What happened
On February 21, 2021, a Japan Airlines flight, operating as JL48, departed Helsinki-Vantaa Airport for Tokyo Haneda. Approximately 1 hour and 20 minutes into the flight, the captain decided to divert the Boeing 787-9 Dreamliner back to Helsinki due to vibrations detected in the right engine. The decision to return was made to ensure technical assistance could be obtained, as the upcoming flight path through Russian airspace would have made accessing maintenance facilities at alternate airports difficult.
Following standard operating procedures, the crew did not shut down the affected engine but significantly reduced both altitude and airspeed. During the approach to Helsinki, all engine indications appeared normal. The aircraft landed on runway 04L at 20:02. Following the landing, the aircraft underwent a technical inspection by the airline's maintenance organization.
The investigation
The investigation focused on the cause of the engine vibration and the physical condition of the engine components. During the post-flight inspection, maintenance personnel discovered that a piece of coating had detached from a fan blade on the right engine. This fragment, measuring approximately 5 x 20 cm, was found bent and lodged in the first-stage stator vane of the bypass duct.
Findings
- The vibration was caused by an imbalance in the engine resulting from the detached coating fragment on the fan blade.
- The aircraft's systems successfully detected the anomaly; however, the engine manufacturer had only established specific vibration limit values for the takeoff phase, not for other phases of flight.
- While the technical issue necessitated a return to base, the decision to divert was considered an example of proactive safety management, as it prioritized technical oversight over continuing a long-haul flight with an unidentified engine anomaly.