What happened
On December 31, 2021, at approximately 1715 EST, a Cessna 501, registration N75TL, experienced an uncontained failure of its number one engine during the takeoff roll from Miami-Opa Locka Executive Airport (OPF) in Opa-Locka, Florida. The aircraft, operated by Interstellar Air LLC under Part 91 regulations, was flying in visual meteorological conditions with 10 miles of visibility. The pilot was not injured during the event. Post-event inspection showed that damage was confined to the left engine and the nacelle structure.
The investigation
Engine serial number PCE-76844 was disassembled at the Pratt & Whitney Canada (PWC) facility in Bridgeport, West Virginia. Investigators found the engine fan was loosely secured in the fan case upon arrival. The engine outer bypass duct exhibited a hole spanning 14 inches axially at the 12 o’clock position and 9 inches at the 8 o’clock position, with edges petaled outward, indicating high-energy radial uncontainment.
All fan blades showed tip bending and leading edge material loss. The high pressure compressor (HPC) impeller had separated into several large and small pieces. This failure caused secondary damage throughout the engine gaspath, including a sheared low pressure shaft.
A metallurgical analysis performed in Montreal, Quebec, identified a low cycle fatigue (LCF) crack that initiated at a circumferential machining groove on the aft face of the HPC impeller. While the impeller met all drawing requirements for geometry and material, crack striation counts estimated that the component had accumulated approximately 6,380 cycles between the crack's initiation and the final overload failure.
Findings
- The HPC impeller failure was caused by a fatigue crack originating from a machining groove created during manufacturing.
- The machining grooves on the impeller would have passed standard overhaul inspection criteria, aside from the damage caused by the failure.
- Previous borescope inspection criteria, specifically a May 2004 inspection, were insufficient to detect the presence of these specific machining grooves due to the technology available at that time.
- PWC released service bulletin 7655 to address the issue, and regulatory bodies have moved to mandate improved inspection procedures via airworthiness directives.