B777 flight crew reported an N2 over-speed on the left engine while on initial approach.
Synopsis
B777 flight crew reported an N2 over-speed on the left engine while on initial approach.
Narrative
We were flying the ZZZZZ-STAR into ZZZ. We had been on a; 'FMC-Path;' descent for quite some time with the power at idle or near idle. As aircraft slowed and auto-throttles pushed power up at ZZZZZ1/6;000 feet we got an EICAS warning and the secondary engine display came up.I was the pilot flying and declared; 'I have the aircraft.' Instinctively; I pulled the left engine back because the N2 was red boxed and just over 104%. Now the engine was in limits; however; after a brief discussion we decided to pull the QRC and run the; 'ENG LIM/SURGE/STALL L;R;' procedure.When I started slowly advancing the left throttle the left engine N2 shot up above 100% again. This time I left it at or near idle with the auto-throttle off. Worth noting that no other engine indications were abnormal and everything on the left engine; except N2; matched up with the right engine.We notified ATC we were working an issue and after some quick discussion decided not to declare anything. After some more quick discussion we decided on a Flap-20 landing; changed data in FMC; the relief pilot checked landing data; discussed Flaps-5 go around procedures and continued the approach. We were at a reduced power; 3-degree; decent/glideslope all the way from around 6;000 MSL. ATC got us right in for the visual.Approach and landing were stable; normal and uneventful.Cause: Other notes; comments: I didn't take the radios the way we usually do in the sim; I was busy flying the approach and watching the engine parameters. Captain handled all of that.Somehow; we missed going; 'TA;' on the transponder; not a big deal for this day; but was procedure.Could have easily switched to longer runway; but never really felt the need; had data and we were already set up for XXL.Entire event was expedited but didn't feel rushed; we could have decided to get extended vectors or hold to go thru a ton of checklist but it seemed like the right decision was to just continue the reduced power descent/approach and get the aircraft on the ground in case the engine started getting worse.
Second reporter narrative
On ZZZZZ 6 arrival into ZZZ we got a L Engine Control EICS message followed by a N2 limit exceedance on the left engine. We ran the non annunciated checklist from the QRC. Subsequently ran the left engine at idle throughout the approach and landing. We opted for a flap 20 landing; checked landing data; and made a normal landing. After shutdown at the gate I made an AML (Aircraft Maintenance Logbook) entry; informed maintenance and debrief event with dispatch. Cause: I completed training and had a similar event during training and I was very familiar with the SOP and checklist management for this event. Along with an excellent FO and relief pilot this event was calm and under control throughout the landing.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.