The US National Transportation Safety Board (NTSB) has requested Boeing redesign the fan cowl on all 737 Next Generation (NG) series of aircraft following the deadly Southwest Airlines accident aboard Flight 1380 on 17 April 2018. The NTSB wants operators of the type to retrofit their aircraft with the new cowl design, as investigators found that the old fan cowl structure on 737NGs were “susceptible” to failure.

The NTSB determined that the engine failure on Flight 1380 was caused by a broken fan blade from one of the two CFM International CFM-56-7Bs powering the aircraft.

The Board noted that the accident occurred when portions of the left fan cowl separated in-flight. Fan blade No.13, which had fractured due to a low-cycle fatigue crack, impacted the engine fan case at a position 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 went into the engine 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.

Sections of the blade caused the engine inlet and fan cowl to break apart, which damaged the fuselage, which resulted in rapid cabin depressurisation. Fragments of the cowling hit the fuselage near a cabin window – one passenger died after being partially sucked out of the blown-out window. Eight other passengers were injured.

The accident occurred as the Southwest flight departed New York’s La Guardia Airport bound for Dallas Love Field, Texas. The flight crew were forced to divert and make an emergency landing at Philadelphia International Airport. These were 144 passengers and five crew members on board.

As a result of the investigation, the NTSB issued seven new safety recommendations: five were sent to the FAA, one to the EASA and one to Southwest Airlines. The primary recommendation was the need to “ensure the structural integrity” of the fan cowl on 737NGs and assess whether other airframe and engine combinations have critical fan blade impact locations.

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