NTSB determined probable cause of fatal Southwest accident and issued safety recommendations

Last updated: 20. 11. 2019 20:42 CET

The National Transportation Safety Board (NTSB) met today (November 19, 2019) to determine the cause of an engine failure on a Southwest Airlines flight last year. Flight WN1380, registration N772SW, from New York to Dallas had to make an emergency landing in Philadelphia due to engine failure on April 17, 2018.

There were 144 passengers and five crewmembers aboard. One passenger died and eight others suffered minor injuries during the incident.

According to NTSB preliminary findings a fractured fan blade from a CFM International CFM-56-7B engine, powering a Southwest Airlines Boeing 737-700, 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 depressurized. As a result of the incident, one passenger, a 43-year-old banker and mother of two, was blown halfway out of the plane. Two passengers helped bring her back inside, but she died from the injuries.

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 traveled 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.

Metallurgical examinations of the fractured fan blade found that the crack had likely initiated before the fan blade set’s last overhaul in October 2012. At that time, the overhaul process included a fluorescent penetrant inspection to detect cracks; however, the crack was not detected for unknown reasons. The crack on the fan blade accident was also not detected during the on-wing fan blade visual inspections (subsequent to the overhaul) that were conducted as part of fan blade relubrications.

As a result of the investigation the NTSB issued following safety recommendations:

1. Require Boeing to determine the critical fan blade impact location(s) on the CFM56-7B engine fan case and redesign the fan cowl structure on all Boeing 737 next-generation-series airplanes to ensure the structural integrity of the fan cowl after a fan-blade-out event.

2. Once the actions requested in Safety Recommendation [1] are completed, require Boeing to install the redesigned fan cowl structure on new-production 737 next-generation-series airplanes.

3. Once the actions requested in Safety Recommendation [1] are completed, require operators of Boeing 737 next-generation-series airplanes to retrofit their airplanes with the redesigned fan cowl structure.

4. Expand the Title 14 Code of Federal Regulations Part 25 and 33 certification requirements to mandate that airplane and engine manufacturers work collaboratively to (1) analyze all critical fan blade impact locations for all engine operating conditions, the resulting fan blade fragmentation, and the effects of the fan-blade-out-generated loads on the nacelle structure and (2) develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.

5. Develop and issue guidance on ways that air carriers can mitigate hazards to passengers affected by an in-flight loss of seating capacity.

6. Include the lessons learned from the accident involving Southwest Airlines flight 1380 in initial and recurrent flight attendant training, emphasizing the importance of being secured in a jumpseat during emergency landings.

NTSB determined probable cause of fatal Southwest accident and issued safety recommendations
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