Navy student pilots have not trained in the T-45 Goshawk aircraft for more than two months due to a mysterious oxygen system problem that has puzzled the service, Vice Adm. Paul Grosklags, commander of Naval Air Systems Command, said on Tuesday.

“To date, since the beginning of April, end of March, we have not flown any training events with the students, the students have not flown at all,” Grosklags told a Senate subcommittee.