After two deadly crashes grounded Boeing's 737 Max planes in March, the airline faces new safety concerns about the older version of the same jet.
Inspections of Boeing's 737 NG planes uncovered cracks that required repair and replacement, the company said Thursday.
Of the 810 planes inspected worldwide, 38 jets - or 5% - were found to have cracks in the part of the aircraft that attaches the wing to the fuselage.
The Federal Aviation Administration asked airlines to immediately inspect all 737 NGs that have flown more than 30,000 flights. They have a year to complete the inspection of crafts with fewer flights.
On Thursday, U.S.-based Southwest Airlines grounded two of its 737 NGs and Brazil's Gol Linhas Aereas grounded 11.
It is not know when the grounded planes will return to service.
American Airlines, United and Delta said none of the 737 NGs in their fleets have more than 30,000 flights and none of the planes they have inspected so far have any problems.
Boeing's stocks fell 1% Thursday, closing at $371 per share.