The Southwest Airlines engine that failed Tuesday, leading to one passenger death — the first for a U.S. airline since 2009 and the first for Southwest since it began flying in 1971 — has long been considered among the most reliable, and until recently, had almost a perfect record, experts said.

“This is the single most-used engine both now and ever,” said Richard Aboulafia, vice president for analysis at Teal Group, an aerospace and defense consulting company. “It is the ultimate workhorse. It powers almost all flying Boeing 737s aside from a few antiques.”

Just about every person who has ever flown has likely been on a narrow-body aircraft fitted with a CFM56 engine. The engine, produced as part of a more than four-decade old joint venture between GE Aviation and Safran Aircraft Engines, a French company, had been the 737’s exclusive engine for decades, and it’s on about half of the world’s flying Airbus A320s, Aboulafia said. CFM said this engine, called the CFM56-7B, has powered 6,700 aircraft worldwide since its 1997 introduction.

Airlines like them because they’re the “best in class” for fuel burn, noise emissions and product support, Aboulafia, said. “It is just an incredibly reliable engine,” said Scott Hamilton, an aviation industry consultant with Leeham company.

But Tuesday’s incident, in which a passenger died after a mid-air engine explosion shortly after Flight 1380 departed from New York LaGuardia to Dallas Love Field, came only two years after a similar one episode, also involving a Southwest Airlines Boeing 737-700.

On Aug. 27, 2016, Flight 3472 from New Orleans to Orlando suffered an uncontained engine failure, forcing it to land in Pensacola, Florida. Uncontained failures typically result in debris flying from the engine casing and posing risks to the plane’s fuselage. No one died in the Florida incident, but it caused a five-inch by 16-inch gash in the left fuselage just above the left wing, according to the National Transportation Safety Board. The FAA proposed in August that airlines perform extra inspections on some engines The FAA wanted airlines to perform “ultrasonic inspection of certain fan blades.”

Late Tuesday, Southwest said it would accelerate its its existing CFM56 engine inspection schedule over the next 30 days “out of an abundance of caution.” The checks, the airline said, “are ultrasonic inspections of fan blades of the CFM56 engines.”

Still, in a briefing earlier Tuesday, Southwest CEO Gary Kelly said, “It’s premature to even link to it other engine failures that may have occurred.” (CFM said in a statement, “CFM cannot provide information about the accident or details related to it.”)

But the NTSB will consider it, likely asking whether whether the two faulty engines came from the same production batch, Hamilton said. Investigators also will consider a range of other possibilities, he added.

“The NTSB will look into whether there might have been a bird strike, whether on takeoff maybe the engine ingested some foreign object from the runway that caused the engine to disintegrate after takeoff,” Hamilton said. “They will look into whether there was a defective part. They will look into maintenance practices. They will look into whether there was an oil pressure situation or a fuel leak. They will look into all kinds of things. Clearly, we don’t know what happened.”

Aboulafia said the failure have been caused by a fluke, like debris on the runway, or something like a manufacturing error.

“It looks like an uncontained turbine failure, but what caused it is really what matters,” Aboulafia said. “You can’t completely rule out a manufacturing error. You can’t rule out an external damage. You can’t rule out unpredictable and hopefully isolated wear and tear. You can’t rule out anything at this point, except given the amazing run this engine has had, you can you rule out a design flaw.”

Southwest’s Kelly said the airplane was last inspected on April 15, but he said he didn’t know exactly what part of the aircraft technicians examined. He said it’s much too soon to know if Southwest will change its policies or procedures for engines because of the two events.

“We will be working with the NTSB so that we understand the root cause, [and]  any further actions that we need to take in terms of maintenance or inspection,” he said. “But at this point it premature to say what changes we will make, if any.”

Uncontained engine failures are rare, but they occur a few times each year. The most recent one that made news was Air France in September, when an Air France Airbus A380 made an emergency landing in Goose Bay, Canada. There were no injuries, but the FAA asked airlines to make more inspections of the engine.

Another major incident happened in October, 2016, when an American Airlines Boeing 767 upon takeoff at Chicago O’Hare, after the NTSB said a turbine disk in the right engine failed. No one was killed, but one passenger was seriously injured, the NTSB said in a report.

“It is unusual that you have this kind of engine failure,” Hamilton said of the Southwest incident. “[The NTSB] said that worldwide they see three or 4 engines come apart every year. When you consider the number of engines and the number of airplanes flying every where, even three or four each year is a very rare event. And in this case, unfortunately there was a fatality.”

The last deaths caused by a U.S. airline came in 2009, when a Colgan Air turbpoprop failed to recover from a stall and crashed near Buffalo, New York. The crash spurred Congress to institute new guidelines for pilot training. Also, in 2013, three passengers died after an Asiana Airlines Boeing 777 crashed while landing in San Francisco.

Reports on Tuesday night identified the passenger killed on the Southwest jet as Jennifer Riordan, a bank executive from Albuquerque, New Mexico, on a business trip to New York.

“Its a very somber day,” Kelly said. “We will support the NTSB on the investigation and do whatever needs to be done.

This story was updated to include Southwest’s comments on its CFM56 engine inspection plans and the identity of the passenger killed in the incident..

Photo Credit: A Southwest Airlines Boeing 737-700 suffered an engine failure on Tuesday. One passenger died, the first-ever fatality on a Southwest flight. Amanda Bourman / via AP