Common Themes

Flawed assumptions

The No. 2 fan disk on the accident airplane had been manufactured using the double VAR process, which at the time of the accident was known to result in a higher defect rate than the triple VAR process which had been adopted. In order to allow double VAR manufactured parts to remain in service, a program of regular inspections had been adopted to monitor the airworthiness of those parts. It had been assumed that the inspection program would detect and remove defective parts from the fleet. In the case of this accident, this fan disk was inspected multiple times, and the crack which eventually caused the disk failure was never detected.

Human error

The No. 2 fan disk was subjected to multiple inspections prior to the accident, and the crack in the fan bore which initiated the failure was never discovered. It was estimated by the NTSB that the crack had grown to almost one-half inch in length at the time of fan disk failure.

Pre-existing failures

It was determined by the investigation that the No. 2 fan disk was manufactured with a defect (internal occlusion) that resulted in a crack that reached the inner bore of the disk hub. This crack went undiscovered during multiple inspections, and eventually led to the failure of the fan disk.

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