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Select the option that best describes a fail-safe designed structure.
  • A
    Multiple load paths through a structure that is replaced after limited usage.
  • B
    One load path is sufficient and the structural element is removed at the end of its lifetime or number of cycles.
  • C
    A structure that has built-in redundancy with multiple load paths.
  • D
    A single load path in a structure that can withstand 1.5 times the maximum design load.

For this question you need to know the difference between Safe-Life and Fail-Safe design.

Safe-Life: This is de minimum life during which it is known that no catastrophic damage should occur. The life of such a structure can be defined in different ways, cycles, flight hours, pressurization events or calendar time. After this interval has been reached the structure should be replaced, inspected or overhauled per manufacturers recommendations.

Fail-safe: When a structure fails there is alternative load-path provided by an adjacent part which is able to take over the loads for a limited amount of time. This normally should be enough time until the next periodic inspection where the damage should be identified and the damaged part replaced.

“Multiple load paths through a structure that is replaced after limited usage”
Fail-safe does have multiple load paths, but it is not replaced after limited usage but will be replaced or repaired when damages is found during scheduled maintenance.

“One load path is sufficient and the structural element is removed at the end of its lifetime or number of cycles”
This would be considered a Safe-Life design.

“A structure that has built-in redundancy with multiple load paths”
This complies with a fail-safe design.

“A single load path in a structure that can withstand 1.5 times the maximum design load”
Fail-safe design has multiple load paths so this statement is incorrect.

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