Summary of Event
On July 16, 2014, at 2130 Pacific daylight time, a Beech B100, registration N6756P, sustained an uncontained failure of its left engine while cruising at flight level 190 (19,000 feet mean sea level) approximately 30 miles east-northeast of Bakersfield, California. The commercial pilot, who was the sole occupant, was not injured. The airplane, operated under 14 CFR Part 91 for personal flight, landed uneventfully at Meadows Field Airport after the pilot executed engine failure procedures. Post-flight inspection revealed damage to the left nacelle and external damage to the left engine, consistent with an uncontained engine event.
Engine Examination
Examination by an NTSB powerplants specialist and the engine manufacturer identified failure of the second stage turbine wheel. The wheel assembly consists of a turbine wheel and a rotating labyrinth curvic seal (knife edge seal ring) joined by a tight interference fit. The knife edge seal ring was fractured and separated from the hub, with severe rotational scoring on the inner faying diameter. Approximately one-third of the forward edge was heat eroded, resulting in a wedge-shaped edge. One of two castellations on the seal ring was completely eroded by hot gas; the remaining castellation had small cracks at its inner corners. Only the hub portion of the second stage turbine wheel remained, with the web, platform, and blades missing. The forward outer curvic shaft diameter showed severe rotational scoring, consistent with contact against the fractured seal ring. The curvic teeth of the forward and aft couplings were undamaged.
A materials laboratory evaluation of the first stage stator vanes revealed weld repairs and microstructural evidence of sustained operational temperatures exceeding 1650°F at leading edges and 1840°F at trailing edges for an extended period. The second stage stator assembly, which includes a main nozzle casting with a brazed outer support baffle, was examined. The braze joint defines a cavity for compressor discharge air that cools the aft face of the first stage turbine wheel and the knife edge seal ring. Weld repairs near the braze joint present a risk of oxidation and porosity due to high temperatures. If the braze joint separated, cooling air would escape, leading to reduced cooling of the knife edge seal ring and increased temperatures. The secondary cooling air passing by internal turbine temperature (ITT) probes would cause a lower ITT indication, masking the true operating temperature from the pilot.
The second stage turbine wheel exhibited wear/rub that reduced web thickness by over 0.4 inches at the separation location, consistent with contact against the fractured seal ring. Fracture surfaces showed overload separation. Metal temperatures of the seal ring were estimated at 1446°F at the forward end and 1263°F at the aft end, well above the nominal cavity temperature of 1060°F. Intergranular fractures initiated after exposure to these high temperatures.
Repair History
A review of the Authorized Release Certificate (FAA Form 8130-3) revealed that the second stage stator assembly, part number 894528-15, serial number 0-01345-3454, was repaired by Texoma Turbines in Durant, Oklahoma on February 17, 2009, in accordance with process 72-IR-10 #9 and repair process specification (RPS) SP003 Rev. 2. At the time, the RPS was applicable only to stator assembly part numbers -1, -2, -3, -5, -6, -10, and -11, not to the -15 and -16 parts, making the repair unapproved. According to Honeywell's Instructions for Continuing Airworthiness, no instructions or authorizations exist for vane weld repairs on the second stage stator; components that fail service limits should be removed from service.
The owner of the repair station stated that documentation for the repaired part was destroyed after the FAA-prescribed two-year retention limit (14 CFR Part 145.219). The regulatory requirement allows disposal of records, and the true history of the part is no longer available.
Regulatory Aspects
The original technical documents used to substantiate the repair processes could not be located. An FAA technical staff member from the Fort Worth ACO Branch believed that no technical substantiation data was written for this repair, with only process steps documented by the original Designated Engineering Representative (DER). It is unknown whether the DER considered the braze joint's sensitivity to welding heat or the impact of cooling air loss on adjacent engine components. The technical substantiation data should have been retained by the original DER and repair station, but the DER was no longer active and not listed in the DER directory. FAA Order 1350.15C prohibits destruction of original documents, but no guidance exists for document transfer when a DER leaves or dies.