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Southwest Airlines previously challenged engine maker CFM and federal regulators over proposed engine inspections after a fan blade separated from the engine and caused it to fail in 2016, public documents obtained by Reuters show.

The incident in 2016 was similar to what occurred to the engine on Southwest Flight 1380 in which a fan blade broke off from the engine, causing it to explode, killing one passenger and injuring seven others on board Tuesday. Federal investigators have cited “metal fatigue” as the preliminary cause of Flight 1380’s fan blade break and subsequent explosion.

In 2016, federal investigators also cited metal fatigue as the cause of the uncontained engine failure and cabin depressurization that forced an emergency landing of Southwest Flight 3472 in Florida. The National Transportation Safety Board’s report on the incident says the aircraft’s “left engine inlet separated from the engine during the flight. Debris from the engine inlet damaged the airplane fuselage, wing and empennage,” and that a hole nearly a foot-and-a-half long was found “in the left fuselage, just above the wing.”

The NTSB’s report for the 2016 incident also says that after a metallurgical examination, investigators found a “fatigue crack” on the surface of the missing fan blade, similar to its initial findings in the engine of Flight 1380.

After the 2016 incident, the engine maker CFM, the Federal Aviation Administration and its European equivalent proposed safety checks of the all similar CFM56 engines, which are the best-selling in the world. CFM suggested airlines inspect all of their engines within a period of no more than 12 months.

Now, public documents from that time show that Southwest told the FAA that not all of the 24 fan blades in each engine should be inspected, only certain ones, Reuters reports. The airline also said the inspection requirement would affect 732 engines in its fleet.

“The affected engine count for the fleet in costs of compliance … appears to be vastly understated,” Southwest said, according to the Reuters report.

United Airlines told federal regulators something similar. “The maintenance burden and cost for operators to inspect all effective fan blades is much more significant than proposed,” United told the FAA, according to Reuters’ citation of the documents.

Industry experts say when FAA proposals are written, it’s not uncommon for airlines to request new timeframes or estimate increased costs based on several factors such as the availability of parts, man power, testing time and aircraft taken out of service.

American Airlines, which has 304 Boeing 737-800 aircraft operated by the CFM56-7B engine in its fleet, said in the documents that its “fan blades have been removed, repaired, reworked, and then relocated.”

In an email, an American Airlines spokesperson told TPG: “Our hearts go out to the Riordan family, the passengers and crew of Flight 1380 and the entire Southwest Airlines team. After a notice of proposed rulemaking (NPRM) was published in August 2017, American Airlines voluntarily began inspections of CFM56-7B fan blades under the guidance proposed in the NPRM. We continue to closely monitor the investigation being led by the National Transportation Safety Board (NTSB).”

TPG reached out to Southwest and United for comment but did not receive an immediate response.

Since the incident on Flight 1380, Southwest said it was voluntarily speeding up the inspection of all related engines on its all-737 fleet. The carrier expects to complete that inspection process within the next 30 days. The FAA on Wednesday made mandatory the inspections of fan blades on similar engines it proposed after the 2016 Southwest incident.

Former NTSB chairman Mark Rosenker told Reuters that NTSB investigators likely will be looking at why these inspections weren’t automatically mandatory after Southwest’s 2016 engine failure. He said with regard to finalizing the fan blade inspection requirements “there did not seem to be an urgency.”

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