An aircraft may reasonably be said to be structurally safe if it is known that during the manoeuvres which it is likely to perform

• (i) The stress in no part of the essential structure approaches the failing stress of the material,

• (ii) It does not become unstable or uncontrollable on account of structural distortion.

I intend to discuss in this context the wing structure of the aeroplane.

For many years it was felt both in the design and airworthiness approval of the majority of aeroplane wing structures that the main responsibility for attempting to secure the former desideratum could rest upon the determination of the stresses induced in the wing structural material by load systems appropriate to a rigid aeroplane in certain conditions denned by specified combinations of flight attitude, acceleration, and factor of safety. These combinations — familiarly referred to as the C.P. forward, C.P. back, nose-diving and inverted flight cases—are designed to produce for a particular aeroplane a range of steady load systems which is more severe on the structure than the range of load systems corresponding to the manoeuvres contemplated for the aeroplane.