FedEx Express Flight 597

As the airplane climbed through about FL260, the No. 3 engine low pressure turbine (LPT) stage 1 disk burst when a fatigue crack that initiated in the forward seal tooth serration arm ruptured. Fragments from the burst disk partially severed the LPT case and struck the fuselage in multiple locations. The airframe vibration increased to a severe level until the LPT rotor separated from the engine, about 14 minutes 30 seconds after the uncontained LPT stage 1 disk burst, when the high pressure turbine aft shaft and the fan mid shaft failed due to contact damage and torsional loading. Metallurgical examination of the portion of the LPT stage 1 disk that was imbedded in the right wing found that the disk burst due to a mixture of high cycle and low cycle fatigue. The fracture was located in the vicinity of the forward seal tooth serration, although the exact fracture origin could not be identified because the entire seal tooth and part of the seal tooth arm were missing at the fracture location. Examination of the forward seal tooth serration revealed the presence of two previous weld repairs that extended into the forward arm. Review of the manufacturing records revealed that the disk had been dabber weld repaired twice to correct anomalies and non-conformances found during the original manufacturing process. The dabber weld procedures did not authorize welding in the forward seal tooth serration, therefore, the weld repair performed on the failed LPT stage 1 disk did not conform to the approved repair instructions. Based on testing and analysis of sample, post-manufacture dabber weld repaired disks, the suspect population of CF6 disks was confined to those that had been repaired at original manufacture. Records indicated that, in addition to the accident disk, four other disks had been dabber weld repaired at original manufacture to correct forward seal tooth serration anomalies and non-conformances. None of these disks are any longer in service. Based on the estimated crack propagation rate, it is likely that either no crack existed at the last inspection of the disk or the crack size was below the reasonably detectable threshold. After the disk burst, the flight crew completed all required checklists; however, the crew could not manually shutdown the affected engine because the burst disk fragments had severed the fire shutoff valve cable, the power off cable, and the emergency shutoff cable. The engine eventually shutdown on its own due to airflow disruptions within the engine. The flight crew’s procedural coordination and use of available resources, consistent with cockpit resource management best practices, helped to ensure the safe landing of the airplane.

Verbatim NTSB analysis and probable cause from the NTSB dataset