WASHINGTON — Shortly after the day’s final bell rang and hundreds of youngsters ran outside Lickdale Elementary School with their book bags and lunchboxes, a military drone fell from the sky.
The 375-pound Shadow reconnaissance drone skimmed the treetops as it hurtled toward the school in Jonestown, Pa. It barely missed the building, then cartwheeled through the butterfly garden and past the playground.
The aircraft kept rolling like a tumbleweed and collided with a passing car on Fisher Avenue. People called 911. The rescue squad arrived in a hurry. Luckily, no one was hurt.
The April 3 near-disaster was the latest known mishap involving a military drone in the United States. Most US military drone accidents have occurred abroad, but at least 49 large drones have crashed during test or training flights near domestic bases since 2001, according to a yearlong Washington Post investigation.
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A 63-foot-long QF-4 target drone exploded into a fireball at Tyndall Air Force Base in Florida on July 17, forcing authorities to close a nearby highway. A Global Hawk, the largest drone in the military’s fleet, crashed on Maryland’s Eastern Shore in June 2012.
Two years earlier, the military almost had to shoot down a runaway Navy drone after it penetrated some of the most guarded airspace in the country, over Washington, D.C.
The number of accidents has jumped as the military has brought back drones from overseas and operated them more frequently in airspace shared with civilian planes. The military has almost tripled the number of hours its drones have flown annually in shared US airspace since 2011, according to federal data.
Now, the military and the federal government are preparing for a far bigger expansion of drone flights that will transform US aviation — but could also pose the biggest challenge to safe air travel in decades.
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Thanks in part to a new federal law that will open the national airspace to drones of all kinds, the Pentagon is planning to operate thousands of drones from at least 110 bases in 39 states, plus Guam and Puerto Rico, by 2017.
Budget documents show the military has spent or allocated $1.6 billion for drone-related construction projects in the United States. The Defense Department has about 10,000 drones worldwide, from 1-pound Wasps and 4-pound Ravens to the 15-ton Global Hawk.
At the same time, under orders from Congress, the Federal Aviation Administration is preparing rules that will allow civilian drone flights across much of the country. The agency predicts that as many as 7,500 small commercial drones could take to the skies by 2018.
American airspace is safer than ever. Fatal accidents involving passenger carriers have become exceedingly rare. Military and FAA officials pledge that standards will not be compromised to accommodate drones. The Pentagon says that accident rates per flight hour have declined steadily since it began deploying drones to war zones in the early 2000s and that it takes extra precautions when flying at home.
Air Force Colonel James Marshall, safety director for the Air Combat Command, said the military has trained regular pilots for generations in the United States, with relatively few accidents, and that flying drones at home will be no different.
‘‘We’re really big into risk management,’’ he said. ‘‘I’m comfortable with what I see with the mitigation approach that we’re taking.’’
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Michael Huerta, the FAA’s administrator, said the agency would integrate drones into the national airspace in stages and only after a careful review of each step.
‘‘We have an extraordinarily safe aviation system, and that’s been through the hard work of a lot of people in industry and their public-sector counterparts to continually raise the bar on safety,’’ he said in an interview. ‘‘As we integrate unmanned aircraft into the national airspace system, I believe that the public expect us not only to maintain but to continue to enhance the levels of safety that we’ve been able to achieve for conventional aircraft.’’
But thousands of pages of military investigative reports, obtained by the Post under the Freedom of Information Act, show a pattern of pilot errors and mechanical failures that have caused drones to crash in the United States again and again — including drones flying in civilian airspace.
The documents also reveal that some military personnel harbor an ingrained distrust of the flying robots.
After a Predator crashed at Cannon Air Force Base in New Mexico on July 28, 2010, an unidentified instructor pilot told investigators that commanders had been queasy about having remotely controlled aircraft on the base, fearful that they might crash into buildings.
Scattered opposition to drones has built across the country in the form of protests and campaigns to stop drone strikes overseas or prohibit surveillance by government aircraft.