Boeing Details CFM56-7 Nacelle Inlet And Cowl Redesign Effort

Boeing has finalized its planned redesign of 737 Next Generation nacelles to make them less susceptible to breaking apart and causing airframe damage when fan blades fail, but has asked for more time to validate specific maintenance-related failure scenarios.

• The OEM is redesigning older 737 nacelles
• The effort is linked to two accidents in which parts damaged airframes
• The NTSB recommended the redesign based on its investigations

In a July 19 regulatory filing, Boeing confirmed that it would meet the FAA’s July 31 deadline for submitting design changes and issuing service bulletins related to the redesign. The changes are needed to address issues spotlighted in two fan blade out (FBO) events, both involving CFM56-7B-powered Southwest Airlines 737-700s, that led to unexpected aircraft damage and one fatality caused by pieces of nacelles breaking free.

The NTSB in 2019 recommended that the FAA require redesigns of the 737NG-family nacelle that would factor in analytical modeling developed since the aircraft was certified in the 1990s, and then follow up to make sure the changes are retrofitted onto the 737NG fleet. Boeing agreed and has been working with the FAA on the issue for nearly four years.

In August 2021, Boeing asked for seven years to develop and certify the new design. The FAA formally responded in July 2022 by breaking the proposed program up, giving the company a year to address agency concerns about the proposed design and develop baseline retrofit instructions.

“By July 31, 2023, Boeing will have submitted all design changes to the FAA and will have released service bulletins to provide inlet modifications, fan cowl modifications, fan cowl support beam modifications and exhaust structures modifications for retrofit of these design changes,” the company said in its most recent FAA filing.

“However, Boeing is aware of a limited number of incidents in which operator maintenance errors . . . have led to fan cowls and [integrated drive generator] doors departing during takeoff or landing,” the filing said. “Therefore, Boeing is submitting this request for an additional extension of the previously granted exemption to allow Boeing to work with operators and the FAA to address these undetected maintenance errors and ensure full compliance.”

Boeing has asked to be allowed 17 months to analyze the in-service incidents and develop a risk-mitigation strategy. “The additional time will allow Boeing adequate time to develop appropriate solutions to address the potential for these maintenance errors and have scheduled discussions with the FAA,” the company wrote.

The nacelle redesign must demonstrate compliance with applicable Part 25 regulations, the FAA says. Among the regulations is a requirement to show “no single failure or combination of failures will jeopardize the safe operation of the airplane,” unless the failure in question is both a structural part and one calculated as being “extremely remote.” This regulation plus another concerning extended operations (ETOPS) are the ones related to the possible maintenance mishaps that Boeing said it needs the additional time to satisfy.

Fan cowl door-related incidents have plagued airlines for years. Many of them were linked to designs or procedures that did not take human factors issues into account, such as latch designs that made it difficult to notice when a door was not properly fastened. The issue, common to all mainline narrowbodies but more prevalent on Airbus A320-family models, led the European Union Aviation Safety Agency to introduce a new special condition covering engine cowl retention and ensuring single points of failure, such as unlatched doors, do not lead to cowl losses.

Boeing’s design changes are part of a multipronged strategy to address safety concerns raised by the two accidents involving CFM56-7B-powered Southwest 737-700s that started with broken fan blades. Engine-maker CFM also revised the inspection requirements and fan blade life limits.

“Taken together, these service bulletins, design changes, increased fan blade inspections and life limit for fan blades protect against future FBO events and mitigate any impact of an FBO event, should one occur,” Boeing told the FAA. “Consequently, the FBO-related design issues have been resolved. The incorporation of the fan blade inspection along with the FBO modifications would significantly reduce the likelihood of occurrence of an in-service failure of an engine fan blade to a level consistent with engines of the same generation that are currently operational on other transport category airplanes.”

In each accident, cracked fan blades fractured, triggering a series of events that included parts of the inlet structure breaking away. In the second event, an April 2018 accident that led to an emergency landing in Philadelphia, parts of the fan cowl structure also broke off.

Investigators determined that the blade struck the fan case and transmitted loads to a latch mechanism. Part of the latch broke away, struck a window and dislodged it, causing a rapid decompression and leading to a passenger fatality.

Boeing’s changes address risks flagged during investigations of the two accidents plus an internal analysis using the company’s safety management system. The seven-year request would give Boeing time to develop and roll out design changes as quickly as possible without having to comply with Part 25 rules that cover an entire system—in this case, the nacelle.

By definition, Boeing’s new nacelle design would not comply until all changes are made. Forcing operators to wait and implement all changes at once, instead of as they are available, would mean incremental safety benefits of individual improvements would not be realized, Boeing contended.

“By allowing separate approval of these modifications using this time-limited exemption, U.S. operators have the flexibility to incorporate the modifications independently and earlier than waiting for all design changes to be available,” the FAA said in its July 2022 granting of the partial exemption. “Granting this time-limited exemption also makes these modifications available for the FAA to mandate under potential airworthiness action, if found necessary.”

Boeing’s proposed changes would affect engine inlets, fan cowls and exhaust nozzles (AW&ST July 26-Aug. 8, 2021, p. 26). The company plans to add a spacer to the inlet attachment bolts to improve structural integrity. It says it also will upgrade the inlet aft bulkhead fasteners.

Fan cowls will get modified radial restraint fittings and added venting to help ensure they stay closed, intact and attached to the aircraft during FBO events, Boeing says. Fan cowl support beam bolts are also being upgraded. Finally, Boeing says it will change the exhaust nozzle attachment area structure to improve its load handling ability.

“Boeing has developed a set of design changes which are intended to keep the nacelle structure from separating in the event of fan blade failure by providing additional structural support for the loads resulting from such a failure,” the FAA said in its August 2022 response to Boeing’s design proposals.

The failures, coupled with a similar series of events that affected Pratt & Whitney-powered 777s, prompted the NTSB to urge regulators to reexamine how parts such as nacelles are certified alongside their engines (AW&ST March 8-21, 2021, p. 24).  An FAA review of its Part 25 large-airplane certification rules concluded that new guidance, but no rule changes, were needed, the agency told the NTSB in a July 2022 update on its work.

The NTSB was not convinced. “We would like to know if the specifications contained in Part 25 related to protecting the airplane against a fan blade off event explicitly address the structural integrity of secondary structure elements (including engine nacelle elements) and, if not, how you intend to address this concern,” the board told the FAA in a December 2022 response.

No FAA follow-up has been made public.