IMPROPER use of an aircraft safety system that warns pilots when they risk flying too low can give crews a misleading idea of their position. The problem can arise in remote areas when the warning system is used on aircraft that are not equipped with a GPS satellite navigation system.
A preliminary analysis of a near miss involving a British aircraft in April this year has shown that a type of Terrain Avoidance Warning System (TAWS) in use on a flight from London to Ethiopia was unaware of the aircraft鈥檚 true position, owing to erroneous data from a ground-based beacon. The wrong position was displayed on the TAWS screen.
There are a variety of TAWS systems on the market. All use radar to monitor the ground ahead of, and below, an aircraft and give an audible warning when it gets too close. But many TAWS systems also display a 3D image of the surrounding terrain to give the pilot greater awareness of what is around the plane. The aim is to avoid a common accident known as CFIT 鈥� Controlled Flight Into Terrain 鈥� in which pilots, thinking they are at safe altitude, plough into hills or mountains. But TAWS was not designed to be used as a navigational aid.
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The near miss in April involved a British Mediterranean Airways Airbus A320, which missed the ground by just 17 metres as it tried to land at what its pilots were led to believe was Addis Ababa airport. The runway was in fact two-and-a-half miles away. While the TAWS system managed to avert a disaster by warning the pilots they were too low, investigators are trying to establish whether the TAWS terrain map was also being used as a navigational aid.
According to safety expert Peter Ladkin at the University of Bielefeld, in Germany, the TAWS systems鈥� 3D terrain display can tempt pilots into using it as a navigational aid, rather than solely as a ground proximity alert. The trouble with that is that the terrain display can become misaligned with the plane鈥檚 actual position. 鈥淚t鈥檚 extremely disconcerting, especially when you鈥檙e flying an approach amongst mountains,鈥� he says.
The UK鈥檚 Air Accident Investigation Board is assisting the Ethiopian authorities in investigating the Addis Ababa incident. But preliminary evidence suggests that ground-based beacons sent out inaccurate location signals, which in turn caused the plane鈥檚 flight management system to feed the TAWS system an incorrect position. So the system displayed the wrong piece of 3D terrain on the pilot鈥檚 mapping screen.
This would not have happened if the plane had been equipped with a GPS receiver, the UK鈥檚 Civil Aviation Authority is warning. Pending the results of the investigation, the CAA is recommending that only planes with GPS should be allowed to fly long-haul to remote areas.
Under International Civil Aviation Organization rules, all new aircraft capable of carrying more than six passengers must have a TAWS system 鈥� and older planes must have it fitted by 2005. 鈥淏ut there is no mandate for airliners to have GPS,鈥� says Les Dorr of the US Federal Aviation Administration.
Plane makers Airbus and Boeing say all their new long-haul aircraft come fitted with GPS as standard. But there are already more than 12,000 civil aircraft in the global fleet, many of which do not have GPS.
The reason GPS is not compulsory is twofold, says Carolyn Evans at the British Airline Pilots Association. It is not yet an aeronautically-mandated aid, she says, and it is completely operated and controlled by the US government, which can in theory turn off or degrade the GPS signal at any time. This weakens the case for its adoption by aviation authorities in some countries.
Another problem with using TAWS for navigation, says Ladkin, is that its world map database is of variable quality: 鈥淭here are many countries in the world that have not yet surveyed themselves to the agreed world model.鈥