Agence France-Presse, About one-quarter of the U.S. military’s P-3 Orion aircraft fleet have been grounded for inspections and repairs after the discovery of a structural fault, the U.S. Navy said Jan. 11.

The move affects 39 out of a fleet of 161 aircraft, spokesman Lt. Clay Doss said. The structural problem could cause cracks in the plane’s wings, according to the Pentagon.

“The analysis and corrective measures are going to take between 18 and 24 months per aircraft to complete,” Doss said. “The intention is to bring them back to the fleet.”

Defense manufacturer Lockheed Martin’s Orion maritime patrol aircraft first went into service for the U.S. Navy in 1962 and have been modified over the years, with the latest model built in 1990.

The land-based, turboprop aircraft is designed for long-range maritime patrols and anti-submarine operations.