As the U.S. military scrambles to get more robotic warplanes like the Predator drone aloft, it is confronting an unexpected adversary: human error.

The drones are prized by the Pentagon for their ability to provide reconnaissance imagery and close-air support to ground commanders in Iraq and Afghanistan.

But an Air Force researcher has found that operator mistakes are responsible for a growing number of Predator mishaps in recent years, a period in which the drones have been flown by increasingly inexperienced crews.

"The Air Force has increased the sheer volume of pilots put through the training pipeline and shipped them off to war with the bare minimum training required," researcher Lt. Col. Robert P. Herz said in an e-mail.

Herz investigated the Predator's record earlier this year in a doctoral dissertation that has circulated among military planners and safety experts. He provided a copy of the research to The Associated Press.

The military can continuously operate 29 of the Predator and Reaper surveillance planes, which are flown by remote control from the United States. Each crew consists of a pilot, who is an officer, and a sensor operator, who is an enlisted airman responsible for running the plane's cameras and weapons.

Seven Predators have been destroyed this year, all in combat zones. The causes are still under investigation.

In the last few years, the number of "Class A mishaps" — those resulting in $1 million or more in damage — has generally been between four and six.

The planes cost about $4 million each.

Early in the Predator program, most crashes were blamed on equipment breakdowns, many of which have now been resolved. Herz found that 71 percent of Predator mishaps from 2003 to 2006 could be attributed to "human error factors."

Operator error periodically causes the drones to go down behind enemy lines, where fighters must then bomb them so prized technology does not fall into the wrong hands. Other times, the planes slam onto runways, damaging optics and landing gear. On rare occasions, pilots bank the aircraft so steeply that the drones briefly lose contact with the satellite feeding them commands.

Herz's findings come at a time when the military relies more than ever on remote-controlled warplanes.

Federal funding for unmanned aerial vehicles has increased from $3 billion in the 1990s to more than $12 billion through 2009, according to Herz, who completed the dissertation earlier this year at Northcentral University in Prescott Valley, Ariz., where he studied for a doctorate in business administration with a concentration aerospace operations.

The aircraft are packed with camera gear and can monitor battlefields for more than 20 hours straight. They can also be equipped with two Hellfire missiles capable of blowing up a car.

The military can't get enough Predator operators "mission-ready" fast enough to meet the demand for intelligence requested by ground commanders, Herz said. Currently, only a third of such requests can be met, he said.

In the early Predator program, the typical sensor operator arrived with midlevel enlisted rank and had at least two previous assignments, many of them aviation-related, Herz wrote.

Today, half of new sensor operators "have come directly from basic training as their first exposure to the professional workforce," Herz found.

Col. Chris Chambliss, commander of the 432nd Wing at Creech Air Force Base, Nev., said in a telephone interview that many of his sensor operators are 18 or 19 years old.

Predator pilots, too, are less experienced than before. In the past, they typically had two, three or more tours with a warplane before coming to the drone. Now the average Predator pilot now has one tour, Herz found.

Herz reported that most Predator mishaps were the result of three types of human errors: inadequate skills and knowledge necessary to operate the aircraft; lack of teamwork; and lack of situational awareness.

The result, Herz wrote, is that operators are less able to conduct "real-world operations." For example, he said, "lapses of judgment and lack of experience" often lead pilots to continue marginal landing approaches when it would have been better to circle around for another attempt.

Peer pressure, he found, often leads pilots to attempt to "save a landing" rather than face ribbing from their colleagues after a botched landing.

Herz urged military planners to bolster training for pilots, sensor operators and the mechanics who work on the planes. Some of his findings have been incorporated into new training curriculum this year, he said.

"This is stuff we can correct through training, stuff that happens between the ears," Herz said in a telephone interview. He emphasized that his findings represented his own research, and did not necessarily reflect official Air Force views.

Asked about Herz's findings, Col. Eric Mathewson, director of the Air Force's unmanned aircraft systems program, said: "Flight safety is of paramount importance to the Air Force, and every mishap is carefully studied in order to mitigate future risk."

There were five major mishaps involving Predators in fiscal year 2007, and three of the drones were destroyed, according to an AP review of Air Force statistics.

Predators were flown harder than ever during that period — more than 79,000 hours, up from about 58,000 hours the previous year — but the mishap rate actually went down slightly. Still, the Predator's mishap rate — the military's key measure of reliability — is far higher than for "mature" aircraft such as the F-16 and F-15, which have been in the Air Force fleet since the 1970s.