Control of Aircraft

Control of Aircraft

Procedural ATC

17.1Concept. The provision of ATC to air traffic is largely a post WW2 concept. The need for ATC was highlighted by the high loss rate of aeroplanes during WW2 on and in the vicinity of aerodromes due to mid air collisions, collisions with obstacles and inadvertent flight into terrain. This led to the establishment of ATC at aerodromes provided by the control tower but beyond the aerodrome boundary (now replaced by the ATZ) little was provided other than a ‘flight following’ monitoring service essentially to ascertain that the aircraft was still airborne! After an accident in the US involving two Constellations over the Grand Canyon in the early 1950s concern was expressed that two relatively small objects flying over a vast geographic area could be a threat to each other. This led to the establishment of a ‘one-way’ system for east/west flight over the continental US and gave rise to the first established ‘procedural’ ATC service. The service provided separation by requiring aircraft naturally cruising at the same altitude to fly via different routes (lateral geographic separation). It required the pilot to tell the air traffic controller where the aircraft was by passing position reports. The data collected was plotted and as the flight progressed any apparent collision risk was determined and the flights concerned would be asked to alter course to eliminate the problem. The problems with this were that the equipment used to determine the aircraft position was, by modern standards, somewhat crude; the communications equipment and facilities were poor and the availability of flight information including met data was virtually nonexistent. However, the density of air traffic was also low and the application of large ‘buffer’ distances overcame the problems of inaccuracy and poor communications.

17.2Flight Strips. Within the ATC centres, the progress of a flight is tracked with a paper system known as a flight strip. The strip is originated from the ATS Flight Plan and in theory, is transferred from ATCO to ATCO and from centre to centre. Clearly, the progress of the flight strips may not actually be ‘physical’ but will require a new strip to be compiled at centre ‘B’ from information passed by telephone from centre ‘A’. Within a centre the progress may be a physical passing of the strip from one controller to another. This system is virtually fool-proof and over the last 20 years ATC research centres (e.g. Eurocontrol at Brétigny) have tried to come up with electronic replacements.

17.3Procedural Separation. The procedural separation standards are covered in Chapter 16 of these notes.

17.4Communications. Ever since aircraft have been able to carry radios, they have been used for air to ground communications. The present day ATC system relies on VHF two-way communications to make the system work. Over the ocean areas and remote land areas, HF is used together with the ability to maintain a radio watch using the Selcal system which relieves the pilot of having to actually listen to the radio. Each ATC unit has a radio callsign e.g. Oxford Approach, as does each aircraft. If two aircraft have the same or confusingly similar call signs, ATC can ask one aircraft to use another call sign for the time being.

Radar Control

17.5 Concept. The primary use of radar in ATC is to enhance the provision of separation. In theory, radar gives the ATCO the ability to determine the aircraft position with more accuracy than the pilot can! However, SATNAV systems with precision accuracy (+/- 30 cm) are now adding a whole new dimension to ATC. Radar systems cover long range surveillance used in area control (en route traffic), terminal aerodrome radar (TAR) used in the vicinity of an aerodrome or aerodromes to provide a service to arriving and departing traffic, and surface movement radar at an aerodrome to provide the aerodrome controller with information in poor visibility or at night.

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Control of Aircraft 17

Aircraft of Control 17

Figure 17.1 ATC radar head at Clee Hill,

Shropshire

Figure 17.2 ATC radar head at London Heathrow

17.6 Radar Services. At its most basic, radar is used to derive information for the updating of information displayed on the flight strips. In this manner, it is augmenting a procedural system. If, however, the radar response from a specific aircraft can be individually determined (identified) then the provision of separation from other radar contacts (aircraft) can be achieved. In this manner, the radar derived information is used directly to provide separation to a much greater degree of accuracy. The types of radar service are:

•Radar Control for controlled en route aircraft

•Approach Radar Control for arriving and departing controlled traffic

•Radar vectoring: the provision of navigation instructions to an aircraft to achieve a specific aim e.g. positioning to intercept the ILS localizer.

•PAR and SRA as instrument approach systems

As with all ATC procedures, a radar service is only provided inside CAS. ICAO does however permit the use of radar to obtain information to enable the provision of an FIS outside of CAS. Radar may also be used to assist the provision of information as part of the Advisory ATC service provided in class F airspace. At an aerodrome, radar may be used by the aerodrome controller to determine the separation between departing aircraft but not to actually apply the separation.