Aircraft design has to ensure sufficient structural strength to survive extreme cases of encounters with atmospheric turbulence. This requires the formulation of a mathematical specification of turbulence to be used in the design process. The statistical discrete gust (SDG) method, proposed originally in 1968 by J.G.Jones (Glynn Jones), provides such a specification in a form which takes account of the non-Gaussian statistical structure of the more intense turbulence fluctuations and the manner in which these interact with the dynamic response of a flexible aircraft.