THEY CAN fly at more than 100 mph and remain airborne for a day at a time, monitoring traffic, searching for sailors lost at sea or even tracking suspected criminals 20,000ft below, all without a human on board.
Now plans to allow unmanned drones to fly alongside conventional aircraft in the skies over the UK by the end of the decade have prompted warnings from pilots who fear an increased risk of mid-air collisions.
Their warning comes as officials from the Civil Aviation Authority (CAA) and manufacturers of unmanned aerial vehicles (UAVs) are rewriting the rules governing UK airspace to pave the way for a fleet of robotic drones to carry out a wide range of non-military roles.
These range from police surveillance and search and rescue to monitoring the health of crops over farmlands, checking on pipeline networks and tracking pollution after chemical spills.
Pilots say this would mean drones sharing the skies with passenger airlines as well as light aircraft, gliders and hot air balloons, in areas where aviators rely, ultimately, on line of sight - and emergency action - to avoid each other.
Dave Reynolds, a flight safety officer from the British Air Line Pilots Association (Balpa), said: "Although modern manned aircraft spend a lot of time under automatic control, they have the pilot on board to respond to mechanical or technical failure. It's not going to be very easy to replicate that.
"Technology has existed for many years to have trains running without drivers. But not many do so because there's a psychological aspect of reassurance from having a human in control.''
Keith Auchterlonie, from the British Gliding Association, said: "Our major concern is safety. People need to realise it's not something that is far off in the future. We're very aware of the developments and are working closely with the CAA on any decisions.''
There are also industry fears of problems near airports where air traffic controllers would have to deal with some manned aircraft and some unmanned.
UAVs are currently banned from UK airspace - apart from a few, remote locations for test flights - for safety reasons.
But the rules are being overhauled as part of a pounds 32 million project, codenamed Astraea, which has been jointly funded by the Government and seven companies, including the weapons manufacturers BAE Systems and QinetiQ, to develop a new generation of drones with a variety of civilian roles flying pre-programmed routes.
Scientists are confident that they will be able to fit sensors to drones to help identify and avoid other aircraft.
Simon Jewell, chairman of the Astraea steering board, said: "It will be a challenge to evolve people's thinking but these systems will be just as safe, if not safer, than pilots.''
The drones are expected to be far more cost-effective in a number of roles currently carried out by satellites, light aircraft or helicopters.
They may even be fitted with face recognition technology to follow the movements of particular people. Merseyside police have expressed an interest in using them to hover over problem estates to tackle anti-social behaviour.
The most common drone is likely to be the Herti, which has a 13 yard wingspan and can remain airborne for 25 hours.
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