The equations of motion of a rigid aircraft are required before any analysis can be started. In many problems the Earth can be regarded as ideally flat and non-rotating. In the main ESDU 67003 makes those assumptions and the equations for a rigid aircraft@ first in general form@ and then in a linearised perturbation form@ are presented. The latter form is in widespread use for investigating the small disturbances of an aircraft from some datum flight condition@ which is usually taken as a steady state defined by constant linear and angular velocities.