Mechanical Failure
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.
Certified transport aircraft are built so that no single failure is catastrophic. A mechanical-failure accident therefore usually involves either a failure mode nobody anticipated, or a chain in which the redundancy was defeated — a common cause taking out multiple channels, or a latent defect present in more than one system.
Metallurgical examination of the failed part, service history across the fleet, and any prior occurrences of the same defect are the standard threads.
Related terms
The airframe itself broke — commonly through fatigue cracking, corrosion, an improper repair, or loads beyond what the structure could take.
Loss of thrust from one or more engines. Whether it becomes an accident usually depends on when it happened and what the crew did next.
A legally binding instruction from a regulator requiring an inspection, repair or modification on a type. Unlike a recommendation, compliance is mandatory.