Common Themes

Organizational Lapses

The fan hub is a "safe life" part and was certified by the FAA, based on Pratt & Whitney engineering data, to operate safely for its total design life of 20,000 cycles. The hub does not require inspection unless it is removed from the engine. Fluorescent Penetrant and visual inspections were conducted on fan hubs at the Delta overhaul facility only if they were removed during engine overhaul or disassembly. Delta stated that with a "safe life" part, it is assumed that the part is defect-free on delivery and inspection techniques are selected to find normal wear and tear and abuse not pre-existing damage. Delta also stated if they were required to assume pre-existing damage was present then an alternate inspection technique such as eddy current or ultrasonic would have been employed. Safe life assumes the part will operate safely throughout its design life without an inspection.

Human Error

The fan hub fatigue crack was large enough to have been detectable during the accident hub's last fluorescent penetrant inspection at Delta.

Volvo production inspection personnel found indications during performance of Blue Etch Anodize inspection. Subsequent screening of this indication was to employ a geometric inspection (i.e. use of stylus probe). The defect in question was identified as a material anomaly, not a geometric indication.

Flawed Assumptions

Drilling damage in this accident hub extended much deeper into the tierod hole sidewall material than previously anticipated by Pratt & Whitney.

Preexisting failures

It was determined that the accident fan hub was produced and released into service with a manufacturing induced defect caused by a drilling operation that resulted in altered microstructure and ladder cracking in a tiebolt hole sidewall. Although the altered microstructure was detectable by blue etch anodize inspection methods by Volvo, they did not identify it as rejectable because the appearance of the tierod hole did not match any of the existing inspection templates showing rejectable conditions.

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