FAA Initiates Mandatory Retrofit Process for Boeing 737NG Nacelle Design Changes
The Federal Aviation Administration (FAA) has embarked on a crucial process to mandate nacelle design modifications for the Boeing 737 Next Generation (NG) fleet, a move spurred by a tragic accident and subsequent recommendations from the National Transportation Safety Board (NTSB). These alterations are anticipated to become global mandates.
Three notices of proposed rulemaking (NPRMs), slated for release on December 11, will formalize a comprehensive plan that Boeing has collaborated on and which received FAA approval over the past two years.
The FAA’s proposed timeline allows for 45 days of public input on these modifications. Subsequently, the final regulations will require affected operators to implement the changes by July 31, 2028. Additionally, Boeing is tasked with developing and issuing maintenance instructions related to mitigating risks associated with unlatched fan cowl doors and other access panels by December 31, 2029.
Boeing initiated these changes and presented them in an exemption request submitted to the FAA in 2022. These exemptions are pivotal for Boeing to introduce the modifications in phases, demonstrating compliance with regulations once all upgrades are completed. This approach accelerates the integration of retrofits into the fleet compared to waiting for simultaneous approval and implementation of all changes.
The design enhancements target issues highlighted in extensive reviews following two incidents involving Southwest Airlines’ 737-700 aircraft, where fan blades failed, causing damage to the fuselage. One of these incidents, in April 2018, resulted in a passenger fatality. Subsequent findings from investigations led to the NTSB’s recommendation for a redesign of the 737NG nacelle.
Boeing’s plan encompasses three key modifications: new inlet spacers and fasteners, a fan cowl support beam, and a reinforced exhaust nozzle.
Inlet attachment bolts will be equipped with spacers to bolster structural integrity. Upgrades have been devised for inlet aft bulkhead fasteners.
Fan cowls will undergo alterations with the introduction of modified radial restraint fittings and enhanced venting. These changes aim to ensure fan cowls remain closed, intact, and securely attached to the aircraft in the event of a fan blade failure.
The structural area surrounding the exhaust nozzle attachment will be fortified to enhance load-handling capabilities.
Boeing has committed to delivering all necessary service information to operators by the end of 2024. Furthermore, the company must satisfy the FAA that the resultant nacelle “system” aligns with Part 25.901(c), which stipulates that any single failure or combination of failures, excluding “extremely remote” events, should not pose unacceptable risks.
The FAA’s mandate directly impacts approximately 1,900 U.S.-registered aircraft. Nevertheless, these upgrade requirements are anticipated to become a global standard. According to the Aviation Week Network Fleet Discovery database, there are currently 6,500 737NGs in operation or storage worldwide. Some of these may be retired before the expected 2028 upgrade deadlines.
Sources: AirGuide Business airguide.info, bing.com, aviationweek.com