Boeing 777 Nacelle Changes Expand To Thrust Reversers

flight 328 failure
Credit: NTSB

Boeing has expanded its proposed nacelle changes for Pratt & Whitney-powered 777s to include thrust reverser modifications, the company told the FAA in a regulatory filing.

The manufacturer said in a progress report that it “has completed comparative risk assessments that support” a request to phase in the nacelle modifications, filed in August. The assessments “have considered the effect of the nacelle modifications, including an additional thrust reverser-related modification,” Boeing said. 

Boeing did not provide additional details in its filing. 

Design changes to the reverser include additional "ballistic shielding," Boeing said in response to an inquiry. "A service bulletin for the mod will be ready when the exemption is approved," the company added.

The latest filing backs Boeing’s request to phase in several changes to PW4000-powered 777 nacelles in response to concerns raised by fan blade failures, including one in February 2021 that has left the global fleet grounded. The broken fan blades caused damage to the engine nacelles, which cascaded into airframe damage when pieces broke away from the structure surrounding the affected engines.

Pratt has developed new inspection protocols to address the primary cause of each incident—undetected fan blade cracks. The ancillary damage prompted the FAA to order changes to nacelle structure.

Boeing, in its August petition to the FAA, outlined proposed changes to the fan cowl and inlet, as well as engine modifications that Pratt is developing. 

Under FAA certification regulations, the entire nacelle must stand up to required testing as a single system. Boeing’s petition requests its proposed changes to be phased in as they are developed, which would preclude testing all the modifications together. 

Boeing’s approach means safety improvements will get into the fleet more quickly, but it also requires FAA’s blessing to allow the fleet to operate for a set period of time before the new nacelle design can be certified. Boeing’s petition requests five years for regulatory approval and subsequent phase-in to affected aircraft.

Boeing’s structural and risk assessments looked at the ramifications of a fan blade failure on the proposed design changes “prior to implementation of all modifications required to show full compliance,” the filing said. Boeing said it provided results to the FAA in a proprietary document.

This story has been updated with information 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.