(CNN) -- Hundreds of drones fly dangerously close to manned aircraft in U.S. airspace, forcing pilots to take evasive action sometimes, a new study shows.
Experts reviewed 921 cases involving drones and manned aircraft between December 2013 and September this year.
Of those, 327 incidents fall under the definition of close encounters, according to Bard College's Center for the Study of the Drone.
It defined "close encounters" as drones coming within 500 feet of aircraft.
In close encounters, pilots "maneuvered to avoid a collision with a drone" at least 28 times, the study says. Such encounters involved various aircraft, including passenger jets and helicopters.
"Our findings indicate that incidents largely occur in areas where manned air traffic density is high and where drone use is prohibited," experts said in a statement.
Wandering into flight paths
The study released Friday gives a detailed overview of drones wandering into aircraft flight paths.
It defines close encounters as instances when a drone flew close to a plane and is not limited to cases where pilots took evasive action.
Aside from the close encounters, there were 594 sightings in which drones were spotted near or within manned aircraft flight paths but did not pose a danger of collision.
Unmanned aircraft systems are neither supposed to fly within five miles of an airport without notifying the control tower. Nor are they supposed to go above 400 feet.
But the FAA says it gets about two reports per day from pilots saying they spotted a remotely operated aerial vehicle.
"What is happening now is there are some stiff prosecutions being handed out -- including jail time -- for lawbreakers," said Mary Schiavo, a CNN aviation analyst.
"As the people get the word, they won't do such idiotic things anymore."
Terrorism risk
Drones flying too close to commercial flights pose a serious threat to larger aircraft, and can be sucked into the engine or crash into the cockpit window, injuring or killing a pilot.
Then there's the risk of terrorism.
CNN analyst Bob Baer worries about use of drones to attack planes.
"You can take these drones and, with a 3D printer, make them out of explosives," he said. "They're very dangerous and they're advancing pretty quickly."
Airports should consider jamming nearby drones or simply "knock them out of the air," Baer said.
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