In the following paper it will be convenient to adopt the definition of inverted flight used by the Accidents Committee in R. & M. 617 (Advisory Committee for Aeronautics), “Reports on the behaviour of aeroplanes when flying inverted, with special reference to some accidents on ‘A,’” which was as follows:—“ In normal straight flight the pressure of the pilot on his seat is equal to his weight. During inverted straight flight he would hang on his belt with an equal force. It is therefore considered useful to call inverted flight such flight only as occurs when the weight of the pilot no longer presses on his seat.”