ENG21LA011

An uncontained left engine failure caused by a low cycle fatigue crack that originated from a manufacturing machining groove on the high pressure compressor impeller aft face. Contributing to the incident was service bulletin borescope inspection criteria that was insufficient to detect the presence of machining grooves on the high pressure compressor impeller at the time the incident engine was inspected in May 2004.

The No. 1 (left) engine high pressure compressor (HPC) impeller developed a low cycle fatigue (LCF) crack that originated from a machining feature (groove) on the aft face of the impeller. The machining groove was created during the part manufacturing process. The LCF crack progressed to overload and resulted in an uncontained HPC impeller failure and secondary impact damage throughout the engine gaspath. Pratt & Whitney Canada (PWC) released a service bulletin (SB) in May 2003 that recommended a one-time borescope inspection of the HPC impeller aft face to check for evidence of machining grooves. The borescope inspection was subsequently mandated by Transport Canada and Federal Aviation Administration (FAA) airworthiness directives (AD). The incident engine was inspected in accordance with the SB inspection instructions in May 2004, with no HPC impeller machining anomalies noted. Following the incident, the HPC impeller fragments were inspected at the PWC facility in Bridgeport, West Virginia in accordance with the overhaul manual procedures. Aside from impact damage created during the incident event, the machining grooves present on the HPC impeller aft face would have passed inspection criteria. It is unlikely the machining grooves would have been visible during the SB borescope inspection performed in May 2004, particularly given the technology available at the time. A crack striation count was performed as part of the materials laboratory analysis of the incident HPC impeller. Assuming each crack striation correlates to one engine cycle, a 1/32 inch crack (thumbnail shape) would have formed on the HPC impeller aft face surface at approximately 1,970 engine cycles. The engine had accumulated 6,803 cycles since new when the HPC impeller borescope inspection was performed in May 2004. The crack would have been detectable by fluorescent penetrant inspection as outlined in the recently released corrective actions discussed in the factual section of this report.

History of Flight On December 31, 2021, about 1715 eastern standard time, a Cessna 501 airplane, N75TL, equipped with two Pratt & Whitney Canada (PWC) JT15D-1A turbofan engines, experienced a No. 1 (left) engine uncontained failure during takeoff roll from Miami-Opa Locka Executive Airport (OPF), Opa-Locka, Florida. The pilot was not injured during the incident. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. Post-event examination of the airplane revealed that damage was limited to the No. 1 engine and nacelle structure. Test and Research Engine Examination and Disassembly The incident engine, serial number, PCE-76844, was shipped from OPF to the PWC Bridgeport, West Virginia facility for examination and disassembly. The engine was received with the fan loosely secured in the fan case with a board and zip ties. A trash bag containing engine fragments including a recovered section of the high pressure compressor (HPC) impeller and assorted small debris recovered from the engine nacelle and runway was also in the engine shipping container. All fan blades exhibited leading edge material loss and tip bending opposite the direction of rotation. There was a hole in the engine outer bypass duct from the 8 to 1 o’clock positions that extended 14 inches axially at the 12 o’clock position and 9 inches axially at the 8 o’clock position. The hole location was coincident with the HPC impeller plane of rotation. The edges of the outer bypass duct around the hole circumference were petaled outward from the engine centerline consistent with high-energy radial uncontainment (Photo 1). Photo 1- Engine Serial Number, PCE-7684, Radial Uncontainment Damage The HPC impeller was separated into two major segments and an additional three small pieces were recovered from the debris collected in the engine and nacelle (Photo 2). The larger section of the HPC impeller, which made up approximately two-thirds of the impeller circumference, was loose inside the engine and was removed by hand. All remaining HPC impeller vanes exhibited impact damage, deformation, and material tearing. The HPC impeller failure resulted in secondary impact damage throughout the engine gaspath, including a sheared low pressure shaft. Photo 2- High Pressure Compressor Impeller (Photo Courtesy of PWC) Materials Analysis The HPC impeller, part number, 3020365, was shipped to the PWC Materials Investigation Laboratory in Montreal, Quebec, Canada for analysis. The metallurgical analysis identified a low cycle fatigue (LCF) crack that initiated along a circumferential machining groove on the aft face of the HPC impeller. A fluorescent penetrant inspection (FPI) of the HPC impeller revealed multiple indications along the circumferential machining groove where the fatigue crack initiated and propagated. There were no material anomalies identified near the crack origin and the HPC impeller met drawing requirements for part geometry and material specification. A crack striation count estimated the HPC impeller had accumulated approximately 6,380 cycles between crack initiation and overload failure. The striation count also determined that a 1/32 inch surface crack (assuming thumbnail shape) would have formed at approximately 1,970 cycles. Photo 3- HPC Impeller Fracture, (Blue Arrows) Inner Machining Groove, (Red Arrows) Outer Machining Grooves (Photo Courtesy of PWC) The HPC impeller was inspected at the PWC Bridgeport, West Virginia facility in accordance with the engine overhaul procedures. Aside from impact damage related to the event, the machine grooves on the HPC impeller would have passed overhaul inspection criteria. Corrective Actions In response to the incident investigation, PWC released service bulletin (SB) 7655 on April 14, 2022, applicable to all JT15D-1/-1A/-1B engines with HPC impeller P/N 3020365 installed. The SB has a compliance category 3- recommend completion at the next schedule engine hot section inspection (HSI) or when Flange C (gas generator case to low turbine support case) is separated. The SB references special instruction (SI), 38-022 with improved endoscopic FPI inspection procedures. Transport Canada was briefed on the proposed corrective action in February 2022 and subsequently formed a Corrective Action Review Board where it was ruled that the inspections called out SB 7655 would be mandated by an airworthiness directive (AD). The AD is tentatively scheduled for release in the second quarter of 2022. The FAA concurs with the proposed TC AD and have begun the process of issuing an equivalent AD for United States registered airplanes with JT15D-1/1A/1B engines. As part of the risk assessment performed by PWC, the population of affected engines is estimated to be 334.