The ups and downs of Australian air traffic control

Key text

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:

• the tower in Perth gave instructions for runway taxiing and scheduling of take-off;

• a departures controller in Perth gave instructions for the climb out of Perth;

• an en route controller in Perth tracked the aircraft's progress as it headed east across the State;

• an en route controller in Adelaide tracked the aircraft as it traversed South Australia;

• an en route controller in Sydney tracked the aircraft once it entered New South Wales;

• an approach controller in Sydney gave the flight instructions for a safe approach to Kingsford-Smith airport; and, finally,

• the tower at Kingsford-Smith issued landing and taxiing instructions.

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:

Main screen. A map of the sector shows the location of all aircraft in controlled airspace, as reported by one of several data sources – radar data processing, flight data processing and automatic dependent surveillance. Weather radar display. Voice communications control panel. A touch-sensitive screen allows controllers to choose the radio frequency they need to talk to pilots and ground staff, or the intercom for talking with other controllers. Auxiliary flight data display. The controller can call up a wide range of information such as weather forecasts and other material to relay to pilots.

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:

• reduced cost because facilities can be consolidated;

• the ability to handle the increasing supply of electronic navigation information; and

• the ability to safely maintain an optimal separation of aircraft, thereby increasing the efficient use of airspace and the number of aircraft that can safely occupy it.

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

But there were plenty of other hazards. The flying machines were less reliable than they are today and information on weather conditions was scant. In addition, navigation was hindered by poor maps, basic instruments, and no radio contact with the ground. True, help was sometimes obtained at ground level, but in a way that would be considered somewhat cavalier today. For example, the story is told of one Qantas pilot flying in the Northern Territory in the 1920s who swooped low over a bushman and switched off his engines while he yelled for directions.

Early air disasters were catalysts for air traffic control

It was only after a few major air disasters that people started to consider the need for the control and monitoring of planes in the air. One of the most significant was that of the Southern Cloud, which crashed on a flight from Sydney to Melbourne in 1931, killing the two crew and six passengers on board. Although its wreckage was not discovered for 27 years, the disappearance of the Southern Cloud prompted an investigating committee to advise that all passenger aircraft should be equipped with two-way radios. In addition, ground radio stations should be established to communicate with the planes and to monitor progress towards their destinations.

The loss of the Kyeema in 1938 was another catalyst for the introduction of air traffic control. Due to a navigation error, this airliner crashed into the side of Mt Dandenong near Melbourne, killing the four crew and fourteen passengers on board. An inquiry later suggested the need for a system 'whereby the movements of aircraft could be checked by a competent person on the ground'.

Techniques and technologies from World War II

Air traffic control really kicked off in Australia after World War II, when technologies and techniques developed during the war effort were adapted to civilian use. One of these was radar, used by air traffic controllers to pinpoint the location of aircraft. Radar assisted in aircraft separation near airports and was also useful for 'talking down' an aircraft which had suffered an instrument failure or whose pilots were unable to navigate visually due to darkness or bad weather.

Another technique was the instrument landing system (ILS), improved from the wartime 'standard beam approach' and installed at all major airports in Australia shortly after the war. The ILS operates in the following way. A transmitter located at the airport sends out two beams that can be picked up by an approaching aircraft. One beam, called the localiser, keeps the pilot from moving away from the correct approach path. This path projects as an extension of the runway centreline. An instrument in the cockpit shows divergence from the correct path with a vertical needle. The other beam, the glideslope, gives the path the plane should take downwards – a horizontal needle shows the pilot whether the plane is above or below the correct glide path. In theory, pilots don't need to look out the window (until very close to the landing strip) – all they need to do is keep the localiser needle vertical and the glideslope needle horizontal to arrive safely at their destination.

Radar, ILS and other techniques formed the basis of post-war air traffic control. They are still in use today, although they have been modified in various ways.

