The Air Force isn't just running short on fighter pilots. Air Mobility Command is concerned that a shortfall of mobility pilots is likely to grow over the next few years, further stressing the force.

The timing is particularly bad, as AMC is already running hard worldwide. Nearly every three minutes, one of its planes takes off somewhere around the world to refuel aircraft, move personnel or supplies, conduct medical evacuations or other missions. And nine out of 10 refueling tanker missions flown as part of the war against the Islamic State involves AMC aircraft — more than 33,000 sorties in 2016.

AMC Commander Gen. Carlton Everhart said Thursday that he sees warning signs on the horizon, and the Air Force is looking for new ideas to reverse the trend.

The command is now short 315 pilots, out of 7,940 total force pilots — active duty, guard and reserve. That's roughly a 4 percent shortfall overall.

But the deficit is especially acute among the ranks of Air Force Reserve and Air National Guard pilots. AMC has 2,012 reserve pilots, which represents a 7 percent shortfall from where the command wants to be. And on the guard side, AMC has 2,094 pilots, about 13 percent short.

Over the next four years, up to 1,600 pilots will become eligible to separate, Everhart said. AMC typically hopes to retain 60 percent of those airmen, but lately it's been holding on to less than half of them. So, by 2020, another 800 crucial pilots could be gone.

"This is just the start of a waterfall that’s going to cascade down," Everhart said. "We’re watching the forecast, and it’s going to start dropping precipitously, starting at the end of this year. It will be as bad as we’re seeing it in other weapons systems."

This is a problem the entire Air Force is facing, especially as commercial airlines have boosted their efforts to recruit from the ranks of military pilots to replace a generation of retiring Vietnam-era pilots. To help stem the losses, the Air Force has upped the maximum annual retention bonus from $25,000 per year to $35,000. It has also tried to eliminate or reassign additional duties that pilots hated.

Everhart said Air Force leaders such as Chief of Staff Gen. David Goldfein are trying to think of more creative ways to hold on to their valuable pilots. One method under consideration, he said, is to allow pilots to essentially moonlight as airline pilots for a period, and then return to active duty.

The Air Force is talking to airline CEOs to "work hand-in-hand" and try to figure out ways to do this, Everhart said.

"Are there alternative ways that we can do [pilot] hiring?" Everhart said. "Are there alternative ways that we can keep people to stay in longer? Are there alternative ways that we can hire people on active duty, and then they can go out to the airline job, come back to active duty, or vice versa?"

Everhart said he will meet with Goldfein in May to discuss whether the Air Force wants to pursue this plan, and if so, how it might move forward

In a September interview, Lt. Gen. Gina Grosso, the Air Force's personnel chief, said the air forces of several allied countries, such as the United Kingdom and Australia, already share their pilots with airlines as part of a successful retention strategy. Grosso said the Air Force was starting to look at whether the United States could emulate it.

"Bonuses help, but we can't compete. We just can't," Everhart said. "We got a limited defense budget, and so what we're trying to do is incentivize people to stay in more, with various opportunities that are provided to them, that they have a choice to make. Not just when to get out."