What happened
On 23 August 2000, a Westland Scout AH1, registration G-BZBD, was performing a private ferry flight from Thruxton to Welshpool. The crew, consisting of a pilot, a pilot's assistant, and a ground handler, were cruising at 1,500 feet when the aircraft encountered severe turbulence near the Chiltern Hills. During this turbulence, the helicopter pitched forward and to the right, and the pilot noted a significant increase in the physical effort required to move the cyclic pitch control.
Misinterpreting these heavy control loads as a hydraulic failure, the pilot manually switched off the hydraulic system. This action caused the helicopter to pitch violently down and to the right. In an attempt to regain control, the pilot re-engaged the hydraulic power, which triggered a violent pitch up and to the left. During this sequence, a mechanical failure occurred, and the main rotor blades struck the tailboom, severing the tail rotor drive shaft. The pilot initiated an autorotation to land in a nearby field. As the aircraft approached the ground at approximately 50 feet, the pilot applied collective pitch, causing the helicopter to yaw uncontrollably and strike the ground heavily. The impact resulted in one serious and one minor injury among the passengers, while the pilot sustained minor injuries. The aircraft was destroyed.
The investigation
AAIB investigators examined the wreckage at a Gloucestershire airstrip and conducted system tests. They found that the tailboom had been struck by the main rotor blades, which had removed the rear section of the tail rotor drive shaft. The engine showed no signs of internal failure, and the hydraulic systems functioned normally when tested on a ground rig.
While the investigation considered whether a 'runaway' cyclic trim or inadvertent interference from the passenger in the left seat could have caused the initial increase in control loads, no definitive technical malfunction was identified. Testing suggested that while a map or hand movement from the left seat could theoretically impede the controls, it was unlikely to have occurred without the passenger noticing. The investigation focused on the pilot's decision-making process regarding the hydraulic switch.
Findings
- The primary cause of the accident was the pilot's misinterpretation of increased cyclic control loads as a hydraulic failure.
- The pilot took inappropriate remedial action by deactivating the hydraulic boost without first verifying the status of the hydraulic warning panel.
- The subsequent violent maneuvers and the loss of the tail rotor were consequences of the pilot's manual selection of the hydraulic-off mode during high-speed flight.