The Federal Aviation Administration is demanding answers from Boeing after receiving a 2016 electronic message exchange in which a test pilot talks of unknowingly having lied to regulators and discloses "egregious" problems with the flight control system that figured in two 737 Max crashes.

FAA Administrator Steve Dickson sent a terse letter Friday to Boeing CEO Dennis Muilenburg requesting to know why the messages had only been delivered the day before, not months ago when Boeing had uncovered them. "I expect your explanation immediately," he writes.

The 737 Max, the latest version of the jetliner that has evolved since first being flown in the 1960s, has been grounded worldwide since an Ethiopian Airlines flight crashed in March, killing 157 aboard. It followed another accident involving a 737 Max flown by Lion Air by five months, claiming 189 passengers and crew.

In both crashes, blame has focused on the performance of the Maneuvering Characteristics Augmentation System, or MCAS, which was software added to the jets to make them fly like previous versions of the 737. It was deemed needed because the Max had larger engines than previous 737s that were repositioned on the wing, making the jet perform differently in some circumstances.

Pilots in both the Ethiopian and Lion Air jetliners wrestled with MCAS, which automatically switches on in certain situations, as it overrode their actions. MCAS kept pushing the nose of the planes down as they struggled to keep them aloft.

In the November 2016, message exchange, 737 chief test pilot Mark Forkner — Boeing describes him as a "former employee" — writes that MCAS is "running rampant in the sim on me," a reference to a flight simulator in which it was being tested at the time. "I am levelling off at like 4000 feet, 230 knots and the plane is trimming itself like crazy. I'm like, WHAT?" he said.

He quipped, "granted, I suck at flying, but even this was egregious."

Forkner said he "basically lied to the regulators (unknowingly)" based on the simulator experience though the co-worker is quick to counter, "it wasn't a lie, no one told us that was the case."

In a batch of emails that the FAA later sent, Forkner notes in a March 2016 missive to the FAA that mention of the MCAS system in flight crew operating manuals is unnecessary because MCAS is present in both of the plane's flight control computers, operates in a transparent way and was designed to kick in only in rare circumstances. Pilots have complained that the existence of the MCAS system was kept secret from them until after the Lion Air crash.

The discovery could become a huge complication for Muilenburg, who is set to testify Oct. 30 before the House Committee on Transportation and Infrastructure for the first time about troubles with the 737 Max.

The FAA, the House committee and other authorities are investigating how MCAS was developed and approved for the 737 Max.

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In reaction to the disclosures, Boeing said it released the document to the House committee as part of its continued cooperation with its investigation as it strives to get the 737 Max back in service.

The ranking member of the House committee, Rep. Sam Graves, R-Mo., said the messages raise "a lot of questions" about conditions at the time of the test, what engineers were doing as the test was conducted and what Boeing did with the information.

Pilots union officials at two of the largest U.S. airline operators of the 737 Max, American and Southwest Airlines, reacted with dismay.

“This more evidence that Boeing misled pilots, government regulators and other aviation experts about the safety of the 737 Max," said Jon Weaks, president of Southwest Airlines Pilots Association, in a statement. "It is clear that the company’s negligence and fraud put the flying public at risk."

And Dennis Tajer, spokesman for the Allied Pilots Association for American, said "it's very serious if Boeing had someone within who is describing something egregious." He added, "We want to know under what conditions (MCAS was malfunctioning) so we can determine if something is new within the MCAS system."

He also said the union is solidly behind the FAA's Dickson on the matter. "We count on the FAA as a safety-culture partner."

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