The U.S. Navy has announced it will allow a fleet of T-45C training jets to fly again under modified conditions while it determines what’s causing a lack of oxygen in some cockpits. Vice Adm. Mike Shoemaker said 200 T-45C aircraft will resume flights as early as Monday after being grounded for more than a week. Its pilots had become increasingly concerned late last month after   incidents in which some personnel weren’t getting enough oxygen. The concerned pilots had declined to fly on more than 90 flights. Instructors and students will now wear modified masks in the two-seat trainers and fly below 10,000 feet to avoid use of on-board oxygen generating systems. NAS Pensacola is one of three naval air stations where the T-45C is used for training.