B757 flight crew reported returning to departure airport after experiencing severe vibration in the left engine. Engine was operated at reduced thrust for the return to base operation.
Synopsis
B757 flight crew reported returning to departure airport after experiencing severe vibration in the left engine. Engine was operated at reduced thrust for the return to base operation.
Narrative
Prior to entering track at FL380; mach .80 ZZZZZ entry point; a bang and a sharp yaw to the right occurred with the left vibration indicator changing to amber with 37 as the reading. We immediately contacted ZZZZ with a [priority request] as to not enter the track. Requested XXR and once away from track got approval to descend to FL250 while the Captain coordinated Dispatch; and Maintenance Control. Keeping flight attendants in loop and passengers. We were also running the engine vibration checklist. The checklist advised to reduce the throttle out of amber range (18-22) turn auto throttle off; and then to select TA. This made us non compliant with RVSM for the tracks. [Requested priority handling]; figured out non normal landing configuration distance; landing flaps 20 and with all in agreement to return to ZZZ with ZZZ1; ZZZ2 as alternates if engine shuts down. Because this flight was an ETOPS verification flight due to a new left engine installed; we kept Maintenance Control in the loop. When descending into icing conditions the left anti- ice value failed; ran that check list which told us to kept the value push button in for the remainder of the flight. Rudder trim was used often with the large difference in power; in addition to crossing feeding of fuel two to three times during flight. Landing ZZZ YYL and taxied to the gate with fire/rescue following behind.
Second reporter narrative
At ZZZZZ; we were at FL 380- we heard a bang then a quick yaw and saw the left N1 vibration was in the amber band at 3.7. Ran the checklist. We reduced the left throttle until it was out of the amber band. Huge split between the throttles. The left engine was operating at reduced thrust. Jointly made the decision with Dispatch and Maintenance Control to return to ZZZ with the understanding that if we had to shut down the L engine; we would divert to a closer airport.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.