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Boeing Co. (BA - Get Report) shares extended declines Thursday following comments from the acting head of the Federal Aviation Administration that suggested there would be no near-term clearance for the grounded 737 MAX aircraft.

The FAA is meeting with scores of national and international airline regulators today in Forth Worth, Texas, to discuss Boeing's recent overhaul of its flight software system last week. Boeing said the MCAS software update, which includes 360 hours of testing over more than 200 flights, will soon include detail on how pilots interact with the airplane controls and displays in different flight scenarios.

"It's a constant give-and-take until it is exactly right," said acting head FAA head Dan Elwell during a conversation with reporters late Wednesday. "It's taking as long as it takes to be right. I'm not tied to a timetable."

Boeing shares were marked 1.64% lower at the start of trading Thursday to change hands at $347.00 each, a move that would extend the stock's one-month decline to just under 9%.

Last month, Boeing officially acknowledged for the first time that its software system played a role in two recent deadly 737 MAX 8 accidents. It also posted weaker-than-expected first quarter earnings and pulled its earnings guidance for the rest of the year until it has clarity on the fate of its 737 MAX program, which has seen 300 planes grounded in markets around the world.

The planemaker said the preliminary report into the cause of Ethiopian Airlines Flight 302's fatal crash, as well as the Lion Air 610 disaster in Indonesia in early October, which took the lives of 189 people, were caused by activation of the Maneuvering Characteristics Augmentation System, or MCAS, in response to "erroneous angle of attack information" from a broken sensor.

The FAA has said it would "evaluate aspects of the 737 MAX automated flight control system, including its design and pilots' interaction with the system, to determine its compliance with all applicable regulations and to identify future enhancements that might be needed."