AAIB investigation to Westland Wasp HAS1, G-KAXT
Loss of collective pitch control resulting in a forced landing, Bishopstone, near Salisbury, Wiltshire, 23 September 2016.
Summary:
With the helicopter straight and level at approximately 1,150 ft, the pilot felt vibration through the collective lever. The vibration ceased after two or three seconds.
Approaching higher ground, the pilot pulled up on the collective lever but the helicopter did not respond. He lowered the lever and again, there was no response. Faced with a loss of collective pitch control, the pilot made a precautionary landing, but was unable to control the flare. The tail rotor struck the ground but the helicopter remained upright. The pilot and his passenger were uninjured.
The investigation established that the collective pitch control rod in the main rotor gearbox had broken. The control rod failure was secondary to a universal joint failure that had occurred in the cyclic control circuit due to lack of lubrication and a build-up of corrosion deposits. The pilot had reported vibration in the preceding months but despite diagnostic efforts, the cause had not been identified.
The Civil Aviation Authority (CAA) issued Emergency Mandatory Permit Directive No 2017-002-E, applicable to UK-registered Westland Wasp and Scout helicopters, to perform a visual check of the condition of the universal joint and introduce periodic lubrication of the joint. One Safety Recommendation has been made to the CAA to review the maintenance requirements for ex-military aircraft.
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