Air Traffic Control - Part 2
by Roger Meyer
In September 1946 the Provisional International Civil Aviation Organisation (PICAO) appointed a Special Radio Technical Committee to evaluate the wartime developments in navigation, communications and air traffic control which could be used by civil aviation in peacetime. ICAO later issued a series of Annexes which established uniform standards to be observed by member States. Interestingly, under Australian Regulations Air Traffic Controllers were not required to be Licensed until 1961.
Australias Air Navigation Regulations were re-drafted to embody, as far as possible, the PICAO recommendations. The new Regulations, issued in August 1947, gave legal authority to the establishment and functions of the Air Traffic Control service. A new Manual of Air Traffic Control was issued at the same time.
A steady growth in air traffic, particularly on the main air routes between Brisbane, Sydney and Melbourne led to congestion in the vicinity of these aerodromes where aircraft were on descent from their cruising height prior to landing. The already overloaded radio communications facilities did not allow a sufficiently rapid means of communication between ATC and the pilots of aircraft flying in these critical areas, especially in conditions of bad weather.
A separate service called Approach Control was introduced, and evaluated on a trial basis at Essendon, Mascot and Archerfield in July 1947. By 1950 it was introduced at other locations where traffic density warranted such units.
At the same time Australian and New Guinea airspace was divided into ten large areas of responsibility, called Flight Information Regions.
The next major innovation was the Flight Progress Board (FPB), which was based on an American model. The Flight Progress Board was introduced from 1950, and performed two separate control functions. First, it looked after aircraft travelling on controlled air routes. At a distance of more than 60 miles from the major airport aircraft were under the control of an Area Controller, who used Aeradio stations for communications with aircraft.
Second, within a 60 mile radius of a major airport, the Arrivals Controller was in direct radio communications with all aircraft so that control was positive and carried out with least possible delay, thus providing control of aircraft converging on a busy airport.
At least four people were needed to work the Board. On one side was an assistant who used an aircrafts Flight Plan to prepare coloured flight progress strips with each flights details prior to its departure. On the opposite side, Air Traffic Controllers actually directed the traffic using the data prepared by the assistant, updated as the flight progressed. Finally, the Senior Area Controller supervised and coordinated all activities. Click here to see photos of Essendon Area Control Centre's FPBs in the late 1950s.
It was soon realised that there was a need for some positive means of coordination between a control tower and the associated air traffic control centre in the assignment of aircraft altitudes. Without this coordination, there was the ever present possibility that the same altitude for different aircraft could be assigned and although the control tower was responsible for the control of aircraft only in a restricted area, there existed the possibility of double assignment of altitudes.
In order to obviate this difficulty, altitude assignment boards were developed, one of which was designed for building into the tower console, and the other for the flight progress board in the ATC Centre. To assign an altitude, the controller in the tower plugged into his board a flight progress strip, and the fact that this altitude was then assigned was indicated by a lamp in the control tower and a corresponding indication was presented on the flight progress board. If an attempt was made at the flight progress board to assign the same altitude to another aircraft by plugging in a flight strip, alarms in both the tower and at the centre operated. Click here to read more about the altitude assignment board.
This method of separating air traffic within controlled airspace continued to be used until the mid-1960s when Area Approach Control Centres (AACCs) were established, which now leads us, inevitably, to the introduction of radar. This quantum leap changed what had been a static display of a dynamic situation with the flight progress board, to a completely dynamic radar display.
Possibly the most dramatic impact on ATC in Australia was the introduction of radar. Radar was invented in Britain in 1935, and its contribution to the Allied war effort is well documented. The Department of Civil Aviation, after the war, experimented with a number of ex-wartime radar installations at Essendon Airport. Some were, by todays standards, positively primitive, such as the Australian Light Weight Air Warning (LW/AW) radar, housed under a canvas tent. Another was the 276 radar with its daylight plotter in the tower. An operator worked in a small, darkened building on the airport, tracking aircraft on his screen. The x and y coordinate voltages, representing the location of the aircraft, were connected to the tower by a control cable, and traced as a brown line on a sheet of chemical-impregnated blotting paper.
In 1959, short-range raw display Cossor radars were provided at Sydney and Melbourne airports to facilitate control of arriving and departing aircraft within up to 50 miles of each respective airport. The displays were located in the tower cab at Melbourne/Essendon, and in a room under the tower in Sydney. Being raw displays, they were only viewable in semi-darkness.
In 1961, DCA produced a long-term plan outlining Surveillance Radar Requirements, recommending the provision of dual-purpose radars for approach and area control at Brisbane, Sydney, Melbourne, Adelaide and Perth. Scan-converted bright radar display systems were incorporated in new AACCs at Sydney, Melbourne and Brisbane. The AACCs were commissioned, respectively, in 1965, 1967 and 1969. In 1964 the upper limit of controlled airspace was increased to 40,000 feet in 1964 to accommodate the introduction of domestic jet services.
A further enhancement was Secondary Surveillance Radar (SSR) which required target aircraft to be fitted with a receiver/transmitter (transponder) which responded to a recognised signal radiated from the ground radar beacon. In early systems only a symbol, different depending on the code being squawked by the aircraft, was displayed superimposed on the aircrafts Primary Radar return.
In later display systems, such as the fully synthetic, computerised ATCARDS system, an aircrafts callsign, altitude and computed ground-speed were displayed against the aircraft target on the operators screen.
This is a very brief overview of the some of the many innovations which ATC and the supporting Airways Engineering organisation implemented in response to the vastly increased volume of air traffic, and the introduction of jet aircraft.
Click here to read Part 1
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