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.
[1][2] Thus, it is mandatory for an aircraft operator to comply with an AD.
ADs usually result from service difficulty reporting by operators or from the results of aircraft accident investigations.
When ADs are issued by the country of registration they are almost always coordinated with the civil aviation authority of the country of manufacture to ensure that conflicting ADs are not issued.
In detail, the purpose of an AD is to notify aircraft owners: ADs are mandatory in most jurisdictions and often contain dates or aircraft flying hours by which compliance must be completed.