The FAA has signed off on Boeing’s proposed 737 Next Generation nacelle retrofit program, setting an August 2028 deadline for operators to complete the modifications, and giving the company until the end of 2029 to deliver maintenance error mitigation plans linked to the new designs.

Under Boeing’s plan, it will finalize service bulletins detailing the modification work by the end of 2024. Operators will then have until July 31, 2028, or another 43 months, to retrofit their fleets.

The FAA formalized the plan via an exemption granted to Boeing earlier in September. The step was needed so Boeing can roll out the changes incrementally, instead of getting the entire redesign approved as a system before retrofits begin. This gets the individual modifications into the fleet more quickly.

Boeing’s redesign focuses on three main areas: new inlet spacers and fasteners, a fan cowl support beam, and a stiffer exhaust nozzle. The combination is meant to make nacelles less susceptible to breaking apart if a fan blade fails. Boeing agreed to make the changes following two CFM56-7B engine failures involving Southwest Airlines Boeing 737-700s, including one in April 2018 with a passenger fatality.

Boeing filed its initial exemption request in July 2021. The company asked for seven years to develop the modifications and get them into the fleet.

The FAA exemption adopts this schedule, with one notable change. Boeing has until the end of 2029 to deliver “solutions to address potential maintenance errors,” such as ensuring fan cowl doors are always secured before flight.

Regulators around the world are likely to mandate the retrofits as well, setting the stage for as many as 6,500 aircraft to undergo modifications. The Aviation Week Network Fleet Discovery database shows 6,300 737NGs of all types in service and another 230 in storage.

Boeing asked for the extra time earlier in 2023, citing an FAA “policy letter” sent in response to a company query. The letter, which has not been made public, relates to new system safety assessment (SSA) requirements the FAA is introducing. Among the myriad changes is reclassifying some human factors-related failure modes as unsafe system operating conditions that must be addressed—and mitigated—by manufacturers. An unlatched fan cowl door is one example.

The exemption also gives Boeing an additional 17 months, or until the end of 2024, to ensure its proposed designs can comply with the new SSA requirements before finalizing them and the related modification instructions.

“Boeing’s requested extension of the exemption maintains the level of safety of the existing exemption by ensuring operators will have service instructions available to begin implementing design changes without change to the original schedule,” the FAA said in granting the exemption. “The revised exemption will allow both Boeing and operators more time to address appropriate solutions to potential maintenance errors, further improving the level of safety of the airplane.”