INTERESTING! THE SAME CAN BE SAID FOR THE LNDG IN SLC. THE ACFT WAS A B757. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THIS WAS THE FIRST TIME THAT HE HAD EXPERIENCED A RUNAWAY RUDDER TRIM IN HIS 5 YRS AND APPROX 3500 HRS OF OPERATING THE B757. HE LATER LEARNED THROUGH HIS EMPLOYEE UNION CHAIRMAN THAT THE TRIM CTL KNOB CTRING SPRING HAD BROKEN CAUSING THE TRIM CTL TO REMAIN CONNECTED TO THE ELECTRICAL INPUTS FOR CONTINUING TO DRIVE THE TRIM MOTOR AFTER THE FLC RELEASED THE CTL KNOB INSTEAD OF RE-CTRING THE CTL KNOB AS DESIGNED. THEREFORE; IT HAD AN AFFECT OF CAUSING A 'RUNAWAY' TRIM. THE CTRING SPRING WAS REPLACED. HE FURTHER STATED THAT THIS WAS THE ONLY KNOWN INCIDENT RPTED THROUGH THE MECHANICAL INTERRUPTION DISCREPANCY RPTING SYS TO THIS DATE!