Fail-safe

In engineering, a fail-safe is a design feature or practice that, in the event of a failure of the design feature, inherently responds in a way that will cause minimal or no harm to other equipment, to the environment or to people. Unlike inherent safety to a particular hazard, a system being "fail-safe" does not mean that failure is impossible or improbable, but rather that the system's design prevents or mitigates unsafe consequences of the system's failure. That is, if and when a "fail-safe" system fails, it remains at least as safe as it was before the failure.^[1][2] Since many types of failure are possible, failure mode and effects analysis is used to examine failure situations and recommend safety design and procedures.^[3]

Some systems can never be made fail-safe, as continuous availability is needed. Redundancy, fault tolerance, or contingency plans are used for these situations (e.g. multiple independently controlled and fuel-fed engines).^[4]