CASA (Australia's aviation authorities) has a report on an airprox between a crop duster and a surveying mini-drone: ATSB incident report
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There are two instances I remember hearing about.
This one happened to a plane landing at JFK, and the FBI was seeking information about it.
They reported seeing a small black "drone aircraft" with four propellers.
I don't think they every found anything more.
And just recently , two planes landing at YVR saw an RC helicopter, one plane coming very close to it.
The RCMP was investigating, but again it's not certain whether they will come up with anything about it.
There is also an investigation into a video taken from a quadrotor showing a plane landing at YVR.
And if you thought that was close, check out this video from Australia.
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It september 2017, a DJI Phantom collided with an US Black Hawk helicopter causing a real damage to main rotor.
Obviously a military helicopter cannot be destroied by a small uav (why develop expensive Sparrow or Maverick rockets when a simple Phantom could land a military object ;-) )
but... surely it would be a true danger...
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This one in Tallahassee with a regional jet got a lot of attention when it happened but there are many more.
The FAA keeps a database, and I expect we'll see about an actual collision with a Part 121 aircraft soon enough as the number of airborne UAVs is increasing rapidly.
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February 2018 - What is thought to be
the first drone collision in the US occurred in February 2018.
The National Transportation Safety Board is investigating a helicopter crash landing in South Carolina that may have been caused by a civilian drone, according to a report from Bloomberg .
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Small civilian mini-drones such as the DJI Phantom or professional surveying drones are becoming increasingly widespread, raising concerns about the potential collision/airprox with manned aircraft (especially GA where flight altitude is typically relatively low).   Have there been actual cases of collision and/or dangerous airprox reported so far? What were the consequences?
Have there been any cases of collision/airprox of small drones with manned aircraft?
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<general-aviation><unmanned-aerial-vehicle><mid-air-collision>
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There have been some collisions/airprox incidents involving drones. Cases of airprox have been reported in Australia and airprox and collisions have been reported in the US. There have been cases of collision/airprox with drones. CASA, the Australian aviation authority, has filed a report relating to an airprox between a crop duster and a surveying mini-drone. There was also an incident at YVR involving two planes and an RC helicopter, which came very close to one of the planes. There is also an investigation being carried out on a  video  taken from a quadrotor, which shows a plane landing at YVR. Other incidents include a  DJI Phantom colliding with a US Black Hawk  helicopter in September 2017 and a helicopter crash landing in South Carolina, which may have been caused by a civilian drone and which is being investigated by the National Transportation Safety Board.
There have been cases of collision/airprox with drones. CASA, the Australian aviation authority, has filed a report relating to an airprox between a crop duster and a surveying mini-drone. There was also an incident at YVR involving two planes and an RC helicopter, which came very close to one of the planes. There is also an investigation being carried out on a  video  taken from a quadrotor, which shows a plane landing at YVR. Other incidents include a  DJI Phantom colliding with a US Black Hawk  helicopter in September 2017 and a helicopter crash landing in South Carolina, which may have been caused by a civilian drone and which is being investigated by the National Transportation Safety Board.