CESSNA 501 near Opa Locka, FL — 2020-12-31

Probable cause

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.

NTSB narrative

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.

NTSB coding

• Uncontained engine failure · Takeoff-rejected takeoff

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