Our engineering habits form slowly, and once formed are slow to change. This is perhaps less true in aeronautics than in other branches of engineering. Nevertheless some aeronautical customs are deeply rooted, so much so that it is easy to confuse the superstructure of habit with the basic foundations. In this article I have reviewed some of the customary methods of specifying strength conditions and have tried to disentangle the underlying principles. Current changes in these methods emphasise the need of a proper appreciation of the principles, and a study of these principles suggests even more radical changes of methods in future.

Initially, supercharging of aero-engines was introduced with a view to restoring the pressure in the induction manifold at altitude to that corresponding to sea level. During recent years this has been changed, and supercharging is now employed not just to maintain sea level pressure in the induction manifold, but to boost engines at altitude, in order to develop high B.M.E.P.s and thereby obtain high power outputs at high power to weight ratios.