History of the Flight
The aircraft, an Agusta A109E registered G-TYCN, departed from a private field in Blandford Forum, Dorset at 1030 hrs on 18 January 2010 for a flight to Henstridge. After refueling, the takeoff weight was calculated to be 1 kg below the maximum takeoff and landing weight of 2,850 kg. The helicopter departed Henstridge at 1105 hrs for the return flight to the same private field.
The pilot, familiar with the site, began a continuous right turn to intercept an approach angle similar to one used earlier. The approach continued normally until approximately 100 ft agl, when the pilot noticed the rate of descent had increased markedly. He attempted to reduce it by increasing collective pitch and raising the aircraft's nose slightly, but by 30 ft agl the rate of descent had reduced only marginally. He applied maximum torque and raised the nose further, but when ground contact became inevitable, he levelled the attitude. The aircraft touched down heavily, skidded briefly, and became airborne again with a rapidly increasing right yaw. The yaw was not correctable with the anti-torque pedal; the pilot assessed that the tail rotor had likely struck the ground. He lowered the collective lever to reduce the yaw, and the aircraft touched down again, coming to rest after rotating through approximately 250°.
Pilot's Assessment
After exiting, the pilot noticed occasional gusts from the south estimated at 10 kt, which had not been present during the departure an hour earlier. He believed the aircraft had entered a vortex ring state during the late stages of the approach. He thought that flying a normal approach at high aircraft mass and low airspeed with a slight tail wind had led to a high rate of descent that he was unable to arrest.
Vortex Ring State
A vortex ring state requires a helicopter to have power applied while descending at slow airspeed. Air re-circulates through the main rotor, reducing total rotor thrust and increasing the rate of descent. This condition is more likely at high aircraft mass and with a tailwind. High mass increases the power requirement, and a tailwind leads to a higher rate of descent for a given approach angle. In early stages, recovery might be achieved by applying excess power, but if the rate of descent is high enough, sufficient power may not be available. Recovery can be accomplished by lowering collective pitch and increasing forward speed, both of which require sufficient altitude to be effective.