Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 216.12; Flight Crew Total 14075.20; Flight Crew Type 3026.63
Troubleshooting; Human-Machine Interface; Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Track / Heading All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Diverted; Flight Crew Overcame Equipment Problem; Flight Crew Landed in Emergency Condition
Aircraft
At 39;000 feet enroute cruising IMC; icing conditions; the left engine vibration reached 7.0. The intensity of the vibration was felt in the airframe throughout the cabin; audible in the flight deck; and shaking the glareshield. In accordance with the Flight Manual; we reduced power on the left engine to 60% N1 and the vibration level dropped below 4.0. After various power setting tests; we determined 60% N1 was the maximum power setting to keep vibration level below 4.0. This power setting was inadequate to maintain altitude and airspeed. Since we were still left of course on a weather deviation; ATC was unable to approve our request to descend until we reported back on route. The Captain was notified and returned to the flight deck. At that point; we sent a CPDLC message with a request to descend. We were cleared and descended to 33;000 feet. At this altitude; the aircraft was able to maintain 250 knots in level flight. After some discussion with Maintenance Control and Dispatch; the degraded engine performance condition required landing at the nearest suitable airport. Dispatch coordinated the ground response and ATC issued a divert clearance. The descent; arrival; approach; and landing were conducted as if we were a single engine aircraft; with the exception of starting the APU.
[Report narrative contained no additional information.]
B787 flight crew reported excessive vibration from the left engine resulted in a diversion.