US aircraft manufacturer Boeing said Monday it had reached a "definitive deal" to buy its subcontractor Spirit, which has faced scrutiny over production quality control in recent months.

"The merger is an all-stock transaction at an equity value of approximately $4.7 billion, or $37.25 per share," the company said in a statement.

Boeing disclosed in March that it was in talks to potentially reacquire Spirit, which it spun off in 2005 to lower costs.

"We believe this deal is in the best interest of the flying public, our airline customers, the employees of Spirit and Boeing, our shareholders and the country more broadly," said Boeing president and CEO Dave Calhoun.

He said by reintegrating Spirit, "we can fully align our commercial production systems", including safety and quality management systems, and "our workforce to the same priorities, incentives and outcomes -- centered on safety and quality".

Spirit AeroSystems builds fuselages and other significant parts for both Airbus and Boeing.

Boeing is by far Spirit`s biggest customer, with around 70 percent of its revenue coming from the American plane maker in 2023.

The two companies have faced intense scrutiny since a near-catastrophic incident in January when a fuselage panel blew off a 737 MAX mid-flight. BSS/AFP