This part of ISO 1151 deals with derivatives of forces, moments and of other quantities characterizing such forces and moments. The term “derivative” designates the partial derivative of a function with respect to an independent variable. These derivatives appear in the terms of the Taylor series representing the variations of functions with the independent variables. This part of ISO 1151 is restricted to first-Order terms. Terms of higher Order would require additional definitions for derivatives of higher Order. The aircraft is assumed to be rigid. However, most of the definitions tan be applied to the case of flexible aircraft. Aerolastic effects would require the introduction of further quantities.