Definition

The Ultimate Load is the Limit Load multiplied by a prescribed Safety Factor of 1.5. Any part of the structure of an aircraft must be able to support the Ultimate Load and, with certain exceptions, be able to do so without failure for at least 3 seconds. However, when proof of strength is shown by dynamic tests simulating actual load conditions, the 3-second limit does not apply. Static tests conducted to ultimate load must include the ultimate deflections and ultimate deformation induced by the loading

Source: CS 25.303 Factor of safety and CS 25.305 Strength and deformation.

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