An Airworthiness Directive (commonly abbreviated as AD) is a notification to owners and operators of certified aircraft that a known safety deficiency with a particular model of aircraft, engine, avionics or other system exists and must be corrected. If a certified aircraft has outstanding aircraft airworthiness directives that have not been complied with, the aircraft is not considered airworthy. Thus it is mandatory for an aircraft operator to comply with an AD. AD's usually result from service difficulty reporting by operators or from the results of aircraft accident investigations. They are issued either by the national civil aviation authority of the country of aircraft manufacture or of aircraft registration. When AD's are issued by the country of registration they are always coordinated with the civil aviation authority of the country of manufacture to ensure that conflicting AD's are not issued. The purpose an AD is to notify aircraft owners: 1- that the aircraft may have an unsafe condition, or 2- that the aircraft may not be in conformity with its base of certification or of other conditions that affect the aircraft's airworthiness, or 3- that there are mandatory actions that must be carried out to ensure continued safe operation, or 4- that, in some urgent cases, the aircraft must not be flown until a corrective action plan is designed and carried out. AD's are mandatory in most jurisdictions and often contain dates or aircraft flying hours by which compliance must be completed. AD's may be divided into two categories: 1. Those of an emergency nature requiring immediate compliance prior to further flight, and 2. Those of a less urgent nature requiring compliance within a specified period of time.