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, leading 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 Afghanistan 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.