This part of ISO 1151 gives basic definitions and deals with aircraft motion relative to the atmosphere, assumed to be at rest or in translational motion at constant velocity relative to the Earth. 1) The aircraft is assumed to be rigid. However, most of the definitions tan be applied to the case of a flexible aircraft. When account is taken of the variations at the Earth’s surface in the direction of the vertical (local direction of acceleration due to gravity), the term given in the sub-clauses and figures in question is qualified by the term “local”.