Procedural control
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Procedural control is a method of providing air traffic control services without the use of radar. It is used in regions of the world, specifically sparsely-populated land areas and oceans, where provision of contiguous radar cover is either prohibitively expensive or is simply not feasible. It also may be used at very low-traffic airports, or at other airports at night when the traffic levels may not justify staffing the radar control positions, or as a back-up system in the case of radar failure.
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[edit] Separation
In air traffic control, the risk of aircraft colliding is managed by applying separation rules. These rules require aircraft to be separated by either a minimum vertical distance, or, if for some reason they cannot be separated vertically, by a minimum horizontal distance defined by various means. One of the means of determining horizontal separation is by a controller observing the radar returns of the aircraft to be at least a minimum horizontal distance apart. This is the essence of radar control and is probably the form of air traffic control most familiar to lay people.
However in times gone by radar was not very common, and in certain parts of the world today it still is not common, on grounds of cost or technical feasibility. Procedural control is a form of air traffic control that can be provided to aircraft in regions without radar, by providing horizontal separation based upon time, the geography of pre-determined routes, or aircraft position reports based upon ground-based navigation aids, for those aircraft that are not vertically separated.
[edit] The fifteen minute rule
The central rule of procedural control is that no aircraft travelling on the same route at the same level shall come within 15 minutes' flying time of another aircraft. There are, however, many cases when this is reduced, for example in areas where pilots can determine the position of their aircraft with some accuracy using radio navigation aids (navaids), this is reduced to 10 minutes.
A particular aircraft will have a clearance to fly a pre-determined route. Controllers then determine separation between aircraft based upon pilots' estimates for and reports over reporting points, which are defined by navaids or specific map co-ordinates. Controllers may use speed control to maintain separation, or may climb or descend aircraft to different levels to allow a faster aircraft to overtake a slower aircraft.
[edit] Procedural approaches
Every runway used by IFR aircraft normally has a procedural approach defined and published. This generally involves the aircraft flying over a navaid (or 'beacon') on or close to the airport (where the aircraft can hold if necessary), flying away from the airport on a set track whilst descending (going 'beacon outbound'), then at some point approximately ten miles from the airport turning back towards the airport aligned with the runway to make an approach (becoming 'inbound track established' or just 'established').
In procedural approach control, separation is maintained by only allowing one aircraft to be making an approach at any one time - the proceeding aircraft must either be in visual contact with the airport or have landed before the next aircraft is allowed to leave the hold and go beacon outbound.
At airports equipped with radar, procedural approaches are only normally used in the event of an aircraft suffering a radio failure, or for training purposes. At non-radar airports or when radar is not available, these become the only means of IFR flights making approaches to the airport.
[edit] Airspace capacity
En-route jet aircraft tend to fly at roughly 8nm per minute: 15 minutes' flying could therefore equate to 120nm. This contrasts with standard separation on radar whilst en-route, which is 5nm. A typical procedural approach allows one aircraft to land every 8 minutes, whereas with radar this could be less than one landing every 90 seconds. It is therefore easy to see what a large increase in airspace capacity the provision of radar-based air traffic control gives.
[edit] External links
- UK CAP493 Manual of Air Traffic Services Part 1 Section 1, Chapter 3, heading 8 onwards (note that the UK does not implement the ICAO 15 minute rule and instead uses a standard 10 minute rule)
