Incident Overview
On 6 January 2004, at approximately 1315 UTC, an Enstrom F-28F Falcon helicopter, registration G-BYKF, experienced an abnormal vibration during flight near Crowhurst Park, Crowhurst, East Sussex. The aircraft was engaged in a private flight with one pilot and two passengers on board. The pilot held a Private Pilot's License and had 537 total flight hours, with 430 hours on type.
Flight Sequence
The helicopter departed from a private helipad after a hover-taxi to a clear area. The pilot initiated a semi-towering take-off to avoid power lines. Following a normal climb, at around 1,000 feet, the pilot felt a kick to the left followed by an abnormal vibration. As the helicopter transitioned to level flight and accelerated to 100 mph, the vibration increased in magnitude, though the controls remained normal and no other abnormal indications appeared. The pilot decided to conduct a precautionary landing, executed a descent, and landed successfully back on the helipad.
Post-Flight Examination
After disembarking the passengers and consulting his maintenance organization, the pilot restarted the helicopter to diagnose the vibration source. No abnormal vibrations were present at ground idle, but above 1,500 RPM the vibration became noticeable and increased with RPM. The pilot shut down the engine and observed a misalignment of the tail rotor drive shaft as the blades slowed.
Aircraft Examination
The helicopter's tail rotor drive shaft runs externally along the top of the tail boom and is supported by five roller bearings located within blocks, each with a rubber bush between the block and bearing. The AAIB examined the aircraft with the maintenance engineer and pilot. The rubber bush from the No 4 bearing (numbered from the forward end) was damaged, had migrated aft from its location, and remained attached to the shaft. Its release allowed radial movement of the tail rotor drive shaft within the bearing, causing the vibration.
Component Analysis
The bearings and rubber inserts are maintained 'on-condition' with no requirement to remove the drive shaft for inspection. Pre-flight inspections include checking for bearing play, and bearings are re-greased every 50 hours. The helicopter was over 20 years old, originally operated in Japan where it had been idle for a significant period, then imported to the UK and purchased by the current owner in 2000. The rubber bushes had not been replaced by the current owner and were likely those fitted during manufacture.
Examination revealed that the No 4 bearing was heavily contaminated with elastomer particles from the rubber bush. The bush had swollen from prolonged contact with grease, causing wear from contact with the block while rotating. The other bushes showed similar swelling. The No 4 bearing exhibited some corrosion, likely from moisture ingress while idle, probably occurring after the bush released. No other in-flight failures of this component have been recorded.
Damage
The incident resulted in damage to the tail rotor drive shaft bearing and rubber bushes. No injuries were reported among the crew or passengers.