The emergency on Southwest Airlines Flight 1380 that killed one passenger, a woman who was nearly sucked out of a window, and injured seven others might easily have been much worse.

The sudden engine failure on the Boeing 737 had similar effects as would a small bomb. A failed fan blade sent shrapnel ripping into the jet’s fuselage, taking out a a cabin window and by doing so causing a sudden loss of cabin pressurization.

Had it also ripped into one of the airplane’s gas tanks or fuel lines that are vulnerable to this kind of impact the result would have been catastrophic.

It is precisely this kind of vulnerability that requires engine fan blades to be encased in a shield that is supposed to contain any debris. What left this Southwest Airlines flight stricken at 30,000 feet is called an “uncontained failure.”

And it does appear that shrapnel hit a fuel line because firefighters who raced to the jet as it made its emergency landing at Philadelphia had to put out a small fire from fuel that was leaking from one engine.

The engines involved, produced jointly by the French company Safran and General Electric in the U.S., have a long history with the Boeing 737. In fact, the engines and the airplane have evolved together through many different versions. They fly more passengers in the world than any other jets—and by 2020 the engines will have flown an amazing 1 billion flight hours.

Following a very similar emergency involving the same engine, the CFM56-7B, on another Southwest 737 in 2016 on a flight from New Orleans to Orlando, Florida, the Federal Aviation Administration advised airlines to increase the frequency of maintenance checks specifically to detect any cracks in the blades.

Engines like this are constantly being developed to increase performance, and pushed to become more and more fuel efficient while providing more power. To meet these targets new materials and technologies are introduced. An engine that had previously been trouble-free can suddenly develop problems and last year’s FAA directive indicates that a change in the specification of the fan blades in these engines may have been causing concern.

Indeed, investigators from the National Transportation Safety Board found that the blades that failed in the 2016 Southwest Airlines emergency showed cracks consistent with metal fatigue. Last night NTSB officials said they found evidence of similar metal fatigue after inspecting the jet on the ground at Philadelphia – at the exact point where the blade separated from its hub.

For Southwest Airlines this event ends an astonishing safety record in which no passenger had been killed in a flight emergency since the airline began flying in 1971. Southwest operates a fleet of more than 700 Boeing 737s, each flying as many as six or seven flights a day, and flies around 150 million passengers every year.

One detail that investigators will look at as they scrutinize the records of this airplane will be the frequency and depth of the maintenance checks on the fan blades. They will also note that the failure occurred at a stage in the flight when the engines are under the least amount of stress, as they throttle back when the jet approaches its cruise altitude.

And that in itself might well have been lucky. The majority of engine failures of this kind occur as the pilots call for maximum thrust at takeoff, a far more critical moment in terms of safety.

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Nonetheless, the 148 people who survived Flight 1380 had a terrifying experience beginning with the shrapnel ripping into the cabin. Jennifer Riordan, a mother of two and a bank vice president from Albuquerque, died in a window seat after nearly being sucked through the window as it blew out. Hit with the sudden depressurization the pilots had to immediately descend rapidly to 10,000 feet to lessen the effects of the depressurization, at the same time making their mayday diversion to Philadelphia where even as the airplane came to a halt on the runway the leaking fuel presented an imminent threat of fire and explosion.

The pilot of the jet, Tammie Jo Shults, did an exemplary job of getting a seriously damaged airplane to a runway as fast as possible.

As the gravity of this situation becomes clearer the passengers who escaped unhurt can consider themselves very lucky. And investigators will now have to urgently determine why the fan blade or blades failed and, just as critically, why the engine casing did not contain the shrapnel.