Though none has to date, the risk of a drone being ingested by a jetliner engine and causing a fatal accident is all too real. Bird strikes are bad enough. One of the most feared birds encountered by aircraft is the common Canada goose, weighing anything up to 6kg. In 2009 a collision with a flock of migratory Canada geese caused a US Airways flight to suffer complete loss of power after takeoff from LaGuardia airport, New York. The bird strike could have easily ended in disaster but for the skill of the pilot, Captain Chesley Sullenberger, who famously brought the stricken Airbus A320 down for a splash landing in the Hudson River without loss of life. The last thing airline pilots need is an additional hazard caused by UAVs weighing as much or more than a Canada goose.

Over the past year, American pilots have reported some 700 close encounters with UAVs of one sort or another. That pales almost into insignificance when compared with the 13,700 bird strikes in America last year, or the 4,000 or so reports of laser pointers being directed at aircraft. But the number of recreational drones is increasing rapidly. If just one percent of those already in circulation were to cause problems, drone threats would exceed bird strikes by a considerable margin.

The biggest aviation threat of all comes when passenger planes are taking off or landing, or when helicopters have to operate close to the ground while fighting fires, rescuing accident casualities or pursuing criminals. Unlike fixed-wing aircraft, which are required in America to fly at least 1,000 feet above the highest obstacle in their path, helicopters can operate at much lower altitudes. At least four instances have been reported over the past six months of helicopters fighting wild fires in southern California having to cease operations because of reckless drone-owners flying dangerously close for their own amusement.