Two days into the investigation into the accident aboard Southwest 1380, the National Transportation Safety Board is increasing its focus not only on the CFM56-7B engine and its fan blades, but the engine’s surrounding structure that tore away, causing substantial damage and a fatality aboard the aircraft.
The NTSB has yet to make any recommendations to Boeing or the Federal Aviation Administration regarding the nacelle design, but NTSB spokesman Keith Holloway said a close review of the structure “will definitely be a part of this investigation.”
There are more than 12,000 engine cowling and inlet structures flying today on Boeing 737s around the world, the most numerous of any commercial aircraft.
With this close scrutiny, industry officials are quietly asking if the Boeing 737’s engine nacelle structural design constitutes an even larger challenge than just ensuring that all vulnerable fan blades are inspected, found and replaced.
Boeing declined to comment.
The NTSB said Wednesday pieces of the inlet structure were found in Pennsylvania, some 60 miles from Philadelphia, where the aircraft landed safely.
A second incident
Tuesday’s accident wasn’t a one-off. Southwest Airlines Flight 3472 in August 2016 was forced to make an emergency landing in Pensacola, Fla. after a similar failure of one of its CFM56-7B engine blades.
Just as in the case of flight 1380, the failure obliterated the engine inlet. In the 2016 incident, the failure sent debris flying into the wing, fuselage and leading edge of the horizontal stabilizer. The extent of the damage to the aircraft was less severe than that of 1380, but the incident no less serious. No one was hurt aboard that flight.
In both cases, fatigue cracking on the fan blades of the CFM56-7B engine has been identified as the focus of each investigation. Southwest is inspecting its entire fleet over the next 30 days and the Federal Aviation Administration is expected to issue new inspection directives to airlines in the coming days.
That’s crucial to the safety of the aircraft, ensuring that any fragments are contained by the casing and safely ejected out of the rear of the engine. In the case of Southwest Airlines flight 1380 and 3462, the titanium blade was safely contained – but the failure set off a chain reaction that significantly damaged both aircraft. And in the case of flight 1380, caused the first fatality on a U.S. airline since 2009.
UPDATE: NTSB Chairman Robert Sumwalt said evidence of both blue and red paint was found on the leading edge of flight 1380’s left wing, an indication of the cowlings trajectory after the initial engine failure.
But containment of the blade isn’t the only aspect of that dramatic test.
According to federal regulations, the violent vibrations and stresses of the failure cannot cause a cascading failure of the structure surrounding the engine. One senior official from an engine manufacturer described the requirement this way:
“The aircraft structure must be capable of the transient load. So the engine is designed, and demonstrated, to contain high energy fragments. But the nacelle and in fact all the aircraft structure has its own requirements. It has to withstand the transient loads of this event. So in a nutshell the engine must not release high energy fragments, and in this event the nacelle must be capable of the imbalance load (limit loads).”
In short, the structure around the engine can deform (according to Federal regulations), but it can’t fail and pose a hazard to the aircraft. (Note – UPDATED: Limit load is defined as the most punishing forces that would be expected in operation. Ultimate load is defined as 150% of limit load.)
While the CFM International engine met certification requirements spelled out in federal regulations, both incidents included an aspect unable to be replicated on the ground.
The failed engines faced a “500 mile per hour wind on top of everything else” when the engine failed, said an industry official briefed on the investigation, “you can only test so much in a test cell.”
Precisely what changes, if any, might be mandated for the nacelle design has yet to be determined at this early phase of the investigation. But the industry official said such a crucial part of the aircraft “goes back to the very basic design of the engine and the airframe.”