A Boeing pilot behind the 737 MAX certification in 2016 told a colleague a key flight handling system was "running rampant" during simulator tests, according to documents reviewed on Friday by AFP.
The Boeing employees quipped about problems with the Maneuvering Characteristics Augmentation System, a flight-handling mechanism that is believed to be at the center of two MAX crashes that killed 346 people.
Boeing's chief technical pilot of the 737, Mark Forkner, said the MCAS system's performance during the simulator tests was "egregious" and that "I basically lied to the regulators (unknowingly)," according to the instant messages.
The messages -- which Boeing knew about for months before sharing with the Federal Aviation Administration -- raised fresh questions about the company's knowledge of problems with the MAX long before the crashes.
And they sent shares sharply lower Friday as aviation experts said the revelations could further delay the plane's return to service.
The crashes and the FAA's certification of the MAX are under investigation from a number of authorities, including the Department of Justice and congressional committees that have scheduled hearings with Boeing Chief Executive Dennis Muilenburg later this month.
In both the Lion Air and Ethiopian Airlines crashes, the MCAS pointed the plane sharply downward based on a faulty sensor reading, hindering the pilots' ability to control the aircraft after takeoff, according to preliminary crash investigations.
The FAA, based on its interactions with Forkner and others at Boeing, believed during certification that the MCAS system would activate only in rare cases and did not pose a threat to plane safety.
The FAA criticized Boeing for learning of the messages "some months ago," but not disclosing their existence to safety regulators until Thursday.
"Last night, I reviewed a concerning document that Boeing provided late yesterday to the Department of Transportation," FAA Administrator Steve Dickson said in a letter to Muilenburg.
"I understand that Boeing discovered the document in its files months ago. I expect your explanation immediately regarding the content of this document and Boeing's delay in disclosing the document to its safety regulator."
The FAA said it flagged its concerns to lawmakers on Capitol Hill and the Department of Transportation's inspector general.
A Boeing spokesman said the document was shared with congressional investors.
"Over the past several months, Boeing has been voluntarily cooperating with the House Transportation & Infrastructure Committee's investigation into the 737 MAX," the Boeing spokesman said.
"As part of that cooperation, today we brought to the committee's attention a document containing statements by a former Boeing employee.
"We will continue to cooperate with the committee as it continues its investigation. And we will continue to follow the direction of the FAA and other global regulators, as we work to safely return the 737 MAX to service."
News of the messages comes as the FAA has taken the lead among international regulators in overseeing the recertification for the MAX in a process that has dragged on much longer than originally expected.
The messages add to the pressure on Muilenburg ahead of an October 30 congressional hearing. Boeing last week stripped Muilenburg of his title as chairman, a moved that analysts said could be a precursor to his removal as chief executive.
In recent days, American Airlines, United Airlines and Southwest Airlines have pushed back their target dates for returning the MAX to service, with all three carriers pulling all flights through January or February 2020.
Michel Merluzeau at AirInsight Research said the latest disclosures could lead to further delays.
"Today's disclosure is potentially going to impact return to service again and likely to lead to prolonged uncertainty for the program and lead to consequences at the enterprise level," Merluzeau said. "This is frankly sobering news."
Shares of Boeing tumbled 5.9 percent to $347.41 in late-afternoon trading.
