History of the Flight
The Robinson R22 Beta, registration G-CHYL, was being used for a trial lesson on 11 August 2003. The flight had been operating away from Bournemouth Airport. Before rejoining the circuit, the commander asked the passenger to remove his hands and feet from the controls and conducted pre-landing checks. The commander applied full carburettor heat prior to initiating a descent from 1,300 feet to the normal circuit height of 700 feet. He kept the aircraft level at 700 feet for about a minute before descending further parallel to the disused Runway 35, with the carburettor heat selected HOT throughout. Before descending below 300 feet, he pushed the carburettor heat back to COLD to ensure best engine performance in the warm ambient conditions (27°C).
Shortly after, the low rotor-speed warning horn sounded and the associated warning light illuminated. The commander heard the warning continue to the ground. He made a slight cyclic flare, lowered the collective lever, and opened the twist-grip throttle. He steered away from the active runway and buildings. The passenger instinctively grabbed the cyclic control, initiating a flare; the commander shouted twice for him to release it, which he did. As the aircraft neared the ground, the commander raised the collective lever to cushion the landing. The helicopter landed heavily and bounced to the right, remaining upright as the right skid gear bent. The commander applied the rotor brake to avoid a rotor strike, instructed the passenger to leave, then secured the cockpit against a possible fire before exiting. The aircraft came to rest on the ground approximately 160° to the left of its initial heading during the final descent.
Examination
Post-accident examination identified severe distortion of the landing gear tubes and minor distortion of the cabin structure. No apparent technical defect was found. The engine started and ran smoothly. An anomaly was noted: the governor switch was found in the OFF position. The commander recalled selecting the governor OFF at the same time as turning off the fuel, master switch, and magnetos after the accident. He also noted that the rotors had stopped quickly after landing.
Photographic Evidence
The passenger, who occupied the right-hand seat, provided photographs taken during the flight. One image, taken during the final descent, showed the LOW RPM rotor speed warning light illuminated, a rate of descent of 400 ft/min on the VSI, and an indicated airspeed of 27 kt. The altimeter and dual tachometer needles were not visible. Based on building aspect, the photograph was taken at a height of approximately 150 to 200 feet. Other photographs indicated a southerly surface wind of about 10 kt, giving a 10 kt tailwind component during the landing beside Runway 35.
Possible Explanations
The evidence from the photograph (low rotor RPM, modest rate of descent, low forward speed) made a simple loss of power unlikely. The commander believed he had applied carburettor heat correctly. If the collective lever was not lowered sufficiently or quickly after the warning, rotor speed would decay further, and the helicopter would probably not recover rotor speed before reaching the ground. With low forward speed and low rotor RPM, preventing a heavy landing would be difficult.
Another possibility is inadvertent switching off of the governor before the descent, although the commander thought he switched it off after landing. With the governor off, it would be possible to start the descent in a low-power, low-rotor-RPM condition. The mechanical correlator should have increased engine power when the collective was raised for touchdown, but its effect was likely insufficient to prevent a heavy landing.
A representative from the aircraft manufacturer commented that the conditions shown in the photograph made vortex ring state unlikely. The yaw string indicated a relative airflow from the right, which would have further increased the power requirement and potentially the rate of descent.
The most likely explanation for the accident was the low energy state of the helicopter on final approach as shown in the photograph. Exactly how and why this situation arose could not be determined from the available evidence.