Reporting from Washington — Drone crews protect U.S. ground troops by watching over them 24 hours a day from high above. Sitting before video screens thousands of miles from their remote-controlled aircraft, the crews scan for enemy ambushes and possible roadside bombs, while also monitoring what the military calls "patterns of life."

Only rarely do drone crews fire on the enemy. The rest of the time, they sit and watch. For hours on end. Day after day.

It can get monotonous and, yes, boring.

It can also be gut-wrenching.

Crews sometimes see ground troops take casualties or come under attack. They zoom in on enemy dead to confirm casualties. Psychologically, they're in the middle of combat. But physically most of them are on another continent, which can lead to a sense of helplessness.

"That lack of control is one of the main features of producing stress," said Air Force Col. Hernando Ortega, who discussed results of a survey of Predator and Reaper crews at a recent conference in Washington, D.C. They ask themselves, he said: "Could I have done better? Did I make the right choices?"

The Air Force is only now becoming aware of the toll — which Air Force psychologists call combat stress — posed by drone crews' job, even as the drone workload is growing.

In recent years, the Air Force has trained more drone pilots than conventional pilots, and the Pentagon is increasingly relying on drones to fight wars and terrorism overseas. Drone crews flew 54 combat air patrols a day over and Iraq last year, up from five a day in 2004. The goal is 65 patrols a day by 2013.

The military is changing its terminology accordingly. What the Air Force used to call UAVs, for unmanned aerial vehicles, are now called RPAs, for remotely piloted aircraft.

"They are not unmanned at all," Ortega said. "They're manned to the hilt."

In civilian jobs, the pressures of working long hours on staggered shifts are wearing enough. But with drone missions, one miscalculation can prove fatal.

Last April, two U.S. Marines were accidentally killed by Predator fire, and at least 15 Afghan civilians died in a mistaken attack by a Predator and helicopter gunships in February 2010.

The Air Force considers drone crews "deployed'' in combat, even though most of them fly planes from U.S. bases. "The most dangerous part of their day is their commute,'' said Peter W. Singer, a Brookings Institution scholar who studies robotics in warfare.

Crews must shift repeatedly between home and combat. "A Predator pilot told me: 'I'm spending 12 hours fighting enemy combatants, and 20 minutes later I'm talking to my kids about homework,'" Singer said.

The three-member crews typically work 12-hour shifts. They monitor the landscape and events on the ground — what the Air Force calls ISR, for intelligence, surveillance and reconnaissance — often through the early-morning hours.

"Humans don't work well at 3 in the morning … we're not nocturnal," said Ortega, a flight surgeon. "And that builds fatigue, which decreases human performance, which leads to more stress."

"It's really kind of a boring job ... it's kind of terrible," Ortega said, paraphrasing comments from the survey.

At the same time, the crews can develop strong emotional bonds with ground troops via text messages and radio, Ortega said. "These guys actually telecommute to the war zone," he said. "The band of brothers is built online."

That contributes to the sense of helplessness when their colleagues are in physical danger.

"There can be guilt even if no shot is fired, just from the fact that you don't feel you can help," said Col. Kent McDonald, an Air Force psychiatrist who helped conduct the recent survey of 900 drone crew members in 2010 and 2011.

In the survey, 46% of active-duty drone pilots reported high levels of stress, and 29% reported emotional exhaustion or burnout. The data included Air Force crews who have flown drones over Iraq and Afghanistan, but not crews who fly drones over Pakistan, Yemen and Somalia as part of CIA programs.

Active-duty sensor operators, who operate camera and surveillance gear, reported high-stress rates of 41% and burnout rates of 21%. Mission intelligence coordinators, who are often separated from pilots and sensor operators, reported high-stress rates of 39% and burnout rates of 20%.

By comparison, a recent Families and Work Institute study found that 26% of civilian workers were "often or very often burned out or stressed by their work," and a Yale University study found that 29% of workers felt "quite a bit or extremely stressed at work."

Combat stress puts many drone crew members at high risk forpost-traumatic stress disorder, Air Force psychologists say, though no pilots and only a couple of camera operators have been diagnosed with PTSD.

By comparison, between 11% and 20% of troops from Iraq and Afghanistan, including some Air Force crew members who fly conventional warplanes, have been diagnosed with PTSD, according to the National Center for PTSD.

At the very least, the stress leaves drone crews at risk for depression and anxiety disorders, potentially affecting their performance, psychologists say.

Often, crew members don't even acknowledge that they're stressed by combat. After all, they're not directly exposed to combat smells or sounds, or the imminent threat of death — all typically associated with PTSD in ground troops.

Crews who feel stress don't say it's "because I was in combat or because we had to blow up a building or because we saw people get blown up," Ortega said. Rather, he said, they complain of shift work, schedule changes, long hours, low staffing and failure to maintain family relationships.

As one respondent complained: "Sustaining vigilance is mind-numbing." Others wrote: "Too much monotony/Groundhog Day" and "Not being around to do stuff at home."

The Air Force is considering assigning more chaplains and psychologists to drone units, said Dr. Wayne Chappelle, an Air Force clinical psychologist. It's also considering workshops on family stresses, and adding psychologists to military clinics so that crew members don't have to be referred to outside specialists.

Combat stress needs to be addressed before it affects crew performance, Chappelle said.

Crew members may not be able to escape combat stress, Ortega said.

"This is a different kind of war, but it's still war," he said. "And they do internally feel it."