Airworthiness Directive (AD)
A legally binding instruction from a regulator requiring an inspection, repair or modification on a type. Unlike a recommendation, compliance is mandatory.
ADs are issued by the certifying authority (the FAA, EASA and their counterparts) when an unsafe condition is found in a type design. They specify what must be done and by when, and an aircraft that has not complied is not airworthy.
Accident reports frequently trace back to an AD that existed, and to whether the operator had complied with it in time.
Related terms
A formal proposal from the investigators to a regulator or operator. Recommendations are not binding: the regulator decides what to mandate.
The airframe itself broke, usually through fatigue cracking, corrosion, an improper repair, or loads beyond what the structure could take.
A component or system failed in service. The investigation asks why the part failed and why the failure was not contained by design, inspection or redundancy.