What happened
On 6 January 2004, an Enstrom F-2 and 8F Falcon, registration G-BYKF, departed from a private helipad in Crowhurst, East Sussex, carrying two passengers. During the initial climb, the pilot performed a semi-towering manoeuvre to clear nearby power lines. While the aircraft reached approximately 1,000 feet, the pilot experienced a sudden leftward kick and subsequent abnormal vibrations. As the helicopter reached a speed of 100 mph in level flight, the intensity of the vibration increased, though flight controls remained responsive. The pilot opted for a precautionary landing, successfully returning the aircraft to the original helipad with no injuries to the crew or passengers.
Following the flight, the pilot attempted to diagnose the issue by restarting the engine. While the vibration was absent at ground idle, it became prominent once the engine exceeded 1,500 RPM. As the rotor blades decelerated during shutdown, a visible misalignment of the tail rotor drive shaft was observed.
The investigation
The AAIB, working alongside the pilot and a maintenance engineer, examined the tail rotor drive shaft assembly. In this aircraft model, power is transferred to the tail rotor via an external shaft supported by five roller bearings housed in blocks, with rubber bushes positioned between the bearings and the blocks.
Investigators discovered that the rubber bush for the fourth bearing had detached from its position and moved toward the rear of the shaft. This displacement allowed the drive shaft to move radially within the bearing, creating the vibration. The investigation also noted that the aircraft, which was over 20 years old, had spent significant periods idle during its previous service in Japan. The rubber bushes had likely not been replaced since the aircraft's manufacture.
Findings
- The primary cause of the vibration was the release of the No 4 bearing rubber bush, which allowed for radial movement of the tail rotor drive shaft.
- The rubber bush had become swollen due to prolonged exposure to grease, leading to wear against the bearing block.
- The No 4 bearing was heavily contaminated with elastomer particles from the degrading bush.
- Evidence of corrosion was found in the No 4 bearing, likely caused by moisture ingress during periods of inactivity.
- The rubber bushes in the remaining bearings showed similar signs of swelling.