Requirements for a new trainer aircraft to replace the T-38 have been released to industry by the U.S. Air Force.

The Air Force plans to award a contract for 350 so-called T-Xs to replace T-38s in the fall of 2017, with initial operational capability by the end of 2023.

The T-X requirements identify three key performance characteristics for the aircraft for advanced pilot training: sustained G, simulator visual acuity and performance, and aircraft sustainment. While there are just over 100 requirements in all, these were the most critical to ensure the T-X Family of Systems closes training gaps and creates strategic agility for the future, the Air Force said.

According to Brig. Gen. Dawn Dunlop, the director of plans, programs and requirements at Air Education and Training Command, or ATEC, the T-38 is no longer practical for preparing pilots for newer aircraft. Currently, 12 of 18 advanced pilot training tasks can't be completed with the T-38, which means fighter and bomber formal training units must complete that training and at a much greater cost.

"Cockpit and sensor management are fundamentally different today in 4th- and 5th-generation aircraft than it was when the T-38 was built in 1961," Dunlop said. "While the T-38 has been upgraded to a glass cockpit, the inability to upgrade the T-38's performance and simulated sensor capability presents a growing challenge each year to effectively teach the critical skills essential to today's military pilots."

A second issue for the T-38, according to Dunlop, is sustainment. The T-38s assigned to Air Education and Training Command haven't been able to meet the 75-percent availability requirement since 2011.