Precursors

There are two precursors that have been identified for the National Airlines Flight 27 accident.

First, American Airlines experienced a similar engine failure during test cell operation of a CF6 engine on November 15, 1972; the bellmouth and exhaust cone separated and all the fan blades were liberated.

This was a precursor because the same failure mode as the National Airlines event was experienced but was not fully understood and the risk for a repeat event to a revenue service engine was not recognized.

The failure investigation put considerable focus on the cause of the failure due to possible improper installation of the bellmouth to engine attachments bolts by American Airlines. This was eventually shown to be irrelevant to the failure. This is another example of a "red herring" that can detract from determining the true root cause.

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