Related sites

• Airways Museum and Civil Aviation Historical Society (Australia)
  • Air traffic control – Part 1
  • Air traffic control – Part 2

• Edward George Bowen (Biographical Memoirs, Australian Academy of Science)

• Frederick William George White (Biographical Memoirs, Australian Academy of Science)

Activities

• Australian Science Teachers Journal
  • Measuring motion using a Macintosh computer (June 1998, pages 44-51)

• Texas A & M University, USA
  • Use and interpretation of radar – an on-line tutorial which discusses the basic principle of operation of weather radars, describes how to interpret radar mosaics and discusses the use of radar in weather forecasting.

• Newton's Apple, USA
  • Jumbo jets – demonstrating Bernoulli's principle

Further reading

Useful sites

Airspace safety: Air traffic control and airline operations in Australia (Australian Parliamentary Library)

This background paper concentrates on air traffic control and airspace safety in relation to major changes in that sector of aviation. Includes the Australian Advanced Air Traffic System and Airspace 2000. Lists many of the acronyms used in the area of air traffic control.
http://www.aph.gov.au/library/pubs/bp/1997-98/98bp10.htm

TAAATS implementation – a safety progress report (Australian Society of Air Safety Investigators)

This 1999 paper summarises the transition to TAAATS and outlines the safety features TAAATS adds to Australia's aviation system.
http://www.asasi.org/papers/taaats.pdf

Air traffic control (ARC Centre for Complex Systems, Australia)

Describes several research projects on air traffic control.
http://www.accs.uq.edu.au/index/members-viewprogram-action?action=view_program&prog=atc

How air traffic control works (How Stuff Works, USA)

Describes airspace and air traffic control in the United States.
http://www.howstuffworks.com/air-traffic-control.htm

Glossary

electromagnetic radiation. Electromagnetic radiation is simply energy which travels through space at about 300,000 kilometres per second – the speed of light. We imagine radiation moving like a wave. The distance between two adjacent wave crests is called a wavelength. The shorter the wavelength, the more energetic the radiation is said to be. Also, the shorter the wavelength, the greater the frequency of the radiation. Other than wavelength, frequency and energy there is no difference between a radio wave, an X-ray and the colour green. They all possess the same physical nature. For more information see Back to Basics: Electromagnetic radiation (Australian Academy of Science) and Electromagnetic Spectrum (NASA Goddard Space Flight Center, USA).

flight data processing. This plots an aircraft's expected position as calculated by computer from the aircraft flight plan (stored electronically by the Australian Advanced Air Traffic System).

frequency. A measure of how frequently an electromagnetic wave goes up and down (oscillates) or the number of waves passing by in a second. A hertz is a unit of frequency – 1 oscillation per second; a kilohertz (kHz) is 1000 hertz – 1000 oscillations per second; a megahertz is 1 million hertz – 1 million oscillations per second. For more information see Sound properties and their perception – pitch and frequency (The Physics Classroom, USA).

Global Positioning System. The Global Positioning System (GPS) is a collection of 24 earth-orbiting satellites which allows any person who owns a GPS receiver to determine their location on the planet. More information on the Global Positioning System can be found How a GPS receiver works (How Stuff Works, USA) and The Global Positioning System: The role of atomic clocks (Beyond Discovery, National Academy of Sciences, USA).

radar. The use of reflected radio waves to determine the location of an object and its speed if it is moving. It is an acronym derived from radio detecting and ranging. For more information see How radar works (How Stuff Works, USA).

radio frequency. This is lowest of the electromagnetic radiation frequencies. Radio frequencies, or radio waves, have wavelengths ranging from less than a centimetre to as long as 100 kilometres. (See also electromagnetic radiation).

We divide the radio wave part of the electromagnetic spectrum into bands that are allocated to different uses. These include AM (amplitude modulation), FM (frequency modulation) and CB (citizens' band) radio, television, aircraft communications, satellites, mobile phones and pagers. Within each band, no two transmissions can use the same part of the spectrum – or frequency - at the same time. For this reason, each band within the radio wave spectrum, itself a part of the broader electromagnetic spectrum, must be managed carefully to ensure the best use of this limited resource. For more information see How the radio spectrum works (How Stuff Works, USA).

VHF (very high frequency) radio. Radios that use frequencies in the range 30 to 300 megahertz (millions of oscillations per second). The wave length of these VHF radio waves range from 1 metre to 10 metres.