A fractured fan blade powering a Southwest Airlines Boeing 737-700 was the cause of an accident in 2018, the National Transportation Safety Board (NTSB) determined during a public board meeting.
The malfunction to the fractured fan blade led to the engine inlet and fan cowl separating and subsequently damaging the fuselage, resulting in a rapid cabin depressurisation.
One passenger died and eight others suffered minor injuries on April 17, 2018, when the fractured fan blade impacted the fan case, causing fan cowl fragments to strike the airplane’s fuselage near a cabin window.
The window departed the airplane, and the cabin rapidly depressurised.
The accident happened after Southwest Airlines flight 1380 departed New York’s LaGuardia Airport, bound for Love Field, Dallas, Texas. The flight crew conducted an emergency descent and diverted to Philadelphia International Airport. There were 144 passengers and five crew members aboard.
NTSB chairman Robert Sumwalt said: “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.
“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.”
The NTSB noted, as part of its probable cause, the accident occurred when portions of the fan cowl separated in flight after a fan blade, which had fractured due to a fatigue crack, impacted the engine fan case at a location that was critical to the structural integrity and performance of the fan cowl structure.
The NTSB found that the separated fan blade impacted the engine fan case and fractured into multiple fragments. Some of the fragments travelled forward of the engine and into the inlet. The impact of the separated fan blade with 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.
As a result of the investigation the NTSB issued seven new safety recommendations with five issued to the Federal Aviation Administration, one to the European Aviation Safety Agency, and one to Southwest Airlines.
These recommendations address the need to ensure the structural integrity of the fan cowl on Boeing 737 next-generation airplanes and assess whether other airframe and engine combinations have critical fan blade impact locations, the importance of having flight attendants secured in a jumpseat during emergency landings, and guidance for mitigating hazards to passengers affected by an in-flight loss of seating capacity.