Aviation Topic of the Week
By Michael Oxner,
September 21, 2003
This week's topic:
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
Changes to Magnetic Variation
Areas of Compass Unreliability
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
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
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
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.
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
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
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 email@example.com.
Thanks once again for reading, and I always enjoy hearing from readers.