The Federal Aviation Administration has finalised airworthiness directives (ADs) aimed at ensuring Boeing 737NG nacelle and engine components can better withstand forces generated by CFM International CFM56 fan-blade failures.
Released on 3 March, the three orders respond to two incidents involving CFM56 failures that caused nacelle components to break loose and cause broader aircraft damage. One of those incidents, in 2018, killed one passenger on a Southwest Airlines 737NG.
The orders affect some 2,000 US-registered 737NGs, including -600s, -700s, -800s and -900s.
“The FAA is issuing this AD to address fan cowls that are not strengthened, which could, in the event of [a fan-blade out] occurrence, depart the nacelle, potentially damaging a stabiliser… [or] strike the fuselage and window,” says one the FAA’s new orders.
The agency is now requiring that carriers modify several nacelle and cowling components on 737NGs, which are powered solely by CFM56 turbofans. They must do so as detailed in Special Attention Requirements Bulletins already published by Boeing.
Specifically, airlines must replace engine-cowl fasteners, modify “radial restraint” assemblies and replace “crushable spacers” used in attachments connecting inlet cowls to fan cases. The FAA has previously said such spacers can reduce “fan-blade impact energy that will be transmitted into the inlet structure”.
Operators of 737NGs must also install “bridge brackets” to reinforce primary exhaust nozzles on some CFM56 engines. The agency says brackets can help prevent nozzles, during engine failures, from breaking lose and damaging an aircraft’s fuselage or stabiliser.
CFM, which does not make the nacelles, defers comment to Boeing.
“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,” Boeing says. “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.”
Turbofans are surrounded by containment rings designed keep engine components, during failures, from exiting the engine and its surrounding structure, preventing the components from causing more damage.
But the two CFM56 failures that prompted the new rules involved blade failures that caused nacelle components to detach and strike other parts of the aircraft. In the case of the 2018 Southwest incident, the components struck and broke a window. One passenger died after being sucked partly out of the aircraft.
The 2016 incident, which also involved a Southwest 737-700, did not cause injuries.
In response to the events, Boeing agreed to develop modification the 737NG’s inlet cowl, fan cowl and exhaust nozzle. The FAA has set a date of 31 July 2028 for operators to incorporate the modifications.
Story updated on 3 March to include comments from Boeing.
