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]
If a certified aircraft has outstanding airworthiness directives that have not been complied with, the aircraft is not considered airworthy.[1][2] Thus, it is mandatory for an aircraft operator to comply with an AD.