What happened
On a scheduled international flight from Newark, New Jersey, to Halifax, Nova Scotia, the crew of a BAe-146-200 (registration C-GRNV) experienced significant engine vibration while climbing through 9,000 feet. The vibration in the No. 4 engine exceeded the maximum allowable limit of 1.2 inches per second, reaching a peak of 3.0 inches per second.
In response to the vibration, the crew initially reduced power, which temporarily lowered the vibration levels. However, as the aircraft leveled off at flight level 190, the vibration increased again. Following established procedures, the crew performed an engine vibration checklist and subsequently shut down the No. 4 engine. After consulting with maintenance, dispatch, and operations departments, the decision was made to proceed to Halifax. The aircraft landed safely in Halifax with no injuries to the 53 persons on board.
The investigation
Technical examinations of the No. 4 engine revealed a fractured third-stage turbine blade. Microscopic analysis showed that the fracture was a result of an overload extension of a high-cycle fatigue crack that had started near the leading edge of the blade. While some scattered microporosity was found at the crack' and origin, it was not considered the primary cause of the failure.
Investigators noted that this was the third incident involving a third-stage turbine blade failure on this specific aircraft, C-GRNV, within a single year. The investigation also looked into the history of engine re-blading practices, noting that previous failures in the ALF 502 series engines were linked to the use of blades that had undergone in-service wear, which created excessive gaps between blade tip shrouds.
Findings
- The primary cause of the engine vibration was the failure of a third-stage turbine blade.
- The failure occurred due to the overload extension of a high-cycle fatigue crack originating near the blade's leading edge.
- It is possible that the use of previously used blades during engine overhauls led to excessive tip shroud gaps, reducing the damping effect and contributing to high-cycle fatigue.
- The flight crew's decision to shut down the engine was a precautionary measure following the breach of vibration limits.