Boeing's defense unit chief, Colbert, is leaving, according to the CEO.

• Boeing's CEO Kelly Ortberg announced that Ted Colbert, the head of the defense unit, is leaving the company and a replacement will be appointed at a later date.
• In the first half of the year, Boeing's revenue was mostly generated by its defense unit, which contributed approximately 40%.

Ted Colbert, the head of 's defense unit, is leaving the company immediately, announced CEO Kelly Ortberg, marking her first major executive change since taking the top job in early August.

"To restore the trust of our customers and meet their high expectations, our priority at this critical juncture is to improve our performance and deliver on our commitments, as Ortberg stated in a staff memo on Friday."

Colbert was thanked by Ortberg for his 15 years of service at Boeing, and Steve Parker, the unit's Chief Operating Officer, will take over until a replacement is named.

Boeing's defense, space, and security unit accounted for nearly 40% of the company's revenue in the first half of the year. However, this unit has faced production issues 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 NASA astronauts who took it to the International Space Station in June. Instead, NASA announced last month that SpaceX's Crew-9 vehicle will take the Starliner back.

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