FAA Set To Issue Pratt-Powered 777 Service Return Requirements

United Airlines flight 328 cowling debris
Credit: NTSB

WASHINGTON—The FAA will soon release draft final rules that details return-to-service requirements for Pratt & Whitney-powered Boeing 777s, adopting Boeing-recommended nacelle modifications and new Pratt-developed fan blade inspection protocols.

Set for publication Dec. 28, the three draft airworthiness directives (ADs) propose mandating 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 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 new fan blade—or first-stage low-pressure compressor [LPC] blade on the PW4000 series—inspection protocol 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. In addition, 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’s input several months ago 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 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 costs 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, inspection fan cowl doors for moisture damage, and the repetitive hydraulic pump checks. Estimated cost for the modification is $14,500, including $4,300 in parts.

"Boeing supports the FAA’s guidance on inspection requirements for Pratt & Whitney PW4000-112-in. engines and will work with our customers and Pratt & Whitney through the process," Boeing said in a statement. "Airworthiness directives are part of the longstanding rulemaking process by which manufacturers, operators and regulators work together to ensure that the safety of the world’s commercial jetliners continues at the highest levels."

The company declined to say who is paying for the work.

The global Pratt-powered 777 fleet totaled about 130 when the grounding occurred, with most of them—52—operated by United Airlines. The U.S. carrier plans to work them back into its network, while several other operators, including Japan Air Lines (JAL), have accelerated plans to retire their aircraft. Cargo and charter specialist Eastern Airlines has 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 777-200s in February 2018 and February 2021 and one on a JAL 777-200 in December 2020—spotlighted previously 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 U.S. regulatory 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 said 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 has cited improved computer-modeling capabilities as key to gaining a better understanding of how fan blade failures, even so-called contained failures that do not eject debris through the fan case, can affect nacelles and put the airframe at risk.

Boeing, CFM International, and the FAA are working on similar changes that target the 737 Next Generation (NG) series, driven in large part by a 2018 Southwest Airlines accident that killed one passenger.

In each scenario, Boeing is proposing phased-in changes that improve the nacelle design. 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 inflight fan-blade out events—are expected to come in early 2022.

The FAA is also looking at bigger-picture ramifications on how the issues affect nacelle certification. 

This story has been updated with a response from Boeing.

Sean Broderick

Senior Air Transport & Safety Editor Sean Broderick covers aviation safety, MRO, and the airline business from Aviation Week Network's Washington, D.C. office.

Guy Norris

Guy is a Senior Editor for Aviation Week, covering technology and propulsion. He is based in Colorado Springs.