Narrative:

Over 3 hours into our flight at FL360; en route to ZZZ; we suddenly experienced a significant loss of thrust and yaw associated with a simultaneous clunk sound coming from the l-hand side of the aircraft. Looking at the engine gauges; we observed a high engine vibration in the amber caution zone from the left engine. I immediately instructed the first officer to declare an emergency with ATC and inform them of our position and intentions; as I simultaneously did the same on primary emergency frequency 121.5 and executed appropriate procedures departing the track airway. I also retarded the left engine thrust lever and requested the engine vibration QRH from the first officer. In the process of executing the QRH items; the engine vibration monitor indicated it was no longer in the caution zone and had stabilized at about 1.7 which led me to conclude that eventual shutdown of the engine might not be necessary as all other engine parameters were indicating they were nominal. As we descended below rvsm; ATC issued us an amended clearance direct ZZZ; which we were already at a cruise altitude of FL200. I had already briefed the flight attendant and passenger and we continued to keep them informed as I consulted with maintenance and dispatch. We continued with normal progress; plotting and position reports keeping dispatch informed of our fuel and flight progress. We landed in ZZZ without further incident or need for instruction to brace but had the ground equipment on standby just as a precaution.

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Original NASA ASRS Text

Title: B757-200 SUFFERS ERRATIC OPERATION AND EXCESSIVE VIBRATION FROM LEFT ENGINE DURING TRANSOCEANIC FLT. DESCEND BELOW RVSM AIRSPACE AND DIVERT TO NEAREST ARPT.

Narrative: OVER 3 HRS INTO OUR FLT AT FL360; ENRTE TO ZZZ; WE SUDDENLY EXPERIENCED A SIGNIFICANT LOSS OF THRUST AND YAW ASSOCIATED WITH A SIMULTANEOUS CLUNK SOUND COMING FROM THE L-HAND SIDE OF THE ACFT. LOOKING AT THE ENG GAUGES; WE OBSERVED A HIGH ENG VIBRATION IN THE AMBER CAUTION ZONE FROM THE L ENG. I IMMEDIATELY INSTRUCTED THE FO TO DECLARE AN EMER WITH ATC AND INFORM THEM OF OUR POS AND INTENTIONS; AS I SIMULTANEOUSLY DID THE SAME ON PRIMARY EMER FREQ 121.5 AND EXECUTED APPROPRIATE PROCS DEPARTING THE TRACK AIRWAY. I ALSO RETARDED THE L ENG THRUST LEVER AND REQUESTED THE ENG VIBRATION QRH FROM THE FO. IN THE PROCESS OF EXECUTING THE QRH ITEMS; THE ENG VIBRATION MONITOR INDICATED IT WAS NO LONGER IN THE CAUTION ZONE AND HAD STABILIZED AT ABOUT 1.7 WHICH LED ME TO CONCLUDE THAT EVENTUAL SHUTDOWN OF THE ENG MIGHT NOT BE NECESSARY AS ALL OTHER ENG PARAMETERS WERE INDICATING THEY WERE NOMINAL. AS WE DSNDED BELOW RVSM; ATC ISSUED US AN AMENDED CLRNC DIRECT ZZZ; WHICH WE WERE ALREADY AT A CRUISE ALT OF FL200. I HAD ALREADY BRIEFED THE FLT ATTENDANT AND PAX AND WE CONTINUED TO KEEP THEM INFORMED AS I CONSULTED WITH MAINT AND DISPATCH. WE CONTINUED WITH NORMAL PROGRESS; PLOTTING AND POS RPTS KEEPING DISPATCH INFORMED OF OUR FUEL AND FLT PROGRESS. WE LANDED IN ZZZ WITHOUT FURTHER INCIDENT OR NEED FOR INSTRUCTION TO BRACE BUT HAD THE GND EQUIP ON STANDBY JUST AS A PRECAUTION.

Data retrieved from NASA's ASRS site as of January 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.