Whenever we step onto a passenger jet, we place our lives in the hands of a few highly skilled people – pilots, mechanics, weather forecasters and air traffic controllers. In Australia, our faith has rarely been misplaced – these days most of us board an aeroplane with no more concern than we do a bus or a train. There has not been an accident involving a major public transport airline since 1968. But our skies are criss-crossed by the jet trails of airliners like never before. How is all this activity controlled? How are aircraft kept apart? It may surprise you to learn that while the technology aboard the jets has changed beyond recognition in the last few decades, air traffic control has changed much less. For the last 50 years, the movement of an aircraft has been tracked largely by air traffic controllers using little more than pencil and paper, with the aid of radar in only a few small areas of the continent (Box 1: The beginnings of air traffic control in Australia). Now, a new computer-based system (the Australian Advanced Air Traffic System or TAAATS) is being phased in. The basics of air traffic control Air traffic control can be defined as the supervision of airborne and taxiing aircraft by ground-based personnel. The task of air traffic controllers is to ensure that aircraft complete their flights safely and efficiently. With the changeover from the old to the new system, this task does not change. Nor does the fundamental principle of separation – keeping aircraft separated in space and time. In both the new and old systems of air traffic control, airspace is divided into controlled and uncontrolled airspace. Controlled airspace is that part of the sky where traffic density is so high that strict control of aircraft movements is required – this is the part that most concerns air traffic controllers. You can think of controlled airspace as a bit like a walkway that leads up a stairway, along a raised corridor and down a stairway on the other side. The stairways represent the airspace above and adjacent to an airport – it is in this space that large commercial airlines either depart or approach the runway. They climb up the 'stairway' before levelling out and traversing the distance (or corridor) between destinations, before descending a 'stairway' again. For the entire trip, all aircraft are monitored and controlled by air traffic control. Uncontrolled airspace is the part of the sky that is outside controlled airspace. This is where most light aircraft and some smaller regional airlines operate – although these aircraft may use controlled airspace during take-off and landing. Collision avoidance in uncontrolled airspace relies largely on the wits of the pilot and on agreed ways of separating traffic, such as by flying at different altitudes depending on the direction of flight. Recent trials of new procedures for this airspace have been the subject of some controversy. Another critical part of air traffic control is what happens on the ground. This is the domain of ground traffic controllers, who direct aircraft as they taxi about the airport. Large aircraft may look graceful in the sky but on the ground they are awkward and unwieldy, and they need to be directed carefully to avoid collisions. Ground controllers also issue airways clearances and coordinate departures with the tower and other controllers. How the old system worked Under the old system, controlled airspace in Australia was divided into six flight information regions, based roughly on State boundaries. So, for example, when an aircraft flew from Perth to Sydney, the pilots would communicate with the following air traffic controllers:
For about the first 50 kilometres after taking off in Perth, and on its approach to Sydney, the aircraft would be tracked by radar. But for the most part, air traffic controllers sitting at their desks would estimate the location of the aircraft based on its flight plan. They would mark the expected position of the aircraft by moving slips of paper – on which were recorded the flight number and other details – across a board. From time to time, the pilot would report the plane's position to air traffic control, confirming its expected position. If the aircraft flew outside controlled airspace (as might happen on some of the more remote parts of the trip), they would pass their position reports to flight services units based at regional centres like Broken Hill, Albury and Kalgoorlie. How TAAATS works The six flight information regions of the old system have been amalgamated into two, divided by a line running roughly east to west through the middle of the continent. Together, the two regions account for 11 per cent of global airspace, extending south to the Pole, west to the Indian Ocean, east to New Zealand and north to Asian-controlled airspace. One of the advantages of reducing the number of flight information regions is that it reduces the number of air traffic control units with which a pilot needs to communicate. For example, an aircraft flying from Brisbane to Jakarta need only deal with controllers in the northern flight information region until it enters Indonesian airspace somewhere over the Indian Ocean. Air traffic under TAAATS is managed by two centres, one in Brisbane (for the northern region) and one in Melbourne. The operations room of both contain 40 individual workstations, divided into groups responsible for different sectors within the flight information region. A number of safeguards have been built into the system to reduce the risk of malfunction. For example, almost all of the electronic systems have been duplicated – standby equipment is ready to switch into immediate operation if the main equipment fails. TAAATS workstations Each workstation has four computer screens:
Communication technologies used in TAAATS The new TAAATS buildings have windows even though they are not necessary for air traffic control. Controllers don't look to the sky for their information; instead, they rely on a range of modern communication technologies to monitor and manage airspace. The TAAATS centres require large quantities of data, which must be supplied via communication facilities. These include aircraft monitoring devices such as radar. TAAATS uses a radar network consisting of 19 radar sensors and is supplemented by radar data from six military radar sites. The network covers airspace in the vicinity of major airports and along the busier air corridors of the east coast. There is also a network of VHF (very high frequency) radios connected into the TAAATS system so that air traffic controllers can keep in touch by voice with pilots. Nevertheless, text messages between TAAATS computers and computers aboard the aircraft will become a more common method of transferring information. The two TAAATS centres keep in contact via intercoms provided by satellite trunk circuits and, when these are saturated or if they break down, via the public telephone system. Keeping aircraft separated With the introduction of TAAATS, the basic aim of keeping aircraft separated continues to apply. But the system takes advantage of improvements in navigation and communication systems to increase the ability of air traffic controllers to pinpoint an aircraft's position at any given time. This should result in a more efficient use of airspace. Despite the electronic wizardry of TAAATS, the system is not foolproof – and treating it as such would probably be a recipe for disaster. But it offers a number of advantages over the old system, including:
As a passenger, you shouldn't notice any difference under the new system, because you're being guided to your destination with the same attention to safety as always. So just sit back, relax, and enjoy the flight. Box 1. The beginnings of air traffic control in Australia
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Posted February 1999.
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