Airworthiness Directive

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 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.
Purpose.
ADs 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 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.
ADs may be divided into two categories:
Issuance.
ADs are issued by most civil aviation regulatory authorities, including:
National procedures.
United States.
The FAA issues ADs by three different processes: 