(Bloomberg) -- Lockheed Martin Corp. continues to produce F-35 jets with flaws discovered after the fighter jets are delivered to US military units, according to the Pentagon’s contracts management agency.

Four F-35Bs at a Marine unit at Cherry Point, North Carolina, have been repaired on-site since August by Lockheed technicians to correct “Category 1” deficiencies, according to people familiar with the problems, including a defense official. Such flaws affect flight readiness but aren’t seen as endangering pilot safety.

The issues at Marine Fighter Attack Squadron 542 indicate that quality problems persist even as production in the $438 billion F-35 program accelerates, with more than 975 jets delivered to the US and international partners, including Israel, the Pentagon’s Defense Contract Management Agency said in a statement to Bloomberg News.

Military customers, mostly recently the US Marine Corps, “are still expressing to us that too many quality assurance defects are escaping to the field” and that’s “an issue for the user community and a major concern negatively impacting” readiness, the contract management agency said.

Separately, the Marine Corps continues to investigate what went wrong last month after the pilot of an F-35B ejected from the plane — an event that mesmerized the world when a local base asked public assistance finding the aircraft before eventually spotting its debris field.

Earlier: F-35 Debris Found After a $100 Million Fighter Jet Went Missing

The Pentagon’s F-35 program office said it’s working with Lockheed to “constantly monitor quality assurance and actively address every concern as identified.” The program office also is “working to understand and mitigate” Marine Corps concerns.

All four jets arrived at the North Carolina unit in unblemished condition and flew 18 to 50 or more flight hours before the issues of concern emerged, said one the people familiar with quality matters.

Without commenting directly on the issues at the Cherry Point unit, Lockheed said in a statement that it is “committed to delivering quality products on time that enhance service members’ capabilities, enabling them to execute their mission and come home safely. Our aircraft delivery quality has continued to see improvement year over year. When a defect is identified, we apply our robust, standardized processes to determine and address root causes to avoid future issues.”

Earlier: Lockheed F-35’s Factory Flaws Persist Even After 800 Are Built

A Marine Corps spokesperson declined to discuss the unit’s problems, saying in a statement that the service “remains committed to extensively training our pilots, maintainers, and aircrew, conducting exhaustive maintenance.” At every step the service “puts in place safeguards and precautions to ensure a high degree of aviation safety,” the spokesperson said.

In-Plant Problems

The contracts management agency said Lockheed “has shown strong performance”  improving the number of hours needed to assemble the aircraft. 

But it said “parts shortages continue to cause production inefficiencies” leading to “delays for many aircraft.” Even as F-35 production increases, in-plant “scrap, rework and defect” rates “continue to limit production efficiency during assembly” because of “workforce turnover and some poor-performing suppliers,”  the contract management agency said.

Lockheed said it’s “focused on achieving quality” in “our production processes, which has improved to 95.7% in the last year.” 

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