Accident Board Findings

Photo of failed engine
Photo of failed engine

The NTSB determined that the probable cause of this accident was the disintegration of the number 3 engine fan assembly as a result of interaction between the fan blade tips and the fan case. The fan-tip rub condition was caused by the acceleration of the engine to a fan speed which resulted in the fan tip and fan case making physical contact (i.e., "tip rub"). This physical contact permitted vibratory coupling to exist between the fan blades and fan case, resulting in very high energy being exchanged between the two components and the eventual fan blade liberation from the fan hub.

The precise reason or reasons for the acceleration and the onset of the destructive vibration could not be conclusively determined during the course of the investigation. Subsequent to the investigation, extensive research identified the precise causes of the vibratory responses of the engine components and identified effective solutions to mitigate other occurrences.

The NTSB identified 14 findings for this accident in report NTSB-AAR-75-2. The findings addressed:

  • Certification, qualification and maintenance of the crew and the aircraft
  • Actions by the crew in relation to the N1 tachometer circuit breaker and the engine operating above normal conditions
  • Damage to the aircraft caused by the liberation of 32 of the 38 fan blades
  • Damage to the wiring and loss of electrical power

Select this link to view the detailed NTSB Findings. Select this link to view the entire NTSB Report.

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