While the vast majority of U.S. Air Force pilots still control their aircraft from inside the cockpit, about 8.5 percent are drone pilots who operate their vehicles remotely. That percentage is expected to grow, but there's a problem: the Air Force can't get enough people to volunteer for the training, according to a new report written by Air Force Colonel Bradley Hoagland for the Brookings Institution think tank.

Here's the challenge: Drones are usually chosen for jobs that are "dirty, dangerous, or dull"—with dull being the key word here. Some surveillance drones require round-the-clock shifts, and the very stressful work is so time intensive that drone pilots often cannot take advantage of additional training and education, which in turn dampens their prospects for career advancement, according to the study.