What happened
On 23 September 2016, a Westland Wasp HAS1, registration G-KAXT, was conducting a private flight from Barton Ashes to RNAS Yeovilton. While flying at approximately 1,150 ft near Bishopstone, the pilot felt a brief period of vibration through the collective lever. Shortly after, the pilot discovered that the collective pitch control was no longer responsive.
Recognising the loss of control, the pilot diverted toward a field in the Chalke Valley. Despite notifying Air Traffic Control and attempting a landing, the pilot was unable to effectively flare the aircraft. The helicopter touched down with significant bouncing, and the tail rotor struck the ground. There were no injuries to the pilot or the passenger.
The investigation
The AAIB examination of the aircraft revealed that the collective pitch control rod within the main rotor gearbox had fractured. This failure occurred at a point where the rod passed through a metal guide, a contact that should not occur during normal operation.
Further inspection of the control hub assembly identified significant corrosion on the universal joint and damage to the cyclic control spindle. The investigation found that the universal joint had been operating with insufficient lubrication for a long period. Additionally, the investigation noted that the gearbox had been installed for 20 years without an overhaul, as it had only accrued 429 hours of use since its last service in 1996, well below the 600-hour interval.
Findings
- The primary cause of the accident was the failure of the universal joint in the cyclic control circuit due to a lack of lubrication and the accumulation of corrosion.
- The failure of the collective pitch control rod was a secondary effect, caused by excessive vibration that led to fatigue and contact with the metal guide.
- The extended period of time since the last gearbox overhaul, caused by low civilian utilisation compared to military usage, led to the depletion of the oil reservoir in the universal joint.
- The pilot had previously noted vibrations in the months leading up to the event, though a recent air test had been successful.
Safety action
Following the investigation, the CAA issued Emergency Mandatory Permit Directive No 2017-002-E, requiring visual inspections and periodic lubrication of the universal joint for UK-registered Westland Wasp and Scout helicopters. A safety recommendation was also made to the CAA to review maintenance and overhaul requirements for ex-military aircraft to ensure they account for lower utilisation and include appropriate calendar-based limits.