Why are runways always 27?

Runways are not always numbered 27. This is a common misconception. Runway numbers are part of a standardized global system designed to indicate their magnetic compass bearing, ensuring pilots have clear directional information for takeoff and landing.

The Truth About Runway Numbering

Runway numbers are derived directly from their magnetic compass heading, making them unique to each airport's geographical and environmental conditions. The numbering system is based on a straightforward calculation:

1. Magnetic Bearing: The actual magnetic bearing of the runway end is determined.
2. Rounding: This bearing is rounded to the nearest 10 degrees. For example, a heading of 267 degrees would be rounded to 270 degrees, and a heading of 273 degrees would also be rounded to 270 degrees.
3. Truncation: The last digit (the zero) is then truncated. So, 270 becomes 27.
4. Range: This process results in runway numbers ranging from 01 (for 10 degrees) to 36 (for 360 degrees, or north).

Therefore, a runway designated "27" indicates that its magnetic bearing is approximately 270 degrees (due west).

How Opposite Ends Are Numbered

Every runway has two ends, and each end is numbered. The opposite end of a runway will always have a magnetic bearing that differs by 180 degrees. To reflect this, its number will be 18 higher or lower than the other end.

For example:

• If one end of a runway is designated Runway 27 (magnetic heading of 270 degrees), the opposite end will have a magnetic heading of 90 degrees (270 - 180 = 90). This end will be designated Runway 09 (90 degrees, or due east).
• Similarly, a runway designated "18" faces approximately 180 degrees (south), and its opposite end would be "36" (360 degrees, or north).

Why Not All Runways Are 27

The primary reason runways are oriented and numbered differently around the world is due to prevailing winds. Aircraft generally take off and land into the wind to generate maximum lift and minimize ground speed during these critical phases of flight, enhancing safety and performance.

Airport planners analyze long-term wind data for a specific location to determine the optimal orientation for runways. Since prevailing wind directions vary significantly from one region to another, so too do the magnetic headings of the runways. This leads to a diversity of runway numbers across different airports, each optimized for its local wind conditions.

Here's a quick look at how different runway numbers correlate to their approximate magnetic headings:

In conclusion, while Runway 27 is a common designation at many airports due to favorable prevailing winds in certain regions, it is far from the only or universal runway number. Each runway's number is a precise indicator of its orientation relative to magnetic north, crucial information for safe navigation.