What happened
On 14 February 2009, a Robinson R2/22 Beta, registration G-TTHC, was performing a solo training circuit near Sandtoft Aerodrome, Humberside. The aircraft, operated by a flying school based at Leeds Bradford Airport, was being flown by a student pilot. After completing several circuits, the helicopter was on its downwind leg when the engine suddenly lost power.
As the engine power failed, the rotor RPM decayed rapidly. This caused the rotor disc to tilt rearwards, leading the main rotor blades to strike and sever the tail cone. The helicopter then entered a controlled descent that quickly became a loss of control, spinning through several rotations before falling vertically into a waterlogged field. The impact resulted in one fatality.
The investigation
Investigators from the AAIB examined the wreckage and the engine components. They found that the tail cone and tail rotor drive assembly had been struck by the main rotor blades. The investigation focused on why the engine had stopped, specifically looking at the ignition system and potential carburettor icing.
Technical analysis of the left magneto revealed a worn carbon brush and a damaged copper outlet tab. The wear pattern suggested that arcing had occurred over time, eventually leading to a loss of high voltage output. While the weather conditions were conducive to carburettor icing, the investigation found no conclusive evidence that ice was the primary cause, as the pilot had been using carburettor heat.
Findings
- The engine power loss was caused by the failure of the left magneto.
- The ignition switch had been inadvertently left in the 'left magneto only' position following pre-flight checks, meaning the engine could not continue running once the faulty left magneto failed.
- The pilot was unable to maintain sufficient rotor RPM to enter a successful autorotation.
- The decay in rotor RPM caused the rotor disc to tilt, resulting in the main blades striking the tail assembly.