What happened
On 9 August 2005, a De Havilland Canada DHC-8 Series 311, registration G-NVSB, was performing a scheduled passenger flight from Manchester Airport to Aberdeen with 33 passengers and 4 crew members on board. Shortly after takeoff, the crew noticed significant airframe vibrations. Upon reducing engine power and deselecting the autofeather system, a loud bang and heavy vibration occurred, accompanied by a rightward yaw. The torque indicator for the No 2 engine dropped to 0%.
While the crew successfully executed the engine shutdown drill, the propeller failed to feather. A ball from a failed bearing had become lodged between the propeller blade shank and the hub, locking the pitch angle at 31 degrees. Despite the engine being shut down, the propeller continued to windmill at approximately 500 rpm. The crew declared a MAYDAY and returned to Manchester. During the approach, the nose landing gear indicated an unsafe status, requiring the use of the alternate release system. The aircraft landed safely, and passengers were evacuated via the forward left door. No injuries were reported, though the aircraft sustained damage to the right engine and propeller assembly.
The investigation
The AAIB investigation focused on the mechanical failure of the No 2 engine and the propeller assembly. Examination of the engine revealed that the Power Turbine (PT) shaft had failed due to torsional loading, which caused the PT blades to be lost and an exhaust baffle plate to be ejected from the rear of the engine. This ejected component was later found by a member of the public near the airport.
Investigation into the propeller assembly identified that a support bearing for the No 1 propeller blade had failed catastrophically. The failure of the bearing's inner race and the subsequent movement of a bearing ball into the hub mechanism prevented the propeller from reaching the full feather position. Metallurgical analysis showed that cracks had developed in the bearing over a period of time, likely originating from brinelling damage.
Findings
- The primary cause of the incident was the catastical failure of the No 1 propeller blade support bearing within the No 2 engine propeller assembly.
- The failure of the bearing caused large, unbalanced loads that led to the fracture of the Power Turbine shaft.
- The propeller could not feather because a bearing ball became jammed between the blade root and the hub.
- The operator had deferred a reported propeller vibration defect in accordance with technical instructions because vibration survey equipment was unavailable or improperly configured.
- The vibration levels had been noted as abnormal in the days leading up to the incident, providing a potential warning that was not addressed due to the lack of functional monitoring equipment.