ORLANDO — The White House and U.S. Air Force are closing in on the final requirements needed to build the next fleet of Air Force One aircraft, the service's top acquisition official said Thursday.

But as questions swirl about whether specifications will drastically change under President Donald Trump's direction, Air Force officials maintain that revisions will be relatively minor in scope.

The service in 2015 selected Boeing to build a heavily modified version of its 747-8 design for the next presidential transport aircraft, which is in the nascent stages of development. However, Trump has placed the Air Force One replacement program under harsh scrutiny since the election, at one point threatening to cancel it altogether over the total cost of the program, which has not been cemented by the service.

One of the options on the table is to alter the plane’s requirements, first discussed by Boeing CEO Dennis Muilenburg after a January meeting with Trump. But acting Air Force acquisition head Darlene Costello said the service, which procures Air Force One on behalf of the White House, had already been hard at work ensuring that no undue expense had been baked into the program.

"The Air Force has been looking at this over the last two years to make sure that we have traded away any requirements for cost, so that we're not doing more than we have to with the aircraft," she said during a briefing with reporters at the Air Force Association’s Air Warfare Symposium. "But with the president's interest, obviously we work with the White House military office over the past few weeks on the [concept of operations] and the requirements that come from those conops, which will then inform our acquisition strategy on the program.

"So over the last few weeks, we've been working over those again to make sure that we all agree that we have the right set of requirements," she said. "I expect that we'll get that finalized from the White House military office relatively soon. Once that is done, then we'll go off and we'll adjust if need be our acquisition strategy."

The White House is not likely to downsize its requirement for two presidential transport aircraft, Costello said. But a wide array of other changes have been considered, from the number of passengers the aircraft can carry to small engineering adjustments.

Because "we've been scrubbing these [requirements] for a little while," changes "tend to be basic," she said.

'What's good is good enough'

In a latter session with reporters, Air Mobility Command head Gen. Carlton Everhart and Brig. Gen. Ty Thomas, director of Air Mobility Command’s strategic plans, requirements and programs, stated that they did not predict major changes to the aircraft.

"There may be a little bit of movement, but probably not a whole lot," Thomas said.

One area the service is focusing on is optimizing the aircraft’s power requirements. For example, the final requirement will likely move away from developmental power generation solutions and toward proven, off-the-shelf products.

"Do you really need this power requirement, and if you do, is there capability out there right now that we can put on that aircraft to support the particular power requirement, for example?" Everhart said. In some cases, "what's good is good enough."

Everhart was present at a December meeting that Trump held with several generals at the president's Mar-a-Lago resort. During that meeting, Everhart said the president asked "a lot of in-depth questions" about the aircraft and acquisition strategy.

Comments from Air Force officials appear to be at odds with the president’s own claims. In February, Trump asserted that his administration’s work to decrease requirements had reduced the total cost by a billion — almost a quarter of the $4 billion program cost that Trump has cited. Such a huge decrease likely could only arise from a massive shift in requirements.

The Air Force, for its part, has not disclosed its estimated program costs. Budget materials show that the service intends to spend $2.9 billion for research, development, test and evaluation through fiscal 2021.

"We’ve not gotten to the phase where we're going to work the cost yet," Costello said. "As soon as the requirements are set and stable, then we'll go off and we'll work our process and then we'll start negotiating with industry on what the price will be. Then we'll know how much we'll be able to actually acquire this for."