Airlines resumed flying the jet just months ago, after it had been grounded for nearly two years because of accidents that killed nearly 350 people.
Just months after returning to the skies, Boeing’s troubled 737 Max jet is facing another setback.
Boeing said Friday that it had notified 16 airlines and other customers of a potential electrical problem with the Max and recommended that they temporarily stop flying some planes. The company refused to say how many planes were affected, but four U.S. airlines said they would stop using nearly 70 Max jets. Boeing would not say how long the planes would be sidelined.
Airlines and Boeing have tried hard in the last several months to convince passengers that the Max is safe. This latest problem is sure to spur further doubt among some travelers about the plane.
“It’s a Max, so everybody is interested and that makes perfect sense, but this is the aviation maintenance system working the way that it should,” said John Cox, a former airline pilot and crash investigator and chief executive of Safety Operating Systems, an aviation consulting firm.
Boeing said the affected airlines should verify that a component of the electrical power system on certain Max planes was sufficiently fastened. Airlines had resumed flying the jet after it was grounded for nearly two years because of a pair of accidents that killed nearly 350 people.
“We are working closely with the U.S. Federal Aviation Administration on this production issue,” Boeing said in a statement. “We are also informing our customers of specific tail numbers affected, and we will provide direction on appropriate corrective actions.”
The F.A.A. said Boeing notified it late Thursday of the problem, which could affect the operation of a backup power control unit. The agency said it was working with the company and its customers. “It is premature to estimate the amount of time required as it could take a matter of hours or a few days,” Boeing said.
Boeing’s share price closed down 1 percent, and analysts said the problem did not appear significant.
“The fix seems to be well understood, and timing would depend on availability of technicians and equipment access,” analysts at Jeffries, an investment bank, said in a note.
Regulators around the world banned the Max from flying in March 2019 because of the crashes, in Indonesia and Ethiopia. After a long review, the F.A.A. allowed the Max to fly again in November, provided that Boeing and airlines made changes to the jet, including updating its flight control software and rerouting some electrical wiring. Boeing and the F.A.A. said the potential electrical issue flagged this week was unrelated to that earlier directive.
Since the F.A.A. review, aviation regulators around the world have also approved the plane. As of late March, 17 Boeing customers had returned the plane to service, using it for nearly 14,000 flights.
American Airlines, Southwest Airlines and United Airlines, the top three operators of the Max, said on Friday that they had removed 63 planes from service. Alaska Airlines said it had removed all four of its Max jets.
Southwest, which operates an all-Boeing fleet, said in a statement that it had not experienced any “known operational challenges” related to the issue Boeing flagged. Thirty of its 58 Max jets were affected by the notification, and the airline said it had only 15 or fewer Max jets scheduled to fly each day, which were being swapped out for other planes in its 737 fleet.
“Southwest anticipates minimal disruption to our operation, and we appreciate the understanding of our customers and employees as safety is always our uncompromising priority,” it said in a statement.
United said that 16 of its 30 Max planes were affected by Boeing’s announcement and that they would not be used to carry passengers. American Airlines said the issue affected 17 of its most recently delivered planes, all of which have been temporarily sidelined. American said 24 Max planes delivered before the 2019 grounding were unaffected.
“As we shared when we returned the 737 Max to commercial service, the safety of our customers and team members comes above all else,” American said in a statement. “It’s with this unequivocal standard that we rigorously maintain and monitor all our aircraft — including the Boeing 737 Max — to ensure every plane in the air is safe.”
Wiring has been a concern on at least one other Boeing plane. Workers at the company’s plant in South Carolina have complained of careless practices there in the past, including debris left dangerously close to electrical wiring of the 787 Dreamliner, a large plane used on long flights.
The families of those killed in the crashes have been critical of both Boeing and the F.A.A., saying neither has done enough to root out the problems that caused the crashes.
“Boeing proclaims to be a changed company, but it’s clear their culture is built around cutting corners and putting profits over safety,” Yalena Lopez-Lewis, whose husband, Antoine, died in the crash in Ethiopia, said in a statement on Friday. “Since the deaths of 346 people, their sole focus has not been safety but to perform the bare minimum for regulators to allow it back in the air. This grounding illustrates that the Max is still unsafe to fly.”
After working to fix the Max and restore its credibility with airlines and regulators for much of the past two years, Boeing has been on an upswing in recent weeks. United said it was speeding up deliveries of the Max and expanding its order to 180 planes in the coming years. Europe and the United States agreed to temporarily suspend tariffs in a long-running dispute over Boeing and its rival Airbus. And February was the first month in more than a year in which Boeing reported net positive commercial airplane sales.
The company’s stock is up about 17 percent for the year.