Story highlights

At least 327 incidents fall under the definition of close encounters

"Incidents largely occur ... where manned air traffic density is high," expert says

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 airport operator and 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 an unmanned aerial vehicle.

“It’s very, very concerning because having drones at JFK or any major airport was illegal even before the latest drone laws came into effect,” CNN aviation analyst Mary Schiavo says. “What is happening now is there are some stiff prosecutions being handed out – including jail time – for lawbreakers. As they people get the word, they won’t do such idiotic things anymore.”

“As the people get the word, they won’t do such idiotic things anymore.”

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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.

“Going into an engine can destroy an engine,” Derner said. “Going into the cockpit window can injure a pilot or even kill a pilot.”

But CNN analysts Bob Baer and Jonathan Gilliam worry about how drones could be used by those seeking to attack planes.

“This is crazy,” Baer said. “You can take these drones and, with a 3D printer, make them out of explosives. They’re very dangerous and they’re advancing pretty quickly.”

Baer says that airports should consider jamming drones close to airports or just simply “knock them out of the air.”

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Gilliam thinks that drones shouldn’t be able to fly as high or as far as they do. “Four hundred feet away from the controller, these drones should shut off,” he said.