The manufacturer of the engine that blew up last week on a Southwest Airlines jet was preparing before the incident to recommend inspections that would have included the one that failed, triggering an explosive decompression that killed a passenger.

Engine maker CFM International Inc., in response to a similar malfunction in 2016, had recommended last year that carriers inspect a limited population of older fan blades that didn’t include the one that failed last week.

But a draft recommendation circulated to some carriers for their input would have expanded the list enough to include the engine that was powering Flight 1380 when a fan blade broke off midair over Pennsylvania. It shattered a window and sucked a woman partly out of the plane.

Those recommendations were the basis of the emergency inspections ordered Friday by the U.S. Federal Aviation Administration. The company’s recommendations “had been in the works for weeks before the incident,” said CFM spokesman Rick Kennedy.

The expansion of engine inspections was part of the process of trying to gather data on an extremely rare failure like the one that occurred two years ago and was not because of any indication of an impending risk of another blade failure, Kennedy said.

“We’re on the very front end of trying to understand this phenomenon,” Kennedy said. “The biggest challenge in our industry is managing something that’s exceedingly rare. You don’t have a baseline.”

After a CFM engine failed in 2016, the company issued service bulletins calling for inspections of specific lots of fan blades with more than 15,000 flights since their last maintenance overhaul. The Southwest engine that failed last week had about 10,000 flights since being overhauled, according to the airline. It also wasn’t among the fan blade serial numbers cited in that bulletin, according to Kennedy.

The broader inspections, which are now aimed at all older fan blades, adds a new twist to the mystery of why two similar, severe failures on one of the world’s most common and most reliable jet engines have occurred within two years. Engines can’t be certified until they demonstrate in tests that they can lose a fan blade without causing damage to an aircraft, but that happened in both incidents on Southwest planes.

Evolving Hazard

The process of deciding which engines should be inspected is typical for how manufacturers and regulators approach an evolving safety hazard, said Sarah MacLeod, executive director of the Aeronautical Repair Station Association.

“They have to make some assumptions in order to narrow the risks,” MacLeod said.

Since the incident, CFM has recommended that engines with 20,000 flights or more should have fan blade inspections by the end of August. The same inspections should be done in cases where airlines don’t know how many flights a fan blade has made. Carriers sometimes move blades from engine to engine during routine maintenance and it can be difficult to track exactly the number of a blade’s flights.

NTSB Probe

Kennedy said he was addressing the inspection process only and couldn’t comment on National Transportation Safety Board’s investigation of the two incidents. The NTSB hasn’t completed its investigations into either accident. In the 2016 case, a fan blade broke off and ricocheted out the front of the engine, causing damage that led the plane to lose pressure. The flight diverted to Pensacola, Florida, but no one was injured.

Southwest Airlines Co. says it plans to inspect all engines of that make regardless of their number of flights. CFM is a partnership between General Electric Co. and France’s Safran SA.

Under the emergency order issued Friday, the engine that failed in the latest case would have been inspected. It had a total of about 40,000 flights, so was above the threshold in the order. The order also doesn’t make a distinction based on previous overhauls.

The FAA, which is starting with the oldest engines first, says it is considering expanding its inspection order. The agency plans on following CFM’s call for more checks by August in a separate action, according to a person familiar with the matter, who wasn’t authorized to speak publicly about the information and asked not to be identified.

More on CFM inspections by Bloomberg Intelligence analysts

The FAA’s order on Friday covers an estimated 352 of the oldest CFM56-7B engines on U.S. carriers, which fell far short of the number of engines CFM had suggested that airlines eventually inspect. “We are considering further rule-making to address these differences,” the FAA wrote in its emergency airworthiness directive.

Last November, Southwest began a program to inspect all its CFM56-7B fan blades every 3,000 hours, going beyond what the manufacturer had suggested, Chief Operating Officer Mike Van de Ven said in a memo sent to employees last Friday. However, the engine that exploded last week hadn’t gone through that inspection, said Brandy King, a company spokeswoman.

The accident April 17 was the first fatal accident involving a U.S. passenger airline since 2009. Investigators say a fan blade at the front of the engine that had been weakened by repeated stresses broke off on Flight 1380. The Boeing Co. 737-700, which was en route from New York to Dallas, made an emergency landing in Philadelphia.

A passenger, Jennifer Riordan, who was a vice president at Wells Fargo & Co. in New Mexico, was killed after being partly sucked out of the opening. The plane made an emergency landing in Philadelphia.

While the initial orders are focused on the overall time of an engine, carriers sometimes move older fan blades onto engines that were purchased more recently. That could complicate how the airlines target which engines they should inspect, said Gary Weissel, managing officer at Tronos Aviation Consulting Inc.

“They’ll have to go through all their engines and calculate how long have the blades been on. It will be a little more of a manual review,” Weissel said.