AS THE skies have become more crowded, accident rates have actually fallen. But maintaining this trend will be a major challenge, given a new breed of hazard facing the aviation world. 鈥淯nmanned aerial vehicles鈥�, or UAVs, are already flying combat missions over Iraq, and in the coming decades we could see swarms of unmanned cargo planes, and remotely piloted passenger jets, criss-crossing the skies. So how do we handle the new dangers?
According to the UK Civil Aviation Authority, 鈥渃ontrolled flights into terrain鈥� 鈥� crashes in which the crew are unaware of their true position and fly their plane into the ground 鈥� are not only the most common type of accident, but occur more than twice as often as crashes due to loss of control, for example. So one of the most important safety improvements will be a new generation of 鈥渢errain avoidance warning systems鈥� that marry data from radar and ground radio beacons with a 3D terrain database.
These systems not only show pilots where they are, but how much room there is between them and the nearest rock. The International Civil Aviation Organization in Montreal, Canada, is pressing airlines to adopt these systems by 2005, but ensuring that the database shows the right bit of terrain is the challenge. The best map in the world won鈥檛 help if a pilot鈥檚 navigation is incorrect (快猫短视频, 23 August, p 8).
Advertisement
Another major challenge is spotting air turbulence and the 鈥渨ake vortices鈥� from other jets. Clear-air turbulence is a random motion of air pockets that occurs at the boundary between moving air layers. Wake vortices are invisible but powerful horizontal whirlwinds stream from the wingtips of planes. Hitting either of these disturbances can injure or kill passengers, or even crash a low-flying plane. But these threats could be all but eliminated with a new generation of detectors. The most promising is a souped-up version of lidar, a laser-based sensor rather like radar, which can measure air turbulence. EADS and Michigan Aerospace in Ann Arbor, are separately building lidar that can 鈥渟ee鈥� clear-air turbulence and wake vortices.
Conventional lidar uses infrared light, which scatters off particles suspended in the air. If the particles are moving, they shift the wavelength of the reflected light, and show up as an abnormal velocity profile 鈥� but only if there are enough particles around. By switching to a shorter wavelength ultraviolet laser, the lidar measures scattering from nitrogen and oxygen molecules. This can measure both wind speeds and pressure, and easily detect vortices and turbulence. Integrate it into a flight control system and a plane could automatically steer around turbulence, says Helmut Zinner of EADS鈥檚 Munich research centre.
Meanwhile, UAVs present their own risks. At the moment, these craft are not subject to the same tough safety standards as manned aircraft, which raises the questions of whether they should be allowed to share the same airspace as passenger aircraft. In 2002, the US National Transportation Safety Board calculated the number of commercial airplane crashes in US airspace at 0.06 per million flying hours. According to the Pentagon, the crash rate for the remotely piloted Global Hawk UAV is 1600 per million flight hours.
Access 5, a lobby group comprising the Pentagon, Lockheed Martin, NASA, Boeing, and aerospace firm Northrop Grumman, is pushing the US aviation authorities to loosen restrictions on UAVs. This has alarmed pressure groups for aviation safety like the National Air Disaster Alliance in Washington DC. They believe that UAVs should not be able to share runways with civilian jets. Pilots are also concerned that unmanned planes will have poor collision avoidance capabilities in flight, and wonder what might happen if a UAV鈥檚 remote satellite radio link is lost.
Whatever form they take, says Pam Drew, engineering chief at Boeing鈥檚 Phantom Works research lab, there is little doubt that in the future the skies will be shared by a combination of piloted and unpiloted vehicles. And as unmanned aircraft become more common, there will be pressure to automate freight aircraft and even passenger planes. Drew鈥檚 engineers are already designing software that uses GPS satellite navigation to allow UAVs to fly in formation. They hope to copy the way birds flock without colliding with each other, allowing them to fly large numbers of unmanned planes together on the same routes for fast yet safe bulk transport. This would be easy. Which is why the next century of flight could be at least as challenging as that of the last.
