WASHINGTON — Lockheed Martin's upgraded T-50A jet trainer successfully completed its first flight test, a key milestone for the aircraft Lockheed is offering for the Air Force's next-generation trainer fleet.

The flight took place in Sacheon, South Korea, as the T-50A is currently manufactured there jointly by Korea Aerospace Industries (KAI) and Lockheed Martin.

"The aircraft in its new configuration with the 5th Gen cockpit and other upgrades performed flawlessly," Mark Ward, Lockheed Martin T-50A lead test pilot, said after his flight. "I have no doubt this aircraft will close the gap which currently exists between the trainer fleet and 5th Generation fighters."

The T-X is a crucial piece of the Air Force's plan to eventually replace all of its fighter jets with the fifth-generation F-35s and F-22s, as the service needs an upgraded trainer fleet to properly train its rising pilots to fly fifth-generation aircraft.

Lockheed earlier this year announced it would offer the T-50A for the Air Force's T-X program to replace its aging T-38 trainers, nixing a plan to offer clean-sheet design. The key advantage of offering the T-50A instead of a clean-sheet design is that the Korean aircraft is ready for production now, Lockheed officials have said. More than 100 T-50s and more than 1,000 pilots are flying in South Korea, Indonesia, Iraq, the Philippines and Thailand.

For the T-X offering, Lockheed added capabilities to the existing T-50 in a block upgrade that includes embedded training, open system architecture, inflight refueling and a fifth-generation cockpit to better train pilots to fly modern aircraft. The jets will be built in South Korea and in Greenville, South Carolina, where Lockheed is currently standing up its T-50A Final Assembly and Checkout (FACO) site.

The Air Force is planning to release a request for proposals (RFP) for T-X in December, with a contract award expected sometime next year or early 2018, according to Lockheed spokesman Rob Fuller. Full operational capability of the T-X fleet has been pushed two years from when it was originally planned, from fiscal 2032 to 2034. Initial operational capability remains as planned in 2024.

As the service prepares to release the official RFP, a field of at least four industry teams is shaping up to compete for the contract. In addition to the Lockheed and KAI team, Raytheon, Finmeccanica and CAE will offer the T-100.

Meanwhile a pair of clean-sheet designs is being put forth by a Boeing/Saab team and a Northrop Grumman-led coalition that includes BAE Systems and L-3. Northrop expects to fly its prototype this year as well.

Textron AirLand had hoped to offer its Scorpion design, or a modified version of the plane, for the T-X competition. But earlier this year, president Bill Anderson said the company had ruled out offering Scorpion and also determined that developing a clean-sheet design for the T-X requirements as they currently exist would be cost-prohibitive for the company.

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