What happened
On 8 February 2012, an Enstrom 280FX Shark, registration G-OJMF, was conducting a navigation training exercise when the pilot noticed a significant reduction in tail rotor authority. Specifically, the aircraft failed to respond to left pedal inputs, though right yaw remained possible. The pilot elected to return to Manchester Barton Airport and performed a run-on landing on the grass runway.
During the landing, the helicopter veered toward the right side of the runway, encountering rough and frozen ground. This caused the aircraft to bounce on its skids, resulting in the tail rotor striking the ground. The impact caused damage to the tail rotor assembly, the rear fuselage, and the left landing gear skid. There were no injuries to the two crew members on board.
The investigation
Investigators examined the aircraft' and the tail rotor pitch change bearing. They found that the left-side yaw control cable on the tail boom had been severed, though this was determined to be a consequence of the landing impact rather than the cause of the flight instability.
Upon disassembly of the tail rotor, the AAIB discovered that the flange on the inboard end of the bronze pitch change bearing had broken off and was missing. Metallurgical analysis of the remaining bearing showed that a crack had initiated in a corner of one of the keyways. The examination revealed that the fracture surface contained corrosion deposits, with chlorine present, likely originating from atmospheric moisture.
Findings
- The tail rotor pitch change bearing suffered an in-flight failure where an integral flange detached.
- The failure was characterized by a crack that initiated due to corrosion and progressed through the bearing wall.
- Corrosion played a significant role in the crack initiation process.
- The loss of the flange resulted in the remaining bearing component being left in a neutral position, which prevented the pilot from achieving left yaw control.