Boeing has completed redesign work and retrofit instructions on 737 Next Generation nacelles to better protect them from broken fan blades but needs more time to address risks linked to human-factors issues such as leaving access doors unlatched, the company said.

In a July 19 regulatory filing, Boeing confirmed that it will meet 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 Southwest Airlines 737-700s, that led to unexpected aircraft damage caused by pieces of nacelles breaking free. 

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

Last August, Boeing asked for another seven years to develop and certify the new design, which it proposed in August 2021. The FAA responded by breaking the 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 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. 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 for 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 said.

The nacelle redesign must demonstrate compliance with applicable Part 25 regulations. Among them 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, or ETOPS, are the ones related to the possible maintenance mishaps that Boeing said it needs the additional time to satisfy.

Boeing’s work is in addition to revised inspection requirements and fan blade life limits put in place by engine maker CFM to address FBO risks spotlighted by the two accidents—one of which resulted in a passenger fatality. 

“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 internal analysis using the company’s safety management system.

“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.