The National Transportation Safety Board (NTSB) determined during a public board meeting held Tuesday that a fractured fan blade from a CFM International CFM-56-7B engine, powering a Southwest Airlines Boeing 737-700 (N772SW), led to the engine inlet and fan cowl separating and subsequently damaging the fuselage, resulting in a rapid cabin depressurization. As a result, NTSB investigators recommend that Boeing retrofit the engines of nearly 7,000 aircraft to prevent a repeat of the accident.
On 17 April 2018, one passenger died and eight others suffered minor injuries 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 depressurized. 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 crewmembers aboard.
“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,” said NTSB Chairman Robert Sumwalt. “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 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.
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:
To the Federal Aviation Administration
- 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.
- Once the actions requested in Safety Recommendation are completed, require Boeing to install the redesigned fan cowl structure on new-production 737 next-generation-series
- Once the actions requested in Safety Recommendation are completed, require operators of Boeing 737 next-generation-series airplanes to retrofit their airplanes with the redesigned fan cowl structure.
- Expand the Title 14 Code of Federal Regulations Part 25 and 33 certification requirements to mandate that airplane and engine manufacturers work collaboratively to 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 develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.
- Develop and issue guidance on ways that air carriers can mitigate hazards to passengers
affected by an in-flight loss of seating capacity.
To Southwest Airlines
- 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.
To the European Aviation Safety Agency
- Expand your certification requirements for transport-category airplanes and aircraft
engines 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.
An abstract of the final report, which includes the findings, probable cause, and all safety recommendations, is available at https://go.usa.gov/xp9kv.
Links to the accident docket and other publicly released information about this investigation are available at https://go.usa.gov/xpg5C.
The final report for the investigation of the accident is expected to post to the NTSB website in the next few weeks.