Explain the damage tolerance design philosophy and its requirements.

Damage tolerance requires that structures remain safe with assumed damage (cracks or discrete source damage) until detected through scheduled inspections. Per FAR 25.571, the structure must demonstrate: residual strength at limit load with maximum undetected damage; slow, stable crack growth allowing multiple inspection opportunities; detectable damage before reaching critical size; and defined inspection intervals with 95% probability of detection. This replaced safe-life philosophy after lessons from Comet accidents. Implementation requires crack growth analysis, inspection method selection (NDI capability), and establishing inspection intervals typically based on half the time for damage to grow from detectable to critical size.