The Maritime Self-Defense Force has grounded the next-generation mainstay P-1 patrol planes due to engine trouble, Defense Ministry sources said Thursday.

On May 13, a P-1 aircraft developed unstable combustion in some of its four jet engines during a test flight over the Pacific off Aichi Prefecture, after diving to an altitude of 8,000 meters from 10,000 meters, the sources said, without elaborating, except to say the pilot managed to restart the engines before landing.

The MSDF grounded the planes from the following day.

The ministry had planned to put the planes into full operation within two years, but the technical trouble may delay this schedule. Kawasaki Heavy Industries Ltd., the manufacturer of the P-1s, will work to resolve the difficulties, the sources said.

The P-1 is the successor to the aging P-3C Orion turboprop, and the ministry plans to deploy around 70 of them.

Three P-1s have been deployed to the MSDF’s Atsugi base in Kanagawa Prefecture since March, but the move was delayed about one year after wing and fuselage cracks were found in the development stage.