In considering the necessity for requirements to establish the airworthiness of helicopters it is essential to take into account the capabilities of the vehicle itself, the environment in which the vehicle is likely to be used and the abilities of the people most likely to be involved with the vehicle.

As envisaged by the inventors the helicopter is a vehicle which is capable of both vertical and translational flight. Unfortunately if the helicopter enters a cloud whilst engaged in vertical flight all visual reference is lost and the pilot is unable to correct any disturbance due to gustiness in the surrounding air. Conversely, if descending in vertical flight a situation can arise where the rotor develops a vortex ring and without forward speed the descent rate cannot be reduced until the helicopter hits the ground. It is therefore apparent that in flying the helicopter the pilot has to know its limitations and must be trained to handle the aircraft properly. Again in the course of flying the helicopter there is a probability that something will fail and if that probability is too high then such failures must be taken into account in assessing the airworthiness.