What happened
On 5 August 2016, a Robinson R4SS Raven, registration G-RFUN, departed Coal Aston Airfield for a private flight in the Peak District. The aircraft, carrying the pilot and three passengers, flew to a boar shooting ground near Crowden, Glossop, to perform a hover for onlookers.
The pilot initiated a downwind approach to an out-of-ground-effect (OGE) hover at approximately 1,000 ft amsl. During this maneuver, the pilot applied full carburettor heat. While checking instruments during the hover, the pilot noticed the aircraft was sinking. Despite increasing the collective, the descent continued, prompting the pilot to attempt an emergency landing on uneven ground. The helicopter struck the ground heavily on the front skids and rolled onto its side. All four occupants escaped, with one passenger sustaining a minor injury.
The investigation
The investigation focused on the aircraft's weight, the pilot's approach technique, and the impact of engine settings on performance. Investigators determined that the aircraft was operating at or near its maximum takeoff weight of 2,400 lbs. Discrepancies in weight calculations were noted, but final established figures placed the aircraft slightly above its maximum allowable weight.
Crucially, the investigation examined the use of full carburettor heat. While the pilot believed this was a safety precaution, the manufacturer's performance data indicates that applying full carburettor heat significantly reduces the maximum hover ceiling. The investigation also reviewed the pilot's experience, noting he had 87 total flying hours, with only 9 hours on this specific type.
Findings
- The aircraft was operating at a weight that exceeded its maximum takeoff weight.
- The pilot performed a downwind approach, which is discouraged due to the risk of losing translational lift.
- The use of full carburettor heat significantly degraded the helicopter's performance capabilities, placing the flight outside the declared flight envelope.
- The aircraft likely entered a vortex ring state (settling with power) during the descent, making the sink rate difficult to arrest with increased power.