Date: 202007
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Daylight
Aircraft: Regional Jet 900 (CRJ900)
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Fan
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Troubleshooting
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Air Traffic Control Provided Assistance; Flight Crew Landed As Precaution; Flight Crew Landed in Emergency Condition; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: While at 34;000 ft. the number two engine N1 vibration gauge indicated that fan vibration had started to exceed the normal operating range target value; entering into the amber arc. The maximum vibration indicated before retarding the right thrust lever was 2 1. We executed the QRH procedures in full. The N1 continued to indicate in the non-normal range after the QRH procedures were complete. With a significantly lower thrust setting required to keep the number 2 engine fan vibration in the normal operating range; we were unable to maintain our airspeed at that altitude. With an ATC clearance; we descended at a normal rate to 25;000 ft.; and then 21;000 ft. At 21;000 ft.; we were able to maintain 280 knots indicated airspeed with the vibration indication on the upper end of the normal operating range (with a small buffer). This still required the use of differential thrust to accomplish. The thrust levers were not matched and the engines were not producing equal thrust to maintain 280 knots indicated airspeed at 21;000 ft. After coordinating with ATC and initiating the descent; we then contacted Dispatch and Maintenance Control to inform them of these developments. It was our opinion that proceeding to [our destination] was not prudent ONLY because significant convective activity was present in its vicinity and was forecast to possibly be a factor at our ETA. This is why we were filed with an alternate airport of ZZZ1. We did not think that it would be the safest course of action to proceed in proximity of that weather during the approach and landing phases with potentially degraded engine performance. I expressed our opinion to the dispatcher and asked for her assessment.She consulted with the [crew] and everyone mutually agreed that a return to ZZZ was the best course of action. We coordinated with ATC for an air return to ZZZ. We had already spoken with the Flight Attendants almost immediately after completing the QRH procedures to apprise them of the situation; to inquire if they heard any unusual noises; and to let them know that a diversion or return was a possibility. They were hearing an unusual deep rumble. We followed up again with the Flight Attendants after the decision was made to return to ZZZ. We also fully explained the nature of the issue. Finally; since we were executing an air return because of a mechanical engine-related issue; I elevated the flight status with the Flight Attendants. We did not think it was necessary to [request priority handling] with ATC; but we did; of course; brief them on the nature of the problem that we were having; and they were very amenable to expeditiously granting our few requests. Once we were given a new clearance for a return to ZZZ; I informed the passengers of the situation with honest but very careful and reassuring wording; apologizing for the major inconvenience; letting them know the time remaining in the flight; that arrangements were already being made to change aircraft in ZZZ and resume the flight to [our destination] as quickly as possible; that I would update them in the gate area when I had additional information; and that customer service agents would be there to meet them after deplaning.The continued use of differential thrust; and maintaining an indicated airspeed of 280 Knots or less; was required for the remainder of the flight to keep the vibration indication in the normal operating range. Once the aircraft was fully configured for landing and descending on the glideslope; we were finally able to move the thrust levers together in unison without the vibration indication exceeding the normal operating range. The landing was normal and we taxied to the gate.I do not know the specific cause of this fan vibration. We were not in icing conditions. Additionally; the procedures in the QRH did not rectify the problem; so that may help eliminate potential causes




Synopsis: CRJ900 Captain reported excessive engine vibration during cruise and a successful air return was conducted.