There have been at least 241 reports of close encounters between drones and manned aircraft that meet the government's definition of a near midair collision, including 28 in which pilots maneuvered to get out of the way, according to a report released Friday.
Ninety of the close encounters involved drones and commercial jets, the majority of which had the capacity to carry 50 people or more.
Two aircraft must fly within 500 feet of each to meet the Federal Aviation Administration's definition of a near midair collision. In 51 of the incidents, the drone-to-aircraft proximity was 50 feet or less, according to the report by Bard College's Center for the Study of the Drone in Annandale-On-Hudson, New York.
The report is based on an analysis of government records detailing 921 incidents involving drones and manned aircraft between Dec. 17, 2013, and Sept. 12, 2015. Researchers cautioned that when flying at high speeds it can be difficult for a pilot to judge the distance between themselves and another object.
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The majority of the incidents, 64.5 percent, were sightings of drones in the vicinity of other aircraft with no immediate threat of collision.
The FAA has previously released data on reports of drone sightings, but the Bard report is the first comprehensive analysis of the sightings by researchers outside the aviation community. Its findings are likely to fuel more debate over how much of a threat drones are to manned aircraft as the government struggles with how to reap the benefits of unmanned aircraft without undermining safety.
Most of the sightings analyzed in the report occurred within 5 miles of an airport and at altitudes higher than 400 feet even though the FAA prohibits flying most drones near airports or over 400 feet.
The locations with the most incidents were New York/Newark, New Jersey, 86; Los Angeles, 39; Miami, 24; Chicago, 20; Boston, 20; San Jose, 19; Washington, DC, 19; Atlanta, 17; Seattle, 17; San Diego, 14; Orlando, 13; Houston, 12; Portland, Oregon, 12; Dallas/Fort Worth, Texas, 11, and Denver, 10.
There have been no confirmed collisions between drones and manned aircraft in the U.S. thus far. Government and industry officials have expressed concern that if a drone -- much like a bird -- is sucked into an aircraft engine, smashes a cockpit windshield or damages a critical aircraft surface area, it could cause an air crash.
"With sufficient speed, bird strikes have been known to penetrate the cockpit," the report said. "It's entirely possible, then, that a drone could also break through into a cockpit, potentially causing serious harm to the pilots or other occupants."
Helicopter blades are considered especially vulnerable. Thirty-eight of the near collisions identified by researchers involved helicopters.
Aircraft engine manufacturers currently test the ability of engines to withstand bird strikes by firing dead birds at the engines at high velocities. The FAA hasn't yet said when it will require engine makers to conduct tests with drones, but officials have unofficially acknowledged they are working on the issue, the report said.
The report cited research by engineers at Virginia Polytechnic Institute and State University in Blacksburg, Virginia, that used data on bird strikes to create computer simulations of drones striking planes in order to identify the riskiest impact locations. They concluded that hobby drones weighing between 2 and 6 pounds "can potentially cause critical damage."
The FAA is in the process of finalizing rules for the use of commercial drones weighing less than 5 pounds. The agency is also expected to shortly issue rules requiring the registration of small drones, including those used by hobbyists, in an effort to help create a "culture of responsibility" among drone operators. The agency is trying to get the registration rules in place before Christmas.