WASHINGTON—The FAA plans to mandate three modifications to Boeing 737 Next Generation (NG) nacelles linked to a pair of inflight fan-blade failures that caused significant damage outside the engine and led to one passenger fatality.
In written correspondence between the FAA and NTSB, the regulator said it will “issue three separate airworthiness directives (AD) to address mandating corrective actions for the modifications of the inlet, fan cowl, and exhaust nozzle.” The letter, sent in April 2021, was recently made public and has not been previously reported. “The FAA is working with Boeing to ensure that the design changes will address the most critical fan blade impact locations for each area of structure,” the agency added.
Boeing is finalizing service bulletins (SB) for the inlet and fan cowl that details changes operators will make and could release them as soon as July.
The exhaust nozzle modification timing is less clear.
“We are working with Boeing on a schedule for the SB on the exhaust nozzle,” the letter said. “As they publish the SBs, the FAA will release a notice of proposed rulemaking AD to mandate each corrective action,” the agency added.
If adopted by other regulators, the mandates could affect 6,740 737NGs of all types in the global fleet, including 576 in storage, Aviation Week Intelligence Network Fleet & Data Services shows. Commercial production of 737NGs ended in 2019, but a version of the 737-800 is still produced for modification into P-8 maritime patrol aircraft used by the U.S. and other countries.
“We are working with Boeing to determine if the redesigned fan cowl structure is appropriate for installation in production on baseline airplanes that will be modified into P-8 airplanes,” the FAA wrote.
Changes to the nacelle assembly were recommended in November 2019 by the NTSB and agreed to by Boeing following probes into the accidents—both involving Southwest Airlines 737-700s powered by CFM56-7B engines. The second accident, Southwest Flight 1380 in April 2018, triggered a chain of events that led to a cabin depressurization and the fatality—the first onboard a U.S. scheduled mainline passenger operator since 2009.
In both cases, fan blades cracked near their roots and fragments spiraled forward, starting a chain of events that caused parts of the engine inlets to break away. The debris caused wing and fuselage damage, and each cabin depressurized.
In the first accident, near Pensacola in August 2016, the aircraft suffered a 5-in-by-16-in hole in the fuselage above the left wing. The fan cowl stayed intact, and the passenger cabin was not penetrated.
In the April 2018 accident, a chunk of the fan cowl from the left-aft area of the engine struck the left-side fuselage and broke the window, investigators determined, though some—including engineers from supplier Collins, disputed some of the NTSB’s conclusions.
“The NTSB’s investigation did not establish conclusively that any portion of the fan cowl struck the window and directly caused the window’s departure,” Collins wrote in a July 2019 submission to the NTSB. “This also contrasts with the Pensacola event, where debris struck the fuselage near a window, yet that window did not depart.”
Regardless of the specific chain of events, damage outside the engine was significant enough in both incidents to raise concerns, especially considering Flight 1380 was classified as a contained failure, meaning debris was not ejected through the engine case.
In that accident, the primary exhaust nozzle stayed attached to the engine, but “exhibited 360-deg circumferential buckling” and was torn in several places, an NTSB analysis found.
The Southwest accidents combined with other recent inflight engine failures with similar event-chain profiles—fractured blades that triggered significant engine nacelle and airframe damage —has led the NTSB to urge regulators to re-examine engine-airframe integration. The Southwest probes spotlighted broader changes needed to incorporate the latest technology and modeling into engine-failure analysis. When the 737NG was undergoing certification nearly three decades ago, protocol for modeling engine-failure damage meant relying heavily on trial and error during tests. Results from a 1994 rig test as part of CFM56-7 development led Boeing to re-design the inlet containment doubler, for instance. The redesign was validated in subsequent testing.
Advances in analytical modeling have changed the process, Boeing propulsion structures expert Torben Syberg explained during a 2018 hearing on Flight 1380. New computer models “enhance our capability to be able to understand the physics of the [blade] impact phase,” Syberg said. “It’s helped us in that understanding, relying on predictions that are validated by test opposed to just waiting until you get the empirical data from the test.”
The new modeling is being used to pinpoint model-specific issues that need addressing now. The FAA in its letter said it “worked with Boeing on a cross-model qualitative safety assessment that included all Boeing commercial airplanes with a similar design. Based on the assessment, the 737NG is the only affected fleet.”
Work on changing regulations and guidance to address future designs—also recommended by the NTSB—is also underway.
“The FAA continues to review [certification] requirements and the associated guidance materials pertaining to airplane and engine level aspects for engine nacelles and containment case integrity following a fan blade out event,” the agency wrote. “We are also supporting engine and airframe manufacturers’ evaluation of current industry practices for analyzing and addressing fan blade failure events. We have focused on identifying critical data needed by the airframe manufacturers to address airplane level hazards associated with a fan blade failure.”
This article has been updated to correct an error concerning how NTSB categorized one of the two 737-700 accidents.
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