THE major problems connected with the development of satisfactory structural strength requirements have long since been solved. We have, in this country, an empirical system of specification which fulfils the fundamental criterion of any strength system, namely, that failures in the air through lack of adequate strength of any component of an aircraft are extremely rare. An analysis of civil accidents (Ref. 1) shows that about 6 per cent are due to this cause. This does not imply any reason for regarding even this small number with complacency, rather the reverse.