What happened
On 06 July 2015, a Sud Aviation SA3/42J Gazelle, registration YU-HET, was undergoing a maintenance check near the hangars at Stapleford Aerodrome. During a ground run with the engine running, an engineer was performing a check of the hydraulic system. While the engineer was activating the hydraulic test switch on the instrument panel, the collective lever moved upward with significant force. This caused the helicopter to lift approximately 10 feet into the air and yaw to the right, rotating 180 degrees. During this movement, a main rotor blade struck the ground. The subsequent heavy landing caused damage beyond economic repair to the canopy, cabin floor, tail boom, right skid, and a main rotor blade.
The investigation
The AAIB investigation focused on the circumstances leading to the unexpected lift-off and the state of the aircraft's hydraulic systems. Testing conducted with a manufacturer specialist revealed that the hydraulic pack was operating correctly, but the low hydraulic pressure warning light and warning horn were non-functional. The investigation determined that a pressure switch had not been fitted to the hydraulic pack. It is believed that during a previous maintenance event in 2000, a replacement hydraulic pack was installed that arrived without the necessary switch, and the component was never subsequently fitted.
Furthermore, the investigation examined the maintenance history of the engine. The Astazou XIVH engine had been overhauled in 2005 by a Serbian organisation. While this organisation held approval for military versions of the engine, it lacked the necessary manufacturer's approval to overhaul civilian versions. The AAIB noted that the Serbian Civil Aviation Department had previously validated the release to service for this engine, despite the lack of specific manufacturer authorisation.
Findings
- The specific reason why the YU-HET became airborne could not be determined.
- The hydraulic low pressure switch was missing from the hydraulic pack, meaning the pilot would have no cockpit warning of a loss in hydraulic pressure.
- The absence of the switch prevented the completion of required pre-flight checks, which mandate testing the low pressure warning system.
- The engine overhaul was performed by an organisation not authorised by the manufacturer to service civilian-model engines.