Design Philosophy Summary

We looked at the three major design philosophies which are defined as follows:

• Safe-Life Design (Safety by Retirement): A philosophy whereby safety is ensured by defining a safe-life as the number of flights, landings, or flight hours which there is a low probability that strength will degrade below the design residual strength.
• Fail-Safe Design (Safety by Design): A philosophy whereby safety is ensured by making a structure sufficiently redundant that it can retain its design residual strength for a period of time after failure or partial failure of a principal structural element.
• Damage Tolerant Design (Safety by Inspection): A philosophy whereby safety is ensured by designing a structure to sustain anticipated design loads in the presence of fatigue, corrosion, or accidental damage for a sufficient period of time in which such damages can be detected and repaired through regular inspections.

Implementing the Design Philosophies

In addition to the design philosophies themselves, we examined the process of selecting appropriate design philosophies and that their implementation is supported by a significant testing effort covering all scales of tests from material characterization to full-scale aircraft tests.