B-757 UNWARRANTED ENG OVERSPD DURING SHORT FINAL. GAR EXECUTED; ENG SHUTDOWN AND AN EMER WAS DECLARED.

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FLT XX; A B-757; DEPARTED EWR AT XA01Z. THE APCH TO LAX WAS FLOWN BY FO. AUTOPLT AND AUTOTHROTTLE OFF. APCH WAS UNEVENTFUL UNTIL APPROX 1400 FT MSL; WHEN THE R ENG WENT INTO AN UNCOMMANDED FULL OVERSPD CONDITION WITHOUT CORRESPONDING THROTTLE MOVEMENT. A GAR WAS ACCOMPLISHED PER SOP AND THE ENG WAS SHUT DOWN PER SOP WHEN IT WAS FOUND TO BE UNCTLABLE. THE AFTER TKOF; THE ENG SHUTDOWN; SINGLE ENG APCH; APCH DSCNT AND FINAL DSCNT CHKLISTS WERE ACCOMPLISHED PER SOP. THE CABIN WAS PREPPED FOR EVAC AND THE COMPANY WAS NOTIFIED. EMER EQUIP WAS REQUESTED AFTER AN EMER WAS DECLARED. AN UNEVENTFUL SINGLE ENG LNDG WAS ACCOMPLISHED AND WE WERE TOWED INTO THE GATE AFTER CLRING THE RWYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ACFT BECAME UNCTLABLE DURING SHORT FINAL AT APPROX 1100 FT AGL FOR WHAT WAS NOT RECOGNIZED AS A 'RUNAWAY' ENG. SINCE THE ENG WAS SUDDENLY PRODUCING MORE THAN MAX ALLOWABLE THRUST; THE OTHER ENG HAD TO BE THROTTLED UP IN ORDER TO CTL THE ACFT YAW. HOWEVER; BECAUSE OF THE RELATIVE LIGHT ACFT AND HIGH PWR THRUST; THE RATE OF CLB WAS OVER 6000 FPM. THEREFORE; WHEN IT WAS FINALLY DETERMINED THAT THE R ENG WAS OVERSPDING; THE RPTR SHUT OFF THE FUEL TO THE ENG; WHICH FINALLY STOPPED THE ENG (TOOK SEVERAL SECONDS TO USE UP THE FUEL IN THE FEED LINES). THE RPTR WENT ON TO STATE THAT; HAD THE FO BEEN FLYING A 'COUPLED' APCH ON AUTOPLT; HE BELIEVES THAT A SERIOUS ACCIDENT WOULD HAVE HAPPENED SINCE THE AUTOPLT WOULD HAVE UNEXPECTEDLY DISENGAGED; CAUSING THE ACFT TO PITCH UP AND YAW AT A VERY LOW ALT RESULTING IN AN UNCOORDINATED LOOP BACK INTO THE GND BEFORE THE FLC COULD REACT AND REGAIN CTL. HE STATED THAT; WITH THE LOW 400 FT CEILING; IT IS USUAL AND STANDARD PROC TO MAKE A COUPLED APCH. HOWEVER; THEY WERE NOT SINCE THE FO ELECTED TO HAND FLY THE ACFT! THE ENG WAS A PRAT AND WHITNEY MODEL PW2037 (B-757-200). HE FURTHER STATED THAT THE ELECTRONIC ENG CTL AND THE FUEL CTL UNIT HAVE BEEN TESTED INDIVIDUALLY AND FOUND OK. HOWEVER; WHEN THEY WERE HOOKED TOGETHER AS USED ON THE ENG; THEY DID PRODUCE THE SAME ACTION. AT FLT IDLE THE ENG WENT INTO THE OVERSPD CONTROLLABLE CONDITION. HOWEVER; THE MANUFACTURER HAS NOT DETERMINED WHY! THEREFORE; THE CAUSE IS STILL NOT KNOWN AND THE INVESTIGATION IS CONTINUING. THE NTSB IS ACTIVELY INVESTIGATING THIS INCIDENT.