Understanding Runways

Runway view (30R) at Sedona Airport, Arizona

Welcome back to Brooke In The Air! This week’s blog is more educational than most have been. We’re going to understand runways and their markings this week!

Firstly, runways are numbered based on their magnetic azimuth (also known as the compass bearing), and these large numbers help pilots and air traffic controllers identify and navigate the runways. There is a logical and easy system for assigning runway numbers.

Runway numbers reference the nearest 10-degree magnetic heading, with the last zero removed.

• Parallel runways use “L,” “C,” and “2 to differentiate between them.

• Some airports may use true north for runway numbers due to high magnetic variation.

• Magnetic north moves over time, causing occasional changes to runway numbers.

Runway numbers are actually a reference to the closest magnetic heading of the runway. The runway heading is rounded to the nearest 10 degrees, and the zero is removed.

This number is then assigned to the runway. Further differentiation can be achieved with letters on parallel runways.

While it is relatively simple, there are a few things to be aware of when discussing runway numbers.

Airfields and airports are actually pretty heavily regulated by how they are named.

BoldMethod’s illustration of a typical airfield’s runway layout

Here’s what you need to know: those numbers are just at the end of the runway aren’t just for decoration. They also are not arbitrarily named. In fact, there is a lot of information held within those huge white digits.

The runway markings tell you where the runway is pointing, referencing magnetic north.

Let’s imagine facing down a runway with a compass in our hand. Our compass reads 092°.

Runway number diagram/slide

What is the runway number?

Well, here’s a step-by-step guide to how it is calculated:

1. The runway heading is rounded to the closest 10° (in this case, 092° becomes 090°)

2. The final ‘zero’ is removed to give a two-digit identifier (so in this case 090° becