United Airlines Boeing 777-200
A new FAA mandate requires more frequent inspections of certain Pratt & Whitney PW4000-series low-pressure compressor (LPC) fan hubs and broadens the population of parts that need the checks.
Set for publication Oct. 3 and based on manufacturer-recommended guidance, the mandates change the recurrent eddy current inspection (ECI) intervals on affected parts to 550 flight cycles (FC). A 2018 mandate still in place has the interval at 2,000 FC. The new requirements supersede the 2018 directive.
The new inspection protocols cover any LPC fan hubs installed on 112-in fan variants: the PW4074, PW4074D, PW4077, PW4077D, PW4084D, PW4090, and PW4090-3 engines—all of which power Boeing 777s.
Pratt’s previous guidance, mandated by FAA, targeted only two part numbers. The revised inspections also eliminate required fluorescent penetrant inspections.
“The FAA determined that affected LPC fan hub assemblies can meet the published certificated life limit without the need for the required repetitive FPI inspections ... and the repetitive ECI inspections require shortened intervals,” the agency said.
Pratt first alerted operators of its recommended changes in a January 2022 service bulletin. The manufacturer issued an updated bulletin this past August.
Expansion of the affected disk population was based on information gleaned from the probe of a 2017 Air France A380 engine failure over Greenland.
“Investigation of this uncontained failure revealed that, due to the similarity of design and material processing for the LPC fan hub assembly, the ECI inspections should be done on all LPC fan hub assembly part numbers installed,” the agency said.
The French BEA-led probe into the accident concluded that a crack developed in a specific area called a macro zone introduced during the part’s forging. The crack formed 1,850 cycles into the part’s 15,000-cycle life and expanded over the next 1,650 due to dwell fatigue until it caused the part to fail.
Dwell fatigue, which caused several premature fan disk failures in the early 1970s, is well understood as a product of a material’s response to constant stress. However, up until the Air France failure there was no evidence dwell fatigue presented hazards to parts made from titanium alloy Ti-6-4, the material in the GP7200 fan hub, because of the material’s properties, the BEA said.
The probe led to a series of inspection protocols on the GP7200 and similar engines.
