Boeing told to redesign 737 NG part after Southwest accident
Following a lengthy investigation into the deadly Southwest Airlines flight 1380 in April 2018, the U.S. National Transportation Safety Board (NTSB) urged Boeing on November 19, 2019, to redesign the fan cowl on all 737 Next Generation family aircraft and operators of the type to retrofit their airplanes with the new design. This comes after investigators found the fan cowl structure on the 737NGs to be more “susceptible” to failure.
The NTSB has determined that the engine failure on the Southwest Boeing 737-700 (N772SW) was caused by a broken fan blade from one of the two CFM International CFM-56-7Bs powering the aircraft.
As part of its probable cause, the board noted that the accident occurred when portions of the (left) fan cowl separated in-flight. A fan blade (No. 13), which had fractured due to a low-cycle fatigue crack, impacted the engine fan case at a location that was critical to the structural integrity of the fan cowl.
The separated fan blade impacted the engine fan case and fractured into multiple fragments, some of which traveled forward of the engine and into the inlet. The impact to the fan case also imparted significant loads into the fan cowl through the radial restraint fitting, which is what caused the fan cowl to fail, investigators described.
The fractured fan blade led the engine inlet and fan cowl to break apart, subsequently damaging the airplane’s fuselage and resulting in rapid cabin depressurization. Fragments of the fan cowl struck the fuselage near a cabin window – one passenger died after being partially sucked out of the blown-out window. Eight others suffered minor injuries.
The accident happened during climb after Southwest flight 1380 departed New York’s LaGuardia Airport (LGA), bound for Dallas Love Field (DAL), Texas on April 17, 2018. The flight crew were forced to divert and make an emergency landing at Philadelphia International Airport (PHL). There were 144 passengers and five crew members on board.
NTSB reveals findings
As a result of the investigation, the NTSB issued seven new safety recommendations: five were sent to the U.S. Federal Aviation Administration (FAA), one to the European Union Aviation Safety Agency (EASA), and one to Southwest Airlines. The recommendations primarily address “the need to ensure the structural integrity” of the fan cowl on Boeing 737 NG airplanes and assess whether other airframe and engine combinations have critical fan blade impact locations.
“This accident demonstrated the susceptibility of the fan cowl installed on Boeing 737 next-generation-series airplanes to a fan-blade-out impact location near the radial restraint fitting and the effects of such an impact on the structural integrity of the fan cowl,” the NTSB revealed in its findings.
“Given the results of CFM’s engine fan-blade-out (FBO) containment certification tests and Boeing’s post-accident structural analyses of the effects of an FBO event on the airframe, the post-FBO events that occurred during this accident could not have been predicted.”
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