Aviation Topic of the Week
By Michael Oxner, September 21, 2003

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Aviation in Canada Blog

This week's topic:
Airport Layouts

Not intended to be a document to help determine why airports are designed the way they are, this week's topic is more to help the uninitiated learn a few things, like why runways are designated the way they are and so forth. I'll go into a little background as to how things are decided, like which direction the runway should be laid out in and so forth, and then get to the real meat of designations.

Airport Design Primer
    Magnetic Variation
    Changes to Magnetic Variation
    Areas of Compass Unreliability
    Parallel Runways
    Other Runway Aspects
    Too Many Taxiways
    High Speed Exits

Airport Design Primer

The general layout of an airport depends on many, many variables. Construction problems (swamp areas, city boundaries, etc), environmental considerations, including noise abatement, and most importantly, the weather. When an airport is being constructed, the weather records are consulted. Often, weather recording instruments are erected on the planned site to record wind direction and speed, among other considerations, over a long period of time to determine a number of operating factors. For example, a place like St. John's, NL, is known for dense fog, so records of visibility would be wanted to determine the level of service required from NAVAIDs. Prevailing winds are a major factor in airport design, as well as the worst conditions experienced. For example, if the winds are frequently from a given direction during a particular type of weather, a secondary runway may be considered to increase airport availability.

Another issue is the "target audience", or "critical aircraft". Who is going to use the airport? What types of aircraft will serve it? What is the longest runway length required for take-off and landing of the various aircraft expected? What level of accessibility is desired? All of these questions have a large bearing on the dimensions of the runways at design time. If the airport is unlikely to be serviced by anything larger than a Dash 8, there would be little to be gained by building a 12,000 foot runway. However, if B747's are the target customer, and the airport elevation is high, and the weather is often quite hot, a longer runway will be needed to serve the aircraft.

Back to weather for a minute, we have to consider just how available we want the airport. if dense fog is a common issue, then we have to start looking at NAVAIDs which increase availability. This could have an affect on the minimum runway length, and even the desired layout of the airport. One wouldn't put a Category III ILS installation on a 3,000 foot runway. Even standard Category I ILS systems, for example, require large areas surrounding glidepath and localizer antennas to be free from interference by outside sources, so the airport property may have to be significantly larger to allow control of construction on nearby lands. Large metal objects near these antennas can affect radiation of signals. Airport designers have so much to consider before an airport can be built, or even before an existing airport can be expanded.


Runway: An rectangular area of defined dimensions used, or intended to be used, in whole or in part, for the take-off and landing of aircraft.

This tells us, in a lawyer-like way, what a runway is. Many of us already interested in aviation know how runways are designated, and many know they are designated, but don't know how they get their numbers. Runway numbers are primarily chosen based on the magnetic heading of the runway. Since a take-off or landing can normally be accomplished on the same piece of pavement in either direction, runways are designated with two numbers, and the number used approximates the direction of travel along the pavement from one threshold to the other. We drop the last number of the heading, rounding up or down as required. If a runway's heading is 020° degrees magnetic, the runway is named as 02 (or just 2 to our American counterparts) if you're on the south end looking north (well, north northeast, actually). Thats the number you'll see painted on the runway threshold as you approach to land. If you approach the runway from the other side, you'll notice a heading which is the reciprocal of 020°. This would end up being 180° off 020°, and that would work out to 200°, so the number on the threshold as you look south southwest would be 20.

For another example, a runway heading of 272° is normally named runway 27. It's reciprocal would be runway 09 (272 - 180 = 092, the '2' being rounded down). Often times, the runway heading is actually something like 145. In this case, it could be either, but one more factor is generally looked at...

If you look at a map that includes a description of magnetic variation, it will also qualify the value given with a date and a rate of change. Something like "variation 20°30' west as of 1985, decreasing 5.4' annually." This means magnetic variation is expected to change by subtracting 5.4' per year, so as of 2003, the variation would have decreased by 97.2 minutes, or 1°37.2'. This means, for the above example, magnetic variation should be (20°30' - 97.2') 18°52.8'. If the runway heading borders like the example above, they might build the runway on the heading of 145°, but knowing the magnetic variation is decreasing, they might choose to designate it Ruwnay 14, rather than call it 15 and be forced to change it, and all the publications, in the next few years.

This can have an impact on what the runway will be considered in the future. For example, Halifax International airport's runways are numbered 15/33 and 06/24. Their runway headings are 145° for runway 15 (and therefore 325° for 33), and 055° for runway 06 (235° for 24). Within a few more years, Halifax will end up renumbering their runways because of magnetic variation change, even though the pavement hasn't moved an inch.

In areas of compass unreliability, like in Canada's far north, it becomes impossible to use magnetic headings to describe runways, just as it is impossible to use magnteic headings to align NAVAIDs and so forth. In such cases, as with the NAVAIDs, True North is used as a reference. If a runway is designated based on true north, the letter "T" will be added to its designation. "05T" could be used for a runway with a true heading of 052° T.

One note: If you look at a chart and try to read runway numbers, you may have to turn the chart to get it right. Given that runway numbers are all based on headings, there can only be 36 possibilities: 01 to 36. Normally, the charts are printed with the numbers oriented to the direction of the runway. A local newspaper attempted to tell its readers that an aircraft "crossed runway 60" as part of its path. It had crossed runway 09.

