More than 300 incidents classified as "close encounters" occurred between drones and manned aircrafts in U.S. airspace over a period of less than two years, according to a new study.

The report by Bard College's Center for the Study of the Drone, found that 327 incidents between December 2013 and September 2015 posed a "proximity danger" where an unmanned aircraft got within 500 feet of a plane, helicopter or other manned aircraft or when a pilot determined a drone was dangerously close.

"We've seen an increase in the number of incidents that have been reported by pilots and air traffic controllers," Dan Gettinger told NPR. He is one of the authors of the study and co-Director of the Center for the Study of the Drone.

"The number of incidents has increased dramatically and we have also recorded a growth in the number of close encounters between drones and manned aircraft," Gettinger said.

The study, "Drone Sightings and Close Encounters" also found that of these close encounters, 51 of them came within a proximity of 50 feet or less to a piloted vehicle. There were 28 separate instances, according to the report, where a pilot "maneuvered to avoid a collision with a drone."

"Close Encounters involved multiengine jet aircraft, 90 of which were commercial aircraft (the majority of which have the capacity to carry 50 or more passengers). We also counted 38 Close Encounter incidents involving helicopters," the report stated.

Another 594 incidents were classified as a "sighting" which is defined in the report as a pilot or an air traffic controller spotting a drone flying "within or near the flight paths of manned aircraft" but do not pose an imminent threat.

The findings were based on an analysis of Federal Aviation Administration and other government agencies incident reports where a pilot or air traffic controller reported too close with an unmanned aircraft.

Of the 921 instances reported during the study's time frame, a majority of the incidents took place within five miles of an airport and more than 90 percent of the occurrences took place above 400 feet. Both are violations of the FAA's model aircraft operations safety guidelines.

Gettinger says he does not know why people are flying their model aircrafts at such high altitudes. He speculates though that the majority of civilian drone operators don't know that they could be flying in the path of a manned aircraft.

"Some people just want to see how far they can go and how far they can take the drone. And some people just want to see what it's like to fly above the clouds," Gettinger told NPR.

The study also found 116 close encounters involved a multi-engine jet, the majority of which can carry 50 passengers are more.

There have been no confirmed incidents where a drone has crashed into a piloted aircraft. But the study points out that the effects of a manned aircraft colliding with a bird are well documented. It adds:

"With sufficient speed, bird strikes have been known to penetrate the cockpit. It is entirely possible, then, that a drone could also break through into a cockpit, potentially causing serious harm to the pilots or other occupants."

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