Date: 201808
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: IMC
Weather elements / visibility: Turbulence; Icing
Light: Daylight
Aircraft: McDonnell Douglas Undifferentiated or Other Model
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Flight phase: Cruise
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: Turbine Engine
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: First Officer; Pilot Flying
Reporter qualification: Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting; Communication Breakdown; Situational Awareness
Anomaly: Aircraft Equipment Problem Less Severe; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; Flight Crew Requested ATC Assistance / Clarification
Contributing factors: Aircraft; Weather
Primary problem: Aircraft

Narrative: While at cruise altitude of FL310 with ENG anti-ice and wing anti-ice one while in IMC while deviating north of our file route due to severe thunderstorms; we received a RIGHT ENG VIB HIGH EOAP (Electronic Overhead Annunciator Panel) message. Conditions were IMC with light rime ice accumulation in light turbulence. I was the pilot flying; and I specifically remember turning the ENG anti-ice on during climb out once out TAT (Turbine Air Temperature) was below 10 Celsius as per procedure. The ENG anti-ice was never turned off until we descended back into ZZZ when the TAT indicated above 10 Celsius as we were in IMC for the duration of the flight until on final approach into ZZZ. Upon receiving the RIGHT ENG VIB HIGH EOAP message I immediately noticed the VIB (vibrations) on the right engine indicated 4.2 and rising slowly. We received the message once briefly at around 4.0 VIB; and then it went away momentarily. Then as the VIB increased the message came back and remained on until we subsequently retarded the right throttle to idle as per the QRH. The ENG HIGH VIB QRH was ran by the CA (Captain) which instructed us to retard the throttle on the affected engine until the VIB showed below 4.0 or idle; whichever occurs first. As I retarded the right throttle slowly I noticed the VIB decreased down to as low as 4.0; but as I retarded the throttle more the VIB began to increase reaching a maximum VIB of 5.6. Once the throttle was retarded to idle; the VIB finally decreased to below 4.0 before stabilizing to a normal VIB (less than 1.0 from what I can recall). The right engine remained at idle for the remainder of the flight. Once we retarded the throttle as per the ENG VIB HIGH QRH; the captain immediately [notified ATC] as we would require a lower altitude and priority handling. We were initially cleared to descend to FL190. For single engine drift down performance we were further required to descend to 17000 feet; which we subsequently did after receipt of clearance from ATC. The CA as per QRH guidance moved to the One Engine Inoperative/Reduced Thrust Landing QRH procedure. I maintained the easterly track that we were previously on while we accessed were the closest nearest suitable airport was. The CA had difficulties establishing clear communication to dispatch due to static on the radio from the IMC conditions we were flying through and due to our low altitude. The CA and I discussed our options for closest suitable airport. [One option] was our closest airport; but due to severe thunderstorms we ruled that out. In addition the line of weather south of our route ruled out [several others]. [Another] was briefly discussed; but we found returning to ZZZ was our most suitable airport. Dispatch concurred with our decision; and I informed ATC we would be returning to ZZZ and we made a left turn back to ZZZ after being cleared direct to ZZZ. We proceeded back to ZZZ without any further abnormal occurrences. The CA made as successful single engine approach to landing in ZZZ and then we taxied to gate under our own power. We had the AARF (Airport Rescue and Fire Fighting) ready and standing by upon our landing in case we needed them; but no additional assistance was required. Throughout the event our CRM was very effective.




Synopsis: Air carrier pilot reported an engine vibration exceedance resulted in a return to the departure airport.