What happened
On 6 August 2003, an Enstrom F-28F Falcon, registration G-BXXW, was conducting a private flight near Wellesbourne, Warwickshire. The pilot was returning to Sheffield, with two passengers on board, following a stop at Wellesbourne Mountford Airfield.
While flying approximately 10 nautical miles south of the airfield, the pilot experienced a sudden vibration, a physical 'thump', and a rightward yaw. This was immediately followed by a burning smell in the cockpit and a low rotor RPM warning. The pilot responded by lowering the collective, adjusting the nose position to reach the recommended autorotative speed, and managing the engine power.
The pilot successfully executed an autorotative landing in a rough grass field. Although the touchdown was gentle, the aircraft sustained significant damage to the tail boom, tail rotor drive train, and main rotor gearbox. Following the landing, a small fire was observed under the gearbox fairing, which the pilot extinguished using the onboard fire extinguisher. No injuries were sustained by the crew or passengers.
The investigation
Investigators examined the wreckage and the main rotor gearbox (MRGB) to determine the cause of the mechanical failure. The investigation revealed that the rear bearing of the MRGB had broken up due to fatigue, leading to the seizure of the bearing. This seizure generated extreme heat, exceeding 650°C, which caused the pinion shaft to fracture in torsion. This fracture subsequently led to the failure of the tail rotor drive shaft and damage to the tail boom.
Engineers also inspected the chip detector system. While the bearing failure had produced enough metal debris to trigger the chip detector, no warning light appeared in the cockpit. The investigation found that the chip detector wiring had been repaired using an improper automotive-style crimp that was placed over the wire insulation, preventing the warning signal from reaching the pilot.
Findings
- The primary cause of the accident was the failure of the main rotor gearbox rear bearing due to inadequate lubrication.
- The reduction in oil levels within the gearbox could not be definitively determined, though the difficulty of checking the oil level via the sight glass was noted.
- The failure of the bearing caused the pinion shaft to fracture, leading to the loss of tail rotor drive and extensive damage to the airframe.
- A defective crimp in the chip detector wiring prevented the cockpit warning light from activating despite the presence of metallic debris.