The rational estimation and derivation of aerodynamic loading data is becoming of rapidly increasing importance in the design of modern aircraft. The great advances made in aircraft performance during the 1939-45 War, together with the generally detrimental effect of compressibility upon stressing loads, have combined to make the task of the structural designer much more difficult. A few years ago, the loads resulting from the often arbitrary stressing cases of A.P.970 (Ministry of Supply Air Publication 970, Design Requirements for Aeroplanes for the Royal Air Force and Royal Navy) were of relatively minor significance and furthermore, any inaccuracies in their estimation and derivation were covered almost always by the ultimate factor if the estimate were on the low side, while if the loads were conservative, the weight penalties and structural difficulties involved were small.