B757 FLT CREW ATTEMPTS TO TAXI WITH NO NOSEWHEEL CTL. ACFT IS STOPPED AND STEERING BYPASS PIN IS REMOVED.

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THE FLT WAS LATE DUE TO IRREGULAR OPS. WHILE PREPARING FOR PUSHBACK; THE GND TUG PERSON WAS HAVING DIFFICULTY ESTABLISHING COMS DUE TO AN APPARENTLY FAULTY HEADSET CORD. HE COULD TALK WHILE DIRECTLY CONNECTED TO THE ACFT BUT NOT WHILE CONNECTED TO THE TUG. ALTERNATE COM PROCS WERE ESTABLISHED AND I INDICATED THAT I WOULD FLASH THE NOSE LNDG LIGHT IF I NEEDED THE PUSHBACK TO STOP. AT THAT TIME; IT STARTED TO RAIN. INITIAL PUSHBACK WAS UNEVENTFUL AND A 'SALUTE AND RELEASE FROM GUIDANCE' WAS RECEIVED. I ELECTED TO USE ONLY THE L ENG TO TAXI FROM THE PUSHED BACK POS. AS I APPLIED PWR TO BEGIN TAXIING; THE ACFT PIVOTED TO THE R BUT FELT AS IF THE TILLER WAS BINDING SOMEWHAT. I INITIALLY THOUGHT THAT THERE MAY HAVE BEEN SOMETHING IMPEDING THE TILLER AREA. SINCE THE ACFT WAS IN MOTION; I MAINTAINED MY ATTN OUTSIDE OF THE ACFT BUT WAS AWARE THAT SOMETHING SEEMED AMISS. WHEN I TRIED TO STRAIGHTEN ONTO THE TAXI LINE; I REALIZED THAT THE NOSEWHEEL WAS NOT RESPONDING TO TILLER INPUTS. APPARENTLY THE NOSEWHEEL HAD CASTERED DUE TO THE L ENG TAXI THRUST. I IMMEDIATELY BROUGHT THE ACFT TO A STOP TO TROUBLESHOOT FURTHER. REALIZING THAT I DID NOT HAVE STEERING I HAD THE FO NOTIFY THE RAMP TWR THAT I DID NOT HAVE CTL OF THE NOSEWHEEL AND WOULD NEED MAINT. AS IT TURNS OUT; THE PUSHBACK CREW DID NOT REMOVE THE STEERING BYPASS PIN. MAINT PERSONNEL REMOVED THE PIN AND I VERIFIED THAT I HAD TILLER AUTH. IT WAS DETERMINED THAT NO FURTHER MAINT ACTION WAS REQUIRED. FORTUNATELY; THERE WAS NO DAMAGE TO THE ACFT; GND EQUIP OR OTHER STRUCTURES. I BELIEVE THAT MY AWARENESS THAT THE TILLER DID NOT FEEL CORRECT IN THE INITIAL TURN AND PWR APPLICATION PERMITTED ME TO SWIVEL YET STOP WITHOUT ABRUPT BRAKING THAT MAY HAVE DAMAGED THE CASTERING NOSE GEAR OR INJURED FLT ATTENDANTS OR PAX ABOARD.