Boeing (BA - Get Report) executives struck an apologetic tone over two deadly crashes of its flagship 737 MAX heading into the start of the Paris Air Show Monday, and insisted that its safe return to action was more important than establishing a speedy timetable.

CEO Dennis Muilenburg told reporters in Paris that Boeing had erred in the way it communicated a faulty cockpit light to both customers and regulators, and said it would take time to win back confidence in the plane's airworthiness as a result. He also told CNBC that while "steady progress" was being made on the 737's certification, he would not put a firm date on its return, saying only it would happen "before the end of the year."

"It is important for us to focus on safety, we will get back up in the air when it is safe, that's the most important thing here," Muilenburg said. "We are very confident in the MAX family and the heart of the market where it is located."

Boeing shares closed at $347.16 each Friday, after falling 0.5% on the session to extend their decline since the March 10 Ethiopian Airlines crash to just under 18%, a move that has loped more than $38 billion from the group's market value.

Last month, International Air Transport Association director Alexandre de Juniac cautioned that the planemaker's grounded 737 MAX may not return to full service before the end of the summer, while Muilenburg told an investor conference in New York hat the company was focused on "safely returning the MAX to flight" and stabilizing its production rate at 42 units per month.

The FAA itself has said there would be no near-term clearance for the 737 MAX following meetings with national and international regulators in Forth Worth, Texas, on May 23 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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