Parallel Runways

In some cases, airports are built with parallel runways. Typically seen in only larger airports, this kind of planning is intended to increase airport acceptance rates. Since the runways are parallel, they should bear the same numbering, right? How would one differentiate one runway from another? There are two common ways.

In the case of parallel runways sharing the same numbering, the use of additional lettering comes in handy. "L" for left, and "R" for right are used. In the case of three runways, "C" is added for center. On the runway itself, the numbers would be written above the letter, if seen from above. Now what if they want to put four runways in place? Or five? We could really go nuts with combinations like "24 Left", "24 Center Left", "24 Center Right" and "24 Right", but that just leads to confusion. More than 3 parallel runways almost cry out for another convention.

Often, changing the numbers arbitrarily is a better way. In theory, since these are normally only found at larger, busier airports, pilots should have higher levels of training to get them into and out of these kinds of places. This should make them more aware of the fact that the runway heading is indicated on charts, and not deterimined from the runway numbers alone. In a case like mentioned above with four runways, they might actually opt for a naming combination of "23L", "23R", "24L" and "24R", rather than get into "24CL" and so on. In such a case, the runway headings of 23L and 23R may actually be 240° as it would be for the others designated 24L and 24R, but it helps reduce the chances of incidents due to naming mix ups in clearances.

Other Runway Aspects

There are some other points of interest about runways that not all airports share. First off, airport authorities are required in many cases to provide a graded area to the sides of the runway, as well as a "clearway" off each end of the runway. This allows for aircraft that dip a little low on the approach end, but more specifically for aircraft on departure. They may get their wheels off the ground, but in some circumstances, especially abnormal ones like engine failures, they may not climb fast enough. Providing a clearway at the end of a runway removes obstacles like trees and prevents construction of buildings or antennas to allow for a delayed or shallow climb.

Many runways, though not normally at higher volume airports, are not served by multiple taxiways or parallel taxiways that run the length of the runway. This means aircraft must taxi along the runway in the direction opposite to take-off, also known as "back tracking" or "back taxiing", before they can start their roll. To facilitate larger aircraft, or other aircraft with large turning radii, there are occasionally turn-around points paved beside the runway. There may be a few of them, maybe one at the runway threshold, and one or more along the length of the runway. These are primarily intended to allow an aircraft on landing to be able to turn around without having to taxi all the way to the end of the runway to do so. These don't provide enough clearance to ensure safety for an aircraft parked waiting in one of these while an aircraft lands or takes-off. One exception is that two aircraft can often taxi back to the threshold at the same time, and one sit in the turning bay to the side of the threshold while the first one starts his take-off run from the threshold. Having the second aircraft sit in a turn around bay further up the runway is unacceptable and should never be suggested by controllers, or accepted by pilots.

Sometimes a Stopway is provided as well. This is an area that is normally paved and is not intended for the ground run of an aircraft for take-off or landing, but is maintained as a place for an aircraft to use for deceleration in the event of an aborted take-off.


I won't get into a foolish definition for taxiways like I did above for runways. These are simply the prepared surfaces used for the ground manoeuvring of aircraft around an airport. Normally, taxiways are controlled by ATC where a control tower is in operation, however that is not always the case. Some taxiways are not under ATC control. There will often be notes in the Canada Flight Supplement regarding this sort of information. If in doubt, ask ATC. Many airport taxiways are used for the take-off and landing of helicopters (typically those fitted with wheels for landing gear, since skid-equipped helos will normally land where they want to). This gives them the pavement they want, and the control of other traffic by ATC.

In order to differentiate taxiways from runways, they are designated by letters, spoken phonetically, normally starting with "A". I say normally, because there are some airports that don't start with Alpha, such as Sydney, NS, CYQY. Part of this sort of configuration rests in history, where the previous taxiways are decommissioned, and it is decided just to leave the current letters stand where they are, rather than rename existing ones and possibly causing confusion. Unlike runways, it doesn't matter which direction you travel on a taxiway, its name remains the same.

At larger, more complicated airports they often run out of letters as the airport grows. With runways, they add letters to the numbers. With taxiways, they add numbers to the letters. Look at any larger airport diagram and you'll see examples: Vancouver, Montreal, and Toronto all are among airports with dual designation on the taxiways. The examples I've seen all tend to have a certain amount of organization: Taxiways beginning with "W" and having numbers attached tend to be in the same general location on the field -- perhaps on the same side of a runway, or in a grouping in one section.

High speed exits, or high speed taxiways, also exist at certain airports where it is considered beneficial to traffic flow. The idea is to increase runway acceptance rate by allowing the aircraft to make the turn off the runway centerline while not having decelerated to a normal taxi speed. This permits the aircraft to exit the runway faster, thereby spending less time on it, allowing following aircraft to land. These taxiways are at an angle to the runway much shallower than the standard 90° seen by many taxiways, typically around 30° to the runway.

There is a basic look at airport surface designations. There is, of course, much more to airport design than mentioned here, and much more to airport layout than discussed here. I've hoped to provide some basics for reference in this part for future topics that will involve airport operations. Questions and comments will be willingly received by e-mail to moxner@nbnet.nb.ca. Thanks once again for reading, and I always enjoy hearing from readers.