FAA-mandated Boeing 737NG engine nacelle changes
The FAA has issued a new airworthiness directive (AD), effective from 8 April, focusing on ensuring Boeing 737NG engine nacelles can better withstand the impact of a CFM International CFM56 engine fan blade failure.
The AD was prompted by two separate engine fan-blade-out (FBO) events “that resulted in the separation of engine inlet cowl and fan cowl parts from the airplane damaging the fuselage, which caused loss of pressurisation and subsequent emergency descent,” explained the FAA.
Building on an earlier notice of proposed rulemaking (NPRM), published in December 2023, the AD will affect some 1,215 US-registered aircraft including 737-600, -700, -700C, -800, -900 and -900ER variants. Certain 737NG models may have an affected primary exhaust nozzle, which will require installation of bridge brackets as a mitigating measure – “or, as an option, an installation of a serviceable primary exhaust nozzle”.
“During an FBO event, primary exhaust nozzles that are not strengthened could depart the engine, potentially damaging a stabiliser or striking the fuselage and window,” continued the AD. Indeed, this was the case during the fatal 2018 engine failure of Southwest Airlines flight 1380, in which a CFM56-7B-powered 737-800 experienced a fan blade failure after departing New York’s LaGuardia Airport.
“The fan blade failure resulted in the loss of portions of the inlet and fan cowl. Fragments from the fan cowl struck the fuselage, a cabin window departed the airplane, and rapid depressurization occurred,” concluded the National Transportation Safety Board (NTSB) in November 2019. Of the 149 people aboard, one passenger suffered fatal engines while a further eight experienced minor injuries.
“This accident demonstrates that a fan blade can fail and release differently than that observed during engine certification testing and accounted for in airframe structural analyses,” added then-NTSB Chairman Robert Sumwalt, speaking in 2019. “It is important to go beyond routine examination of fan blades; the structural integrity of the engine nacelle components for various airframe and engine combinations needs to be ensured”.
Boeing submitted design developments to the FAA in 2022 as part of an exemption request, and the FAA has already issued three NPRMs in September 2023; requiring Boeing to develop modifications to the inlet cowl, fan cowl, and exhaust nozzle for operators to incorporate by 31 July 2028. Under the new AD, specified inlet cowl bulkhead fasteners must be replaced, along with the crushable spacers “used in the attachment of the inlet cowl to the engine fan case; or, as an option, installing a serviceable inlet cowl,” concluded the FAA.
