Boeing said a new problem with the fuselages of some unfinished 737 planes would force the company to rework about 50 planes, potentially delaying their delivery and raising more quality concerns for the manufacturer and its suppliers.

Stan Deal, chief executive of Boeing’s commercial aircraft unit, said in a note to employees Sunday that a supplier had identified last week that “two holes may not have been drilled exactly to our requirements.” He did not name the supplier.

Boeing’s airframe supplier, Spirit AeroSystems, confirmed to Reuters that one of his employees had discovered two poorly drilled holes in some fuselages and alerted a manager. Spirit did not immediately respond to a request for comment.

The issue “was not an immediate flight safety issue and all 737s can continue to operate safely,” Deal said in the memo. He added that all 737s currently in use could continue to fly.

The new problems were another setback for Boeing, which has been under pressure from regulators, investors and its airline customers since Jan. 5, when a panel on a 737 Max 9 plane operated by Alaska Airlines exploded mid-flight, forcing an emergency landing. and the grounding of some Max 9s in the United States.

Quality concerns at Boeing and its suppliers have taken on new urgency after news reports, including a New York Times report, found that Boeing workers had opened and reinstalled the panel that blew up the Alaska Airlines plane. Last week, Boeing declined to provide a full-year financial forecast as planned, an indication that it was trying to assure customers that quality control would take priority over financial performance.

Deal said Boeing would spend several “factory days” this week at the company’s factory outside Seattle to repair miss-drilled holes and finish other work on undelivered 737s. These days allow teams to pause normal work and attend to specific tasks without shutting down production.

“This is what we mean when we say we will go slow to get it right,” Deal said in the memo.

The Federal Aviation Administration said last month it would limit Boeing’s production until it had confidence in its quality control processes, another blow to the company, which had planned to increase production of its Max series of planes to more than 500 this year. year, from about 400 last year. year. The regulator also said it was investigating Boeing’s manufacturing practices and production lines, including those involving Spirit AeroSystems.

The mechanical and safety issues plaguing the Max series have compounded problems facing the broader aviation industry. Airlines have been unable to take full advantage of strong demand for flights in recent years due to shortages of planes, pilots, flight attendants and other workers needed to operate flights.

The Max series was already under scrutiny after crashes in 2018 and 2019 that killed nearly 350 people and led to the grounding of the Max 8 for nearly two years. The latest problems with the Max planes have renewed concerns among some aviation experts that Boeing has prioritized profits and shareholder returns over engineering and safety.