Boeing Co. (BA - Get Report) shares drifted lower in pre-market trading Tuesday after the head of the aviation industry's top lobby group said the planemaker's grounded 737 MAX may not return to full service before the end of the summer.

International Air Transport Association director Alexandre de Juniac said the lobby will attempt to arrange a meeting with Boeing executives, national regulators and airline operators sometime in the next five to seven weeks as the industry moves towards returning the workhorse aircraft back into commercial service. Boeing CEO Dennis Muilenburg, meanwhile, told Bernstein's Strategic Decisions Conference in New York that the company was focused on "safely returning the MAX to flight" and stabilizing its production rate at 42 units per month.

"We do not expect something before ten to twelve weeks in re-entry into service, but it is not our hands," de Juniac said during a media scrum in Seoul, South Korea. "That is in the hands of regulators."

Boeing shares were marked 2.1% lower Wednesday to trade $347.43 each, a move that would extend the stock's one-month decline to around 8%.

Last week, the acting head of the U.S. Federal Aviation Administration suggested there would be no near-term clearance for the 737 MAX amid meetings with national and international regulators in Forth Worth, Texas, that focused on Boeing's recent overhaul of its MCAS flight software system.

Boeing officially acknowledged that its software system played a role in two recent deadly 737 MAX 8 accidents. 

The planemaker said the preliminary report into the cause of Ethiopian Airlines Flight 302's fatal crash on March 10, 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.

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.

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