The FAA in late December issued draft rules that detail return-to-service requirements for Pratt & Whitney-powered Boeing 777s grounded for nearly a year, adopting Boeing-recommended nacelle modifications and new Pratt-developed fan blade inspection protocols.
The three draft airworthiness directives (AD) propose mandating fan blade inspections, nacelle inlet modifications and changes to thrust-reverser components. The modifications and initial fan blade inspections must be done before the affected fleet—grounded since mid-February of 2021 following the third event involving a fractured fan blade that caused airframe damage—can return to service. The directives also call for repetitive inspections for the fan blades and hydraulic pump shutoff valves, meant to reduce flammability risk during an engine fire.
The revised maintenance protocol for the fan blade—called the first-stage low-pressure compressor (LPC) blade on the PW4000 series—adds ultrasonic inspections to Pratt’s existing, proprietary thermal acoustic imaging (TAI) method. Under the revised program, ultrasonic checks of each blade’s flow path region—the area nearest the blade root—are required every 275 flight cycles (FC), while inspections of two “mid-span” areas farther out from the root are done every 550 FC. All blades undergo a TAI every 1,000 FC. Damaged blades must be replaced at an estimated cost of $125,000, the draft directive said.
The instructions were finalized with FAA input in the fall of 2021 and communicated to operators in an Oct. 15, 2021, service bulletin.
“The FAA has been informed that [Pratt] has done some outreach with affected operators regarding the proposed corrective actions for this unsafe condition,” the FAA said. “As a result, affected operators are already aware of the proposed corrective actions and, in some cases, have already begun implementation of the updated inspections on the first-stage LPC blades proposed by this AD.”
The inlet modification includes “adding ballistic shielding and support structure to the inlet outer barrel, revising the outer cowl aft-row fasteners, adding support structures to the aft bulkhead, and revising the inlet attach-ring to A-flange engine bolts and associated barrel nuts,” the draft directive said. Operators also must inspect the outer barrel for “moisture ingression,” thought to have contributed to parts of the inlet composite structure breaking away during the three events. Boeing first flagged the potential issue in a Dec. 7 filing to the FAA that recommended inspections for possible “fluid ingression” in certain composite panels. Neither Boeing nor the FAA have detailed the type of damage or moisture involved. Estimated per-engine cost for the inlet modifications is $418,600, including $362,500 in materials.
The third directive would require installing debris shields on thrust--reverser inner walls, inspecting fan cowl doors for moisture damage and performing the repetitive hydraulic-pump checks. Estimated cost for the modification is $14,500, including $4,300 in parts.
The global fleet totaled about 130 when the grounding occurred, with most—52—flown by United Airlines. United plans to work them back into its network, while other operators, including Japan Air Lines, have accelerated plans to retire their aircraft. Cargo and charter specialist Eastern Airlines purchased some during the model’s grounding—the only other U.S. carrier with Pratt-powered 777s in its fleet.
The engine fan blade-out events—two involving United Airlines 777-200s, in February 2018 and February 2021, and one on a Japan Air Lines 777-200 in December 2020—spotlighted unrecognized risks of airframe damage caused by parts of the nacelle breaking away. Boeing, Pratt and the FAA have been working for months to develop specific changes for the affected 777 fleet, with the agency pressing Boeing on how its proposed changes address the issues and whether they introduce any new risks.
“Boeing’s [safety management system] process has identified a set of all known root causes of the relevant fan blade-out events and resulting impacts to the engine nacelle,” the company wrote in a Dec. 17 letter to the FAA. “Boeing completed a comparative safety assessment showing that the implementation of each modification, and the combination of modifications proposed for certification, prior to return to revenue service will reduce risk of an unsafe outcome as compared to the unmodified design.”
Boeing, CFM and the FAA are working on similar changes targeting the 737 Next Generation (NG) series.
In each case, Boeing is proposing phased-in changes that improve the nacelle design (Inside MRO November 2021, p. 5). The proposed directives are the first set for the Pratt-powered 777 family and target the highest risks, while similar steps for the 737NGs—which, unlike the PW4000-powered 777s, were not grounded following in-flight fan blade-out events—are expected out by mid-2022.
Besides addressing specific aircraft/engine combinations, the FAA is also looking at broader ramifications on nacelle certification requirements.