What happened
On 13 October 2004, a Robinson R22 Beta, registration G-BYTD, departed Sywell Aerodrome for a solo private flight. Shortly after leaving the airfield's air traffic zone, the pilot noticed the carburettor temperature gauge was indicating a value within the yellow avoid arc. In response, the pilot applied additional carburettor heat to move the needle clear of this zone.
After approximately ten minutes of flight at 2,000 feet amsl, the pilot initiated a descent to 1,500 feet amsl. As the aircraft reached 1,600 feet, the tail began to swing violently from side to side. Efforts to stabilize the aircraft failed to stop the oscillations, and the aircraft lost altitude rapidly. The pilot attempted a forced landing in a nearby field, managing to avoid a tree in a hedgerow before touching down at approximately 40 kt.
The landing was heavy, impacting the right skid first. The force of the impact broke the skids from the airframe, and the rotor subsequently struck the ground. The aircraft rolled over, resulting in the destruction of the aircraft. The pilot sustained minor injuries but briefly lost consciousness during the rollover before escaping through the shattered canopy.
The investigation
The investigation examined the engine's carburettor heating system and the meteorological conditions at the time of the accident. The aircraft was equipped with a manual carburettor heater and a heat assist device. Investigators noted that the local humidity was between 82% and 87%, with cloud cover present at low altitudes, creating conditions highly conducive to ice formation.
Investigators also reviewed the pilot's experience and the aircraft's flight manual. The pilot had 77 total flying hours, all on this specific type. The investigation focused on whether the pilot had applied sufficient heat during the descent, as the flight manual and supplementary CAA instructions required full carburettor heat to be selected before entering a descent or auto-rotation.
Findings
- The primary cause of the instability was likely carburettor icing resulting from insufficient heat application.
- The engine governor likely attempted to compensate for power loss by fluctuating fuel flow, causing the rotor RPM to hunt and creating torque fluctuations.
- The pilot's relative inexperience contributed to the development of pilot-induced oscillations while attempting to correct the yaw.
- The heavy landing was a result of the difficulty in controlling the aircraft during the engine power fluctuations.