B787 flight crew reported excessive vibration from the left engine resulted in a diversion.

Attributes
ACN	1599416
Time
Date	201811
Local Time Of Day	0001-0600
Place
Locale Reference	NFFF.ARTCC
State Reference	FO
Environment
Flight Conditions	IMC
Aircraft 1
Make Model Name	B787-900
Operating Under FAR Part	Part 121
Flight Phase	Cruise
Route In Use	Direct
Flight Plan	IFR
Component
Aircraft Component	Engine Vibration Indication
Person 1
Function	First Officer Pilot Flying
Qualification	Flight Crew Instrument Flight Crew Multiengine Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 216.12 Flight Crew Total 14075.20 Flight Crew Type 3026.63
Person 2
Function	Pilot Flying Captain
Qualification	Flight Crew Air Transport Pilot (ATP) Flight Crew Multiengine Flight Crew Instrument
Experience	Flight Crew Last 90 Days 30 Flight Crew Total 15216.73 Flight Crew Type 1463.75
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Track / Heading All Types Inflight Event / Encounter Weather / Turbulence

Narrative:

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.

Original NASA ASRS Text

Title: B787 flight crew reported excessive vibration from the left engine resulted in a diversion.

Narrative: 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.

Data retrieved from NASA's ASRS site 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.