Boeing said 570 planes manufactured between 1993 and 2000 would be subjected to the inspections, which use "eddy current" electromagnetic technology to test for micro cracks invisible to the eye.
Separately, the safety board said that Southwest had complied with all required aircraft inspections and that the 15-year-old plane that developed the five-foot fuselage tear had been "up to date" with maintenance inspections, including a "heavy maintenance" check in March 2010, according to safety investigator Robert Sumwalt.
Boeing, in a conference call Tuesday, said that its testing and analysis had showed the lap joints had not been expected to crack because of metal fatigue until after 60,000 takeoff and landing cycles.
The Southwest jet flying from Phoenix to Sacramento, Calif., and forced to make an emergency landing Friday in Yuma, Ariz., had accumulated 39,780 takeoffs and landings.
The new inspections will focus on a "lower row of fasteners" in a 50-foot-long area on the crown of the airplane, on the left and right sides, said Boeing chief project engineer Paul Richter.
The electromagnetic inspections "can look through the aluminum for disruptions in the magnetic field, which are signatures for cracks that are in the base metal below," he said in a conference call.
Boeing is recommending a one-time inspection, but the Federal Aviation Administration plans to require repeat checks for every 500 takeoffs and landings above 30,000 cycles on some 737 models, Boeing said.
"Until we, Boeing Co., and the NTSB understand the root cause, and we can fully anticipate the rate in which these cracks might grow," the FAA is imposing a "very conservative repeat" check on the fleet "as a precautionary measure," Richter said.
