The head of Boeing‘s defense unit Ted Colbert is leaving the company effective immediately, said CEO Kelly Ortberg, marking his first major executive change since he took the top job in early August.

“At this critical juncture, our priority is to restore the trust of our customers and meet the high standards they expect of us to enable their critical missions around the world,” Ortberg said in a staff memo on Friday. “Working together we can and will improve our performance and ensure we deliver on our commitments.”  

Ortberg thanked Colbert for his 15 years of service at Boeing and said the unit’s Chief Operating Officer Steve Parker would take over until the company names Colbert’s replacement.

Boeing’s defense, space and security unit generated nearly 40% of Boeing’s revenue in the first half of this year, but it has struggled with production problems and cost overruns, including on the new 747s that will serve as Air Force One aircraft. In the space sector, Boeing’s Starliner is returning without the NASA astronauts who took it to the International Space Station in June. They will instead take SpaceX’s Crew-9 vehicle back, NASA said last month.

Colbert did not immediately respond to CNBC’s request for comment.