Unsafe Conditions - continued

The unsafe conditions that contributed to this incident included:

1. The destructive "interactive" fan blade and fan case vibration mode within the engine operating envelope.
2. Engine design and certification standards that require the engine structure to contain damage resulting from a single rotor blade failure at the outermost retention groove. This engine failure resulted in a number of complete fan blades being released from the fan disk at high speed coincident with the fan containment case separating from the engine. This reduced its effectiveness in containing the engine debris and minimizing the hazard to the aircraft. High energy rotors and uncontained engine debris can cause a hazard to the aircraft. Aircraft safety for this type of threat is provided by a combined effort of precluding the uncontained failures and by mitigating the effects to aircraft in the event of a failure. In this failure scenario, the aircraft safety depended upon the DC-10-10 design precautions that were taken to minimize the hazards from uncontained engine debris.

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