The helicopter was being operated at about 40 feet above ground level at an indicated airspeed of 45 knots, when the engine suddenly failed. During the subsequent landing attempt the main and tail rotors struck trees and the helicopter contacted the ground tail first, severing the tail skid and tail rotor blades. Inspection of the engine revealed that the carburettor air filter screen was blocked by dirt and the alternate air source door had been glued in the shut position.