Lessons Learned

Technical Related Lessons:

Large high bypass engines can exhibit hazardous mechanical interaction between vibratory modes of the fan blades and fan case, with the potential for catastrophic uncontained engine failure. (Threat Category: Uncontained Engine Failure)

  • In this accident, harmful vibratory modes between the fan blades and fan case, within the engine operating envelope, resulted in the engine inlet bolts failing and a number of fan blades being released, striking the
    airplane. Although all large high bypass engines have fan case and fan blade vibratory modes, it is important
    that they do not exhibit a common vibratory frequency and order (e.g. fifth order, sixth order, etc) at a fan
    speed within the operating range of the engine.

Hazardous deterioration of pressure anywhere in the passenger cabin should be rapidly detected and result in timely deployment of all passenger oxygen masks. (Threat Category: Cabin Safety/Hazardous Cargo and Pressurization/Decompression Failure)

  • During the rapid decompression, many passenger oxygen masks were delayed in deploying or failed to deploy, causing panic among the passengers. The delay was caused by the severe damage to the number 3 nacelle which contained the number 3 electrical buses. This partial loss of power affected the depressurization warning system and some of the masks in the automatic oxygen deployment system.

  • One liberated blade struck a cabin window which caused it to fail and the cabin to experience a rapid decompression, resulting in a pressure differential between the main cabin and the lower galley. This made the decompression profile steeper for the lower galley, and it decompressed faster. The single aneroid device, which detects unacceptable cabin pressure altitude and deploys the oxygen masks, was located in the forward passenger cabin. This prevented the supplemental oxygen from being immediately available to the occupants in lower galley.

Photo of No. 3 engine fan
No. 3 engine fan

Common Themes Related Lessons:

It is essential that major service incidents are understood and resolved in a timely manner in order to avoid an additional incident or accident due to this same condition. (Common Theme: Organizational Lapses)

  • Prior to this accident, two CF6 engine fan failures had occurred due to this same vibratory condition, both resulting in fan assembly disintegration identical to the National Airlines accident. Although both occurred during test cell testing, understanding their failure modes, recognizing the potential for similar failures during flight, and implementing timely and effective intervention, is critical in order to avoid in-service failures which could lead to an accident.
     

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