Boeing is developing a series of nacelle improvements for grounded Pratt & Whitney PW4000-powered 777s and wants FAA’s blessing to phase them in as they are finalized, instead of delivering a complete, compliant nacelle structure as required by the agency’s certification rules.
In a petition to the FAA, Boeing said its design changes focus on three general areas—fan cowl, inlet, and engine modifications. Meeting applicable FAA regulations for “fan blade out” events requires showing every part of a nacelle design complies with the rules.
“Thus, a showing of compliance for any one part of the structure requires a showing for all, as all are affected by the change,” Boeing said. “A fully compliant design requires simultaneous certification of all changes. The exemption is required to allow certification of incremental improvements.”
Boeing has requested “partial” exemption from two regulations, one special condition, and FAA’s requirement that the final design must comply “with the requirements of part 25 considering the maximum flight time and the longest diversion time for which the applicant seeks approval.”
The agency has formally acknowledged receipt of the petition and plans to publish it on Oct. 14. The public will have 20 days to comment.
The changes stem from three similar fan-blade-out events involving a Japan Air Lines PW4000-powered 777-200 and two PW4000-powered United Airlines 777-200s—including a February 2021 occurrence near Denver, Colorado that led regulators to ground all 128 Pratt-powered 777s in the global fleet. The aircraft remain grounded, awaiting required fixes.
Investigators have determined that each United event started with cracked fan blades that resulted in parts of the affected engine’s nacelles breaking away and causing airframe damage.
Pratt has developed revised inspection procedures for the blades, including a new ultrasonic inspection that complements its proprietary thermal acoustic imaging process that has not been sufficient, sources with knowledge of the situation told Aviation Week.
United and other affected operators are awaiting both the new inspection protocols and regulators’ decisions on nacelle modifications to get their aircraft back in service. United has the most affected aircraft, with 52, and has expressed its desire to keep them in its fleet. A few other operators, including Japan Airlines, accelerated planned retirements rather than wait on regulators, Boeing and Pratt.
Pratt also is developing enhancements to help protect against engine fires, including a flange separation, the Boeing petition revealed. In the most recent event, “the airplane experienced loss of flammable fluid and core compartment fire,” Boeing said.
The inlet modifications “will include ballistic shielding, metallic reinforcement added to the composite aft bulkhead and reinforcement in the region of the inlet that interfaces with the fan cowl and fan cowl support beam,” Boeing said. They are designed to help “ensure nacelle retention” and “provide structural support for the fan cowl and fan cowl support beam for loads resulting from” a fan-blade failure.
Fan cowl modifications are still being developed. Among the considerations: strengthening of the fan cowl support beam and either modifying or redesigning composite panels.
Boeing expects its design changes to be mandated via FAA airworthiness directives.
“All planned design changes are required to show full compliance to the regulations listed” in its exemption request, Boeing said. “However, due to the complexity of changes required, not all design changes will be available simultaneously ... Without an exemption this system level showing could only be made after all of the design changes were available, which would delay the associated service bulletin and hardware kit availability dates. This delay would impact operator capability to plan for and achieve a systematic and nominal incorporation of each change in order to meet the anticipated mandated compliance period of the planned airworthiness directives.”
Boeing is requesting a similar phase-in of nacelle modifications for the 737 Next Generation family. The changes stem from two fan-blade failures involving Southwest Airlines 737-700s that led to nacelle and airframe damage, as well as one passenger fatality. The NTSB as part of its probes into the failures recommended that Boeing redesign the nacelles. Boeing told the FAA in an August filing that it is on track to have a series of service bulletins outlining the fixes ready in 2022.
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