WASHINGTON — The T-X trainer replacement program is one of the US Air Force's top recapitalization programs, viewed as vital by service leaders as they prepare to train the next generation of F-22 and F-35 pilots.

And based on budget documents, the winner could also end up producing a new "Red Air" companion trainer to fill the US Air Force's aggressor requirement — although top service officials stress that no official decision has been made on the Red Air trainer.

The T-X program has drawn significant interest from five different industry competitors. The winner of the program, which will replace the aging T-38 advanced trainers with 350 new production models, will likely be set up as the dominant producer of training aircraft for the next two decades.

Under tThe research, development, test and evaluation (RDT&E) portion of the Air Force's budget, where the T-X program is listed, includes is language for a companion trainer to be developed from whatever the eventual T-X selection is.

"The companion trainer is intended to develop a variant of the advanced pilot training (APT) T-X aircraft to provide a 'Red Air' adversary or aggressor capability for 5th generation fighter aircraft," the budget document reads.

The RDT&E budget wedge starts in fiscal 2018 with just $10,000, then jumps up to $11.2 million in fiscal 2019 and $30 million in fiscal 2020. Overall, the projected total cost to create a Red Air T-X design is $220.4 million.

"The T-X variant will be equipped with radar/data-link and hard-points for weapons and jammer carriage is envisioned for this role," the document adds. "Missionization of the T-X to meet companion trainer requirements is expected to begin in FY2019."

The service has a need for a new Red Air system, as current planes are not able to truly challenge F-22 or F-35 jets during training. Service observers of the service had assumed the Air Force was considering basing a new Red Air jet on T-X, but this is the first real acknowledgement of that option in budget documents.

The presence of a Red Air wedge in the budget was a mystery to two top service officials, both of whom have a direct stake in the program.

Gen. Robin Rand, head of Air Education and Training Command, and Gen. Herbert "Hawk" Carlisle, head of Air Combat Command, both told reporters on Feb. 12 that they had no knowledge of the Red Air presence in the budget.

Rand, in fact, was emphatic that no decision has been made, forcefully saying he was focused on keeping the T-X program on track just as a trainer.

"There is no decision that's been made yet to use the T-X in any other capacity…There will be discussions, but that's not been decided," he said.

Although he didn't use the word, it was clear Rand remains concerned that requirements creep, whether from a Red Air requirement or elsewhere, could saddle the trainers he needs with both cost overruns and timeline delays.

However, Rand said, the requirements — which should be made public within the next few weeks — will leave space for upgrades to the program, whether that means the ability to turn the T-X into an aggressor aircraft or just to keep it up to date with new technologies.

"We don't want buyer's remorse. It's a 50-year-plus aircraft," Rand said. "We think our requirements will allow the airplane to absolutely do those things…. Will it have that capability [to upgrade in the future]? We think it will, but it's got to be done affordably."

Carlisle agreed no decision has been made, but was more enthusiastic about the option of a T-X based Red Air solution.

"I'm looking at it," he said. "If there is an already developed airplane and I know I need more adversary air and we already have an airplane developed, that makes sense to me. But it's not part of the original plan.

"If someone were to ask me if I think that will happen," he added, "I think it probably will."

But if neither general requested the Red Air funding under T-X, why is it there?

An Air Force spokesperson for the Air Force described the funding as "money to think about it," rather than a solid plan to move ahead, and said the funding stream there should be thought of as separate from T-X despite being listed underneath the budget item.

"We're solidifying our requirements for T-X, but at this point we don't know which road we'll take to accomplish the Red Air or companion trainer missions; a T-X variant is just one option," the spokesperson added.

Todd Harrison, a budget expert with the Center for Strategic and Budgetary Assessments (CSBA), said it's "not surprising" that Carlisle and Rand were not aware of the funding wedge, given that it doesn't really start until fiscal 2018.

"I wouldn't read anything into the fact that they didn't know about it," Harrison added. "It's too small and too far in the future — well below their radar."

The challenge is in training fifth-generation pilots, where old aggressor units simply can't keep up. Using a T-X based Red Air design will help train the next wave of advanced aircraft pilots, while also saving significant funds over having to use an F-35 or F-22 as an aggressor aircraft.

Former Air Force official Mark Gunzinger, also with CSBA, said the creation of an eventual T-X Red Air variant makes a lot of sense.

"Exploring the potential for a T-X airframe to support roles other than basic flying training is a great idea," Gunzinger said. "Moreover, thinking about those roles now — at the front end of a program — is far better than changing requirements (requirements creep) in the middle of a program. I do think it's pretty early in the process for a decision on Red Air."

Neither Carlisle nor Rand offered a timetable for when a decision on the development of a Red Air companion trainer would need to be made, although the latter did note there is no "requirement" for that decision to be finalized in 2015.

Harrison warned not to get too excited about the wedge, given the year-by-year budgeting process.

"The only thing that really matters in the budget submission is FY16 funding because Congress only appropriates one year at a time," he cautioned. "All of the projections over the [future years defense plan] FYDP are subject to change with future budget submissions. I would just interpret this as the Air Force leaving the door open to using the T-X for Red Air.

"They certainly had not decided against it at the time the budget was prepared or the funding would not have been there — but it doesn't mean they have definitely decided to do it."

The service plans on issuing a request for proposals on the program in the fourth quarter of fiscal 2016, with a projected contract award in the fall of 2017.

There are two clean-sheet teams going head-to-head for the program in Northrop Grumman and a Boeing-Saab cooperative, while two legacy systems are being submitted offered in the form of Lockheed Martin's offering of Korean Aerospace Industries' T-50, and the T-100, a collaboration between General Dynamics and Italy's Alenia Aermacchi based on the latter's M-346 design. Textron AirLand is also planning to enter a trainer variant of its new Scorpion jet in the competition.

Twitter: @AaronMehta