The man in the Boeing hot seat, chief executive Dennis Muilenburg, has defended the company ’s record and says it is acting to fully ensure the safety of its new Boeing 737 MAX aircraft. In a letter to airlines, passengers and the aviation community, Muilenburg vowed to keep working "to earn and keep the trust people have placed in Boeing". The letter comes as Boeing and the US regulator have been under media fire for their handling of the crashes: Lion Air Flight 610 in Indonesia, which killed 189 people in October, and Ethiopian Airlines Flight 302, which killed 157 on March 10. SEE the video here The crashes led to the grounding of Boeing 737 MAX aircraft around the world and the fall-out has included increased scrutiny of the certification process. Read: Pilot training, skill levels, and automation come under intense scrutiny. In the letter, Muilenburg said the company knew that lives depended on the work it did and its teams embraced that responsibility with a deep sense of commitment every day. “Safety is at the core of who we are at Boeing, and ensuring safe and reliable travel on our airplanes is an enduring value and our absolute commitment to everyone,’’ he said. “This overarching focus on safety spans and binds together our entire global aerospace industry and communities. “We're united with our airline customers, international regulators and government authorities in our efforts to support the most recent investigation, understand the facts of what happened and help prevent future tragedies. “Based on facts from the Lion Air Flight 610 accident and emerging data as it becomes available from the Ethiopian Airlines Flight 302 accident, we're taking actions to fully ensure the safety of the 737 MAX. We also understand and regret the challenges for our customers and the flying public caused by the fleet's grounding.” READ: Black boxes show "similarities" between Boeing 737 MAX crashes. Muilenburg said work was progressing “thoroughly and rapidly” to learn more about the Ethiopian Airlines accident and understand information from the cockpit voice and flight data recorders. Boeing has a team on-site to support the investigation and provide technical expertise but Muilenburg said it would to the Ethiopia Accident Investigation Bureau to determine when and how it was appropriate to release additional details. He said the planemaker would soon release a software update and related pilot training to address concerns discovered in the aftermath of Lion Air Flight 610. “We've been working in full cooperation with the U.S. Federal Aviation Administration, the Department of Transportation and the National Transportation Safety Board on all issues relating to both the Lion Air and the Ethiopian Airlines accidents since the Lion Air accident occurred in October last year,’’ he said. The Boeing chief said he had dedicated his entire career to the company “working shoulder to shoulder with our amazing people and customers for more than three decades” he shared their deep sense of commitment. He had seen first hand at Boeing’s 737 production facility in Washington the pride staff felt in their work and the pain all were experiencing in light of the tragedies. “The importance of our work demands the utmost integrity and excellence—that's what I see in our team, and we'll never rest in pursuit of it,’’ he said.