I am honoured to have been invited to give this Third Cierva Memorial Lecture on a subject which has a general application to all forms of aircraft, and which is central to any consideration of the future of the helicopter.

Reliability is a problem, the extent and urgency of which can only be appreciated fully and at first hand by the operator of a large fleet of aircraft, but most of the discussions and documents which have helped to lay the basis for this paper are not for general publication. I believe that this has helped me to concentrate on the essentials of the problem, but there is a danger that the simple arguments on which the paper is based may not carry the emphasis needed to convey to my colleagues the critical attitude of the operators.