You must have noticed runways at airports have two digit numbers of approximate length 9m marked at the beginning and end of the runway and at a defined distance from each runway threshold as shown in the image below.

Runway - Wikipedia

These numbers are called the runway designators of the runway and they have a defined science behind. With this article let us discover what the science is.

This article consists of the following sections:

1.1 What do the Runway Designators Indicate?

1.2 The impact of wind in the determination of Runway Orientation

1.3 The concept of Wind Rose Diagram

1.4 Conclusion

1.1 What do the Runway Designators Indicate?

Runway Designators indicate the orientation of the runway on the surface of the earth i.e. the Runway Orientation which is the magnetic bearing or compass heading of the centerline of the runway.

A runway designator consists of a two-digit number, which is the whole number nearest to one tenth of the magnetic North when viewed from the direction of approach.

Old stone walls record the changing location of magnetic north

True north is a fixed point on the globe where the longitudes converge. Magnetic north is quite different. Magnetic north is the direction that a compass needle points to as it aligns with the Earth’s magnetic field.

For example, if the azimuth of the centerline is 220 degrees i.e. with respect to the magnetic north then the runway designator will be 22. This is explained with the image below.

Flying by the Numbers: How to Decipher Two-Digit Airport Runway  Designations - 99% Invisible
Source : https://99percentinvisible.org/article/numbers-decipher-airport-runway-designations-anywhere-world/

When this rule results in a single digit number then the designator is preceded by a zero (e.g. if the runway centerline azimuth is 20 degrees, then the designator will be 02).

If the runway is used in both directions, then each of them receives its own designator. This means that although there is one physical surface used for take-offs and landings, it is treated as two runways. The difference between the numbers is 180 ( i.e. 180 degrees), e.g. if one of the runways is 22 then the other will be 02.

Note : 02 here will be the designator the other end of the runway described in the image above the heading of which is received by a subtracting 180 degrees to the heading of 220 degrees runway extremity. Well as an interesting fact most of the runways in the north eastern region of India have the runway designator 02-20. By the end of the article you will know why.

North-oriented runways are designated 36 (not 00).

1.2 The impact of wind in the determination of Runway Orientation

Landings and take off best done into (against/head) wind. Now let us understand what this statement means:

The tail wind accelerates the aircraft to an extent that is more than required and it becomes very dangerous especially while landing an aircraft since due to the greater speed of the aircraft the requirement of runway length to stop the aircraft increases thus increasing chances of runway excursion.

To understand how fatal tail wind can be read the news below of the fatal crash of ill-fated AI-1344 on 07-08-21

https://www.tribuneindia.com/news/nation/slippery-runway-strong-tailwind-likely-caused-kozhikode-crash-experts-123816

The head wind creates drag during landing and lift during take off thus helps in the process of aircraft maneuvering and thus is the desired direction of wind to operate in.

Cross wind is the wind that flows in a direction perpendicular to the runway centerline and thus pushes the aircraft away from the desired flight path.

The image below splits the wind flow in a particular direction into its respective components (Crosswind and Tailwind or Headwind) for a better understanding of the effect of the wind.

File:Aircraft headwind crosswind.png - IVAO - International Virtual  Aviation Organisation

Thus we can infer that while orienting a runway we would choose a direction where there is maximum probability for the aircraft to faces headwinds.

The question here is how is the direction determined. Here we will get introduced to the concept of Wind Rose Diagram.

1.3 The concept of Wind Rose Diagram

For determining the runway orientation wind data is utilized. A typical Wind data comprises of:
– wind intensity,
– duration of movement of wind
– direction of movement of wind

A typical Wind Data looks like as below:

Table 1

The above data is collected for a period of 10 years and is plotted in a wind rose diagram.

Wind rose diagram is a compass shaped diagram which exhibits wind direction, duration of wind
movement at different speed ranges and also shows wind speeds. It is used in working out the orientation of runway. Runways are oriented in direction of prevailing wind to ensure a 95% usability factor (the percentage of times the runway can be used with minimum interference due to cross wind).

There are two types of Wind- Rose Diagram. They are as follows:

Type-I Shows Direction and Duration of wind ONLY

Type-II Shows Direction, Duration and Speed of wind

AIRPORT PLANNING. - ppt download

Procedure to use Wind Rose Diagram Type-I


Step-1: Draw concentric circles showing wind durations
Step-2: Divide the circles into 16 segments
Step-3: Plot the wind duration along respective directions and join the points
Step-4: Runway is oriented along the longest line in the diagram

Procedure to use Wind Rose Diagram Type-II

Step 1: Draw concentric circles showing wind speed

Step-2: Mark 16 directions, each with a wind coverage area of 22.5 degrees

Step-3: Draw radial directions to show wind coverage area. This creates segments in which wind coverage (The percentage of time in a year during which the crosswind component remained below the specified value as in the data determined) is written.

Step-4: Mark Calm Period

(In this case as in Table 1 above the calm period is 100-86.5 (total percentage of time wind below with the speed of 6to 60 kmph=13.5 % )

Step-5: Fill wind coverage in designated segment for each direction from the wind data (data filled as in Table 1)

Step-6: Determination of Runway Orientation
Draw three equi spaced parallel lines on a transparent paper strip in such a way that the distance between the two near by parallel lines is equal to the permissible cross wind component. This distance is measured with the same scale with which the wind rose diagram is drawn the permissible cross wind component is 25kph. Place the transparent paper strip over the wind rose diagram in such a way that the central line passes through the centre of the diagram. With the centre of wind rose rotate the tracing paper and place it in such a position that the sum of all the values indicating the duration of wind within the two outer parallel lines is the maximum. The runway should be thus oriented along the direction indicated by the central line. The wind coverage can be calculated by summing up all the percentages.

Final Orientation
• Wind Coverage = Sum of the percentages under the strip for each direction + Calm Period
• The direction with maximum wind coverage is the direction along which runway is oriented
• In case wind coverage is less than 95% – Another runway is constructed
• For busy airports, the wind coverage may be increased to 98% or 100%.

Runway Configuration is the layout or design of a runway or a system of runway. Sometimes it is not possible to meet the requirement of usability factor of 95% with just on runway. So two or more runways are designed to reach the optimal usability factor. An exclusive example of this could be the orientation of the Mumbai’s Cross Runway (09-27 and 14-32).

Mumbai airport's main runway to shut 6 days a week from November to March |  Condé Nast Traveller India

1.4 Conclusion

So now coming back to the question why most runways in the north eastern region of India are 02-20. General wind pattern for the Indian subcontinent is Easterly or Westerly which makes 09-27 the most commonly found runway orientation in the Indian Subcontinent but this is not the case with the north eastern part of India. The wind flow due to the terrain and various other factors are such that the orientation 02-20 allows a usability factor of more than 95% for the runway or the system of runways thus we find 02-20 as the runway orientation in the north eastern region of the country.

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7 thoughts on “Runway Orientation & The Science Behind it !

  1. Great, except for those weak in numerics may sometimes feel dizzy…But, that’s the reader’s limitation, not the narrator’s…You are on dot as without numerics it may not explain the issue…Keep going…

  2. This is the perfect blog for anybody who wants to find out about this topic. You know a whole lot its almost hard to argue with you (not that I actually would want toÖHaHa). You certainly put a fresh spin on a topic thats been written about for a long time. Excellent stuff, just wonderful!

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