Procedural control is a method of providing
air traffic controlservices 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.
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
radarreturns of the aircraft to be at least a minimum horizontal distance apart. This is the essence of radar controland 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.
Techniques of Procedural Control
The central rule of procedural control is that each aircraft is cleared onto a predetermined route (airway), and no aircraft traveling on the same or intersecting routes at the same level shall come within 10 minutes' flying time of another (or sometimes 15 minutes depending on the accuracy of the available
Using procedural control, the controller must maintain a mental picture of the location of aircraft based on each aircraft's
flight progress strip, which contains its route, altitude and estimated times over reporting points. That information is compared against all other aircraft in the sector to determine if there are any conflicts. For aircraft that conflict, the controller issues an altitude, speed or routing change that both separates the conflicting aircraft from each other, while still remaining separated from all others. After all conflicts have been resolved in this way, the sector is considered "separated" and the controller only needs to check again for conflicts when new aircraft are added, if an aircraft needs to change its altitude, or if the aircraft reaches a reporting point significantly earlier or later than previously estimated.
There are two main techniques controllers use to organize flight progress strips in order to best detect conflicts: grouped together by "altitude", or grouped together by route intersection points ("fix posting area"). Altitude grouping is the easiest and most common method when most aircraft in the sector tend to be level at a cruising altitude, such as in trans-oceanic sectors. Each aircraft's strip is placed in a "bay" (labeled section of the strip rack) depending on its altitude. Because aircraft cruising at different altitudes will never conflict regardless of route of flight or estimated times, they don't need to be checked against each other. Only aircraft at the same altitude (i.e. in the same bay) need to be further evaluated. If an aircraft needs to transition to a new altitude (whether changing cruising level, or descending to or climbing from an airport), it only needs to be checked against others in the bays between the current altitude and destination altitude.
The "fix posting area" method is easier however when there are a lot of altitude changes because of numerous departures and arrivals within the sector. This is a more obscure method today because most airports around the world with significant enough traffic to warrant this technique have at least a radar covering the climbout and arrival area, and only need procedural control for points between (in which case altitude grouping is likely more efficient).
Nevertheless, the "fix posting area" method was the main technique used in U.S. enroute system prior to the 1960's, for example, when enroute radar became more widespread. This method was evolved from similar techniques used in railroad dispatch. It simplifies conflict detection because aircraft on different routes can only possibly conflict at intersection points with other routes, or with aircraft on the same route. Each intersection point ("fix posting area") in the sector is made into a separate bay on the strip rack, and multiple strips are printed for each aircraft, one per bay. The controller then sorts strips within each bay by the estimated time the aircraft is expected over that point. Only aircraft estimated to cross within 10 minutes of each other are considered possible conflicts, at which point altitudes are then compared to determine if there actually is a conflict. Aircraft on the same route however (in either direction) must be checked for conflict regardless of time over the intersection.
As new altitudes are issued, all of the strips for that aircraft must be noted with the new data and separation checked again at all of the intersections it passes through. This is more laborious than the altitude method of grouping where only one strip is needed, but possible conflicts between multiple aircraft climbing and descending are easier to discern or discard this way since all intersection points and times are precalculated and laid out separately. With the altitude grouping method, if two aircraft are determined to conflict based on altitude, the controller must then manually determine the intersection points and times to further determine or rule out the conflict.
Every runway used by
IFRaircraft 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 by the airport's control tower, or have reported that it has landed before the next aircraft is allowed to leave the hold and commence the approach.
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.
At some non-radar airports the procedural approach may be combined with aerodrome control, thus providing a control service for all aircraft at the airport, and all in and outbound flights within its terminal airspace. As most aircraft will be flying to/or from the same point within this airspace (i.e. the airport), procedural controllers use a variety of separation standards to achieve a safe and expeditious flow of air traffic to and/or from the airport. One commonly used separation in these locations is track separation, which states that two aircraft on different tracks to/from a navigational aid will be separated from each other provided at least one is a minimum distance from the navaid. The distance needed will vary dependent on how far apart the tracks are. For example, two aircraft on tracks that are 60° apart do not have to be far from the navaid to be clear of each other in comparison to two aircraft whose tracks are, say 13° apart.
En-route jet aircraft tend to fly at roughly 8nm per minute: therefore 10-15 minutes' flying time separation using procedural control would equate to around 80-120nm distance between aircraft at the same altitude. This contrasts with standard separation on radar whilst en-route, which is only 5nm. Likewise a typical procedural approach might allow one aircraft to land only every 5-10 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.
* [http://www.caa.co.uk/docs/33/CAP493PART1.PDF 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)
* [http://www.faa.gov/airports_airtraffic/air_traffic/publications/atpubs/ATC/CHP6.HTM FAA order 7110.65] U.S. Air Traffic Control rules for procedural separation (called "nonradar" in the U.S.).
Wikimedia Foundation. 2010.
Look at other dictionaries:
procedural control — procedūrinė kontrolė statusas T sritis Gynyba apibrėžtis Oro erdvės kontrolės metodas, pagrįstas anksčiau suderintais ir priimtais įsakymais ir veikimo procedūromis. atitikmenys: angl. procedural control pranc. contrôle aux procédures … NATO terminų aiškinamasis žodynas
procedural control — A method of airspace control which relies on a combination of previously agreed and promulgated orders and procedures … Military dictionary
procedural control — A method of airspace control that relies on a combination of previously agreed and promulgated orders and procedures … Aviation dictionary
procedural law — Law that prescribes the procedures and methods for enforcing rights and duties and for obtaining redress (e.g., in a suit). It is distinguished from substantive law (i.e., law that creates, defines, or regulates rights and duties). Procedural law … Universalium
Procedural Reasoning System — The Procedural Reasoning System (PRS) is a popular agent architecture in artificial intelligence systems. PRS could, for example, be used as a framework when writing a controller for a mobile robot. Some of PRS s features are:* The program that… … Wikipedia
Procedural surface — In computer graphics, a procedural surface is a representation of a surface as a mathematical implicit equation, rather than an explicit representation. An explicit representation, for example, describes a line as the straight segment going… … Wikipedia
Procedural texture — A procedural texture is a computer generated image created using an algorithm intended to create a realistic representation of natural elements such as wood, marble, granite, metal, stone, and others.Usually, the natural look of the rendered… … Wikipedia
Control character — In computing and telecommunication, a control character or non printing character is a code point (a number) in a character set, that does not in itself represent a written symbol. It is in band signaling in the context of character encoding. All … Wikipedia
Control table — This simple control table directs program flow according to the value of the single input variable. Each table entry holds a possible input value to be tested for equality (implied) and a relevant subroutine to perform in the action column. The… … Wikipedia
control — The exercising of influence, regulation, or restriction over an activity, process, or individual. In an auditing context, control is often understood to be an action or practice that assists an organization to achieve its objectives, though this… … Auditor's dictionary