This invention control system having: means (12) for detecting an irregularity in aircraft handling during flight; means (18) for causing temporarily a rigid body excitation in at least a portion of the aircraft; means (20) for monitoring an actual response to the excitation; means (22) for comparing the actual response and a target response to the rigid body excitation; and means (14) responsive to an output from the comparison means for determining the need for a modification of the current flight plan and for generating a corresponding control output. The invention also provides a corresponding method for controlling an aircraft.