Evidence of metal fatigue has emerged in initial investigations into an engine explosion on a Southwest Airlines flight, which killed a passenger.

The plane, a Boeing 737-700 which was bound to Dallas from New York, made an emergency landing in Philadelphia.

The death of 43-year-old Jennifer Riordan on Flight 1380 was the first in a US commercial aviation accident since 2009, according to National Transportation Safety Board (NTSB) statistics. She was almost sucked through a window after depressurisation.

NTSB Chairman Robert Sumwalt told a news conference at the Philadelphia airport that a preliminary investigation found an engine fan blade missing, having apparently broken off, and that there was metal fatigue at the point where it normally attached.

Sumwalt said part of the engine's covering, called a cowling, was found in Bernville, Pennsylvania, about 70 miles from the Philadelphia airport.

The type of engine that blew apart mid-flight was due for an inspection by early next year following an order from European regulators just this month.

There was some concern about the engine, the CFM56,  after another Southwest accident in 2016 where a fan blade separated from the same type of engine, and debris ripped a foot-long hole above the left wing.

That had shocking similarities to Tuesday's incident, where a fan blade had also broken off, according to early investigations. 

Any design issues with the long-established CFM engine could have repercussions worldwide.

It's one of the most common engines, paired with the world's most-sold plane - the Boeing 737.

CFM and Boeing both say they'll help with the investigation into the incident.

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