Structural Failure
The airframe itself broke, usually through fatigue cracking, corrosion, an improper repair, or loads beyond what the structure could take.
Structural failures are the reason airframe life is measured in pressurisation cycles as well as hours: fatigue is driven by the number of times the fuselage is inflated and deflated, not by time in the air.
Investigations look for the crack origin, the inspection programme that should have found it, and whether a previous repair introduced a stress concentration. Findings here frequently produce airworthiness directives affecting an entire type.
Related terms
How much life the airframe has used. Hours measure time in service; a cycle is one takeoff and landing, and it is cycles that drive fatigue.
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.
A legally binding instruction from a regulator requiring an inspection, repair or modification on a type. Unlike a recommendation, compliance is mandatory.