​​Airlines have reported 38 structural cracks after inspecting 810 Boeing 737NG aircraft in fleets worldwide, the airframer says.

Airlines have reported discovering 38 structural cracks after inspecting 810 Boeing 737NG aircraft in fleets worldwide, the Chicago-based airframer says. The US Federal Aviation Administration (FAA) ordered airlines to inspect 737NGs that had completed 30,000 flight cycles by 10 October.

"Boeing is actively working with customers that have airplanes in their fleets with inspection findings to develop a repair plan, and to provide parts and technical support as necessary," Boeing says in a statement. "We are working around the clock to provide the support needed to return all airplanes to service as soon as possible."

The FAA ordered the inspections on 2 October after Boeing discovered cracks on "pickle fork" hardware connecting the wings and fuselages of 737-800s undergoing passenger-to-freighter conversions in China that had accumulated between 35,600 and 37,300 flight cycles. The airframer has since provided all 737NG customers detailed instructions for conducting the inspections and reporting the results.

Neither Boeing nor the FAA provided details on which airlines discovered cracks in their fleets.

The FAA ordered that if 737NGs have logged between 22,600 and 29,999 flight cycles they must be inspected for cracks within their next 1,000 cycles.

Brazil-based Gol grounded 11 aircraft for repair and replacement after completing inspection of its 737NG fleet that completed 30,000 cycles. The grounding of these aircraft requires Gol to relocate part of its air network "affecting the transportation of approximately 3% of total customers" until 15 December, the airline says in a statement.

"Gol regrets the possible inconvenience caused and stresses that it has worked hard to minimize the potential effects," the airline says.

Southwest Airlines has removed two 737NG aircraft from service because of structural cracks discovered during inspections.

United Airlines and American Airlines are among the carriers that do not have 737NGs in their fleets that have completed 30,000 flight cycles.

The FAA in a statement says it working with Boeing and other international aviation safety regulators including Brazil's National Civil Aviation Agency to better understand the factors that led to the formation of the cracks.

"The FAA will continue to monitor the situation and will consider additional action as necessary," the agency says.

The 1,911 aircraft that meet the FAA short-term and longer-term inspection requirements represent most of the US in-service 737NG fleet of 1,930 aircraft, including -600, -700, -800 and -900 series aircraft, according to Cirium fleet data.