Accident Board Findings

Following are select findings specific to the fan hub failure excerpted from the NTSB report:

  • Some form of drill breakage or drill breakdown, combined with localized loss of coolant and chip packing occurred during the drilling process, creating the altered microstructure and ladder cracking in the accident fan hub.
  • Fatigue cracks initiated from the ladder cracking in the tierod hole and began propagating almost immediately after the hub was put into service in 1990.
  • Although the altered microstructure in the accident hub tierod hole was detectable by blue etch anodize inspection methods, Volvo did not identify it as rejectable because the appearance of the tierod hole did not match any of the existing inspection templates showing rejectable conditions.
  • Although the additional templates will assist blue etch anodize inspectors in detecting potential defects similar to the one that existed on the accident hub, this accident suggests that there may be additional rejectable conditions that have not yet been identified.
  • Drilling damage in this accident hub extended much deeper into hole sidewall material than previously anticipated by Pratt & Whitney.
  • The crack was large enough to have been detectable during the accident hub's last fluorescent penetrant inspection at Delta.
  • Significant questions exist about the reliability of flash drying in removing water from cracks.
  • Better techniques are needed to ensure the fullest possible coverage of dry developer powder, particularly along hole walls.
  • Although it could not be conclusively determined whether this played a role in the nondetection of the crack in the accident hub, the absence of a system that formally tracks the timing of the movement of parts through the fluorescent penetrant inspection process was a significant deficiency.
  • Fluorescent penetrant inspection indications remain vulnerable to manual handling, and fixtures used to support the part during inspection may obstruct inspector access to areas of the part.
  • No personal or physical factors would have prevented the FPI inspector from detecting a visible crack in the accident hub.
  • A low expectation of finding a crack in a JT8D-219 engine fan hub might have caused the FPI inspector to overlook or minimize the significance of an indication.
  • Because of the potentially catastrophic consequences of a missed crack in a critical rotating part, testing methods that evaluate inspector capabilities in visual search and detection and document their sensitivity to detecting defects on representative parts are necessary.
  • Delta's nondetection of the crack was caused either by a failure of the cleaning and fluorescent penetrant inspection processing, a failure of the inspector to detect the crack, or some combination of these factors.
  • Manufacturing and in-service inspection processes currently being used do not provide sufficient redundancy to guarantee that newly manufactured critical rotating titanium engine parts will be put into service defect-free and will remain crack-free through the service life of the part. Further, all critical rotating titanium engine components are susceptible to manufacturing flaws and resulting cracking and uncontained engine failures that could potentially lead to catastrophic accidents.

The probable cause was identified as:

The National Transportation Safety Board determines that the probable cause of this accident was the fracture of the left engine's front compressor fan hub, which resulted from the failure of Delta Air Lines' fluorescent penetrant inspection process to detect a detectable fatigue crack initiating from an area of altered microstructure that was created during the drilling process by Volvo for Pratt & Whitney and that went undetected at the time of manufacture. Contributing to the accident was the lack of sufficient redundancy in the in-service inspection program.

The NTSB identified 30 findings for this accident. The complete list is available at the following link: NTSB Findings

The entire NTSB report (NTSB/AAR-98/01) can be viewed at the following link: NTSB Report

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