Southwest Airlines removed two Boeing 737NG aircraft from service because of structural cracks discovered during inspections mandated by the US Federal Aviation Administration of such aircraft that had completed 30,000 flight cycles.
The FAA ordered the inspections on 2 October after Boeing discovered cracks on hardware connecting the wing and fuselage of 737-800s, known as "pickle forks". Airlines must report their findings to the agency.
The inspection order is separate from safety modifications for the later-generation Boeing 737 Max aircraft the FAA is testing to determine when those aircraft are safe to return to service.
"We did not find abnormalities on the vast majority of our inspected fleet but did identify signs of pickle fork cracking on two aircraft," Southwest says in a statement. "The aircraft will remain out of our schedule until the issues have been fully resolved. Safety is always our uncompromising priority, and our technical operations team is now focused on completing inspections of the remaining portion of the 737 NG fleet."
The 1,911 aircraft that meet the FAA inspection requirement represent most of the US in-service 737NG fleet of 1,930 aircraft, according to Cirium fleet data. Of these 737NGs, including -600, -700, -800 and -900 series aircraft, the FAA mandated that 165 aircraft must be inspected by 10 October because they have accumulated more than 30,000 flight cycles. Aircraft that have logged between 22,600 and 29,999 cycles must be inspected within 1,000 additional cycles.
United Airlines in a statement says none of its 737NG aircraft have flown more than 30,000 cycles.
"Roughly 80 of our aircraft fall between the 22,600 and 29,999 cycles and all of those aircraft will be inspected within the required timeframe," United says.
American Airlines, which operates some 300 737NGs, says none of its aircraft will need inspections within seven days. The carrier anticipates 80 aircraft will need inspections within eight months, but it expects no operational impact.
Delta Air Lines, which operates some 220 737NGs, says it has not discovered structural cracks on its fleet.
The Boeing aircraft that spurred FAA to issue the airworthiness directive were undergoing passenger-to-freighter conversions in China and had accumulated between 35,600 and 37,300 flight cycles, according to the agency's directive.