B777 crew reported high engine vibrations during cruise on engine that had just been changed but not 'balanced' which would have been customary with an engine change. Crew was advised by Maintenance that the vibrations may be higher but were uncomfortable flying plane over water for extended period with limited guidance in their manuals as to flight limits vs. maintenance limits.
Synopsis
B777 crew reported high engine vibrations during cruise on engine that had just been changed but not 'balanced' which would have been customary with an engine change. Crew was advised by Maintenance that the vibrations may be higher but were uncomfortable flying plane over water for extended period with limited guidance in their manuals as to flight limits vs. maintenance limits.
Narrative
Mechanic briefing logbook advised that left engine had been changed and vibrations would most likely be higher than normal because balancing had not been accomplished as thoroughly as customary. While cruising at FL320 felt vibrations on yoke. Vibrations became stronger so I checked the Engine page; which showed left engine N1 vibrations to be at 3.2 while the right engine read 1.5. Vibrations increased steadily over the next 5 minutes to 3.8-3.9. At this point Maintenance was contacted and made aware of the readings. I explained that we had no published limits that I was aware of. He confirmed this and mentioned that 4.0 was the maintenance limit and as long as we were below that we were OK to continue. Shortly after ending the phone call; the secondary engine indications appeared showing the left engine N1 vibration as a black number four on a white background. After several seconds the indication returned to normal. This was repeated at least 10 times; but what concerning is that the high engine vibration display stayed on longer each time. Maintenance was contacted a second time and when asked as to their suggestion if the flight should continue or land he left it to our discretion. Since the situation was not improving and our flight was scheduled to be over open water for approximately 8 hours I advised dispatch that I did not feel comfortable with going oceanic with this condition. Dispatch asked if we felt if it was safe to return to ZZZ; which we agreed to. Diversion was flown at a lower altitude in order to avoid having to jettison fuel. Normal approach and landing at ZZZ.Cause: Secondary engine page being displayed repeatedly indicating a secondary engine parameter is exceeded. Since flight was scheduled to be over open water for 8-9 hours; I deemed it safer to return to ZZZ than risk a possible diversion to ZZZZ or another airport where maintenance would not be easily accessible.Suggestions: Since we have had other aircraft have issues with high engine vibrations which required a shutdown even though there was no guidance on the EICAS; maybe Boeing/GE should be approached so that some guidance can be developed for these situations.
Second reporter narrative
During the logbook briefing from maintenance; the mechanic discussed the engine change they had just completed and mentioned we were likely to see higher than normal vibrations in the left engine because they hadn't had time to balance the engine the way they normally would. The CA (Captain) questioned him about it and he said it would be fine for the flight. We made the initial departure and climb without incident. After initially leveling at FL300; the CA requested a reanalysis of step climb performance from dispatch. The result included a climb to 320 very shortly after. Immediately after the climb to FL320; the CA noticed a vibration in the airplane and pulled up the ENG (Engine) page to view the vibration level. The left engine was significantly higher than the right but within a normal range 'below 4.0'. As we watched it; the Left N1 vibration level continued to increase from 3.2 to 3.9. As the indicator max threshold is 4.0; the CA called Maintenance on the SAT and asked if they were monitoring the engine. There was some discussion about whether a limit existed and maintenance indicated that anything above 4.0 was outside of tolerance. Shortly after disconnecting from the sat call; the Left N1 vibe level reached 4.0; then the background of the displayed vibration number turned white and the secondary engine indications displayed on the EICAS. For the next several minutes in steady state cruise flight the vibration level would decrease to the low 3s then return to 4.0 and the background would inverse to white. With each climb of the vibrations; it would remain in the inverse 4.0 for a longer period. Considering the state of the engine and the upcoming 8+ hour pacific crossing ahead; we discussed divert options and our level of comfort continuing across the ocean versus diverting to a maintenance facility in the U.S. We agreed diverting was the most prudent course of action and dispatch asked if we were comfortable returning to ZZZ. We were about 2+15 into a 13 hour flight and needed to reduce landing weight to remain within limits. As we descended and slowed; the engine vibrations improved significantly and we landed back in ZZZ without further incident.Cause: Engine vibration outside of the normal range/full scale deflection.Suggestions: Consider enhancing guidance or procedures for high engine vibrations inflight.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.