FAA mandates CFM56 engine nacelle modifications after fatal Southwest incident  

The Federal Aviation Administration (FAA) is to issue airworthiness directives (ADs) to US airlines concerning the engine nacelles on older generation Boeing 737 airplanes. It is hoped that the issuance of the ADs will prevent a recurrence of an incident in 2018 in which an engine suffered an uncontained engine failure on one of its Boeing 737-700s and another earlier incident in 2016 involving the same type and the same carrier.

In the April 2018 incident, a Southwest Airlines Boeing 737-700 experienced an engine failure after departing New York-La Guardia Airport (LGA) for Dallas-Love Field Airport (DAL). As a result of the engine failure, fan blades and other components exited the engine nacelle and punctured the aircraft’s fuselage.

A cabin window was shattered in the process, which caused a loss of cabin pressure and the death of a passenger who was partially ejected from the aircraft and sustained fatal injuries. Eight other passengers sustained minor injuries, and the aircraft later made an emergency landing in Philadelphia. The aircraft was substantially damaged and was eventually written off following the accident.

The previous incident in August 2016 also involved a Southwest Airlines Boeing 737-700 operating a flight from New Orleans to Orlando, which suffered an uncontained engine failure 12 minutes after take-off. A fan blade in the engine broke due to a fatigue crack. The separated portion of the blade rotated within the engine, moving forward and struck the engine inlet. Debris from the damaged engine inlet subsequently punctured the left side of the fuselage, causing a loss of cabin pressure and damage to the aircraft wing and empennage. While no injuries occurred, the aircraft was forced to make an emergency landing in Pensacola.

Issued on March 3, 2025, the three FAA ADs refer to specific components of the CFM International CFM56 engines fitted to Boeing 737 Next Generation airliners, including 737-600s, 737-700s, 737-800s, and 737-900s, of which there are still around 2,000 US-registered 737NGs still flying. The ADs address how certain modifications can be made to boost the structural integrity of the engine nacelle in the event of an uncontained engine failure in the future.

The CFM56 turbofan engine inlet fan is surrounded by strengthened containment rings. These rings are supposed to prevent high-energy engine components such as fan blades from exiting the engine and its surrounding nacelle casing structure and damaging other parts of the aircraft structure.

The FAA is requiring that US-based operators of Boeing 737NG family aircraft make these medications to add an additional level of protection for those flying on such aircraft. Specifically, airlines must replace engine-cowl fasteners (which lock in place the cowling to the nacelle frame), modify “radial restraint” assemblies, and replace the crushable spacers that are used to attach interconnecting inlet cowls to engine fan cases. In the past, the FAA has said that these spacers can reduce fan-blade impact energy that will be transmitted into the inlet structure.

In addition to the above measures, 737NG operators will be required to install bridge brackets to reinforce primary exhaust nozzles on some CFM56 engines. According to hr FAA, the use of such brackets can prevent exhaust nozzles from becoming loose or detaching entirely from the engine during catastrophic failures, which can detach and cause further damage to the aircraft fuselage, tail section, or horizontal stabilizer.   

“We support the FAA’s final airworthiness directives that make mandatory a set of service bulletins that Boeing issued to Next Generation 737 operators in mid-2024,” said a Boeing spokesperson. “These ADs outline actions that will increase protections in the event of an engine-fan-blade-out situation. Airlines can continue safely operating the fleet with interim actions until these permanent modifications are made.”

It is understood that Boeing has already issued a series of Special Attention Requirements Bulletins, which require operators to address the issues highlighted by the FAA’s ADs concerning CFM56 engines on 737NG models. While the Southwest Airlines 2018 fatal incident was the most serious involving the failure of a CFM56 engine, there have been others.

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