Boeing Co said on Thursday that airlines had inspected 810 of the company's 737 NG jets around the world and found 38 structural cracks requiring repair and replacement of the affected parts.
The planes will be grounded until the repairs are made, Boeing and airline officials said.
Nearly 5% of inspections have found cracks in a "pickle fork" - a part that attaches the plane's fuselage, or body, to the wing structure and manages forces.
The 737 NG is the third-generation 737 and version before the now-grounded 737 MAX, which is not impacted by the cracking issue.On Wednesday, Southwest Airlines Co and Brazil's Gol Linhas Aereas grounded at least 13 737 NG airplanes after US regulators ordered urgent inspections.
The US Federal Aviation Administration (FAA) last week told US aircraft operators to inspect 165 older 737 NGs for structural cracks, but it now appears the actual number of planes covered is more than 200.
The FAA said late on Thursday "a small number of aircraft based in the United States have been removed from service while Boeing develops instructions for customers to repair or replace the affected parts."
The agency added it "is working with the manufacturer and other international aviation safety regulators to better understand the factors that led to the formation of the cracks."
Southwest spokeswoman Brandy King said of more than 200 high-use airplanes it had found cracks on two planes and grounded them.
She declined to say if cracks had been found on any other jets but said the airline planned an update next week.
"We do not have a timeline for when the airplanes will be returned to service, we are working with Boeing to schedule the upcoming repairs," King said.
Boeing shares fell 1% to close at $371.00 on Thursday.
American Airlines and United Airlines said earlier this week they have not seen any cracks on their airplanes.
Raymond James analyst Savanthi Syth wrote in a research note Thursday that the findings from the 737 NG inspections could "potentially take up to 4% of capacity off-line between mid-Oct and mid-Dec."Planes with cracks "may need to be taken out of the fleet for up to 60 days for maintenance," Syth said.
The FAA last week said inspections would look for "cracking of the left and right hand side outboard chords of frame fittings and failsafe straps."
It added that the cracking "could adversely affect the structural integrity of the aeroplane and result in loss of control of the aeroplane."
Aircraft with more than 30,000 cycles must be inspected within seven days, while planes between 22,600 and 29,999 cycles must be inspected within 1,000 cycles, which typically correspond to the number of flights.
In total, 1,911 US 737 NGs are covered by the FAA directive.
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