If a condition is found that causes a particular design of aircraft, aircraft
engine, propeller, or appliance to fail to meet its certification for
airworthiness, the CAAT, FAA or EASA (Design authority) can issue as
Airworthiness Directive (ADs).
An AD describes the unsafe condition and the corrective action that must be
taken to return the device to the conditions specified for its certification.
ADs prescribe the conditions and the limitations, including inspections,
repairs, or alterations, under which the product may continue to be
operated.
ADs apply to aircraft, aircraft engines, propellers, or appliances when,
• an unsafe condition exists in a product; and
• that condition is likely to exists or develop in other products of the
same type design.