What happened
On 12 June 2014, a Schweizer 300, registration G-BWAV, was performing practice manoeuvres at Dunsfold Aerodrome in Surrey. The pilot, an experienced commander, was preparing for an upcoming air show at Biggin Hill and intended to conclude the session with a simulated engine failure and autorotation, ending in a powered recovery.
At approximately 400 ft aal, the pilot initiated the manoeuvre by lowering the collective and reducing engine RPM. The intention was to create a 'split' between the engine and rotor RPM, allowing the rotor to maintain speed via airflow while the engine remained at a lower setting. However, the pilot did not realize that the engine and rotor remained linked through the clutch system. As the descent progressed, the rotor speed decayed to 300 rpm.
Upon noticing the lack of pedal input required and checking the instruments, the pilot attempted to increase engine RPM by opening the throttle. To avoid further reducing rotor speed, the pilot refrained from raising the collective and instead performed a hard flare. Despite these efforts, the helicopter struck the ground with significant force, resulting in the destruction of the aircraft. The impact severed the gearbox and tail rotor, causing the helicopter to tumble before resting on its side. The pilot escaped no injuries, though the aircraft's inertia reel shoulder harness failed during the impact.
The investigation
The investigation focused on the mechanics of the autorotation attempt and the pilot's cockpit monitoring. The AAIB examined the sequence of the engine and rotor RPM settings and the state of the clutch system. The investigation also looked into the pilot's altitude and concentration levels during the manoeuvre, noting that a previous successful attempt had been conducted from a higher altitude of 800 ft aal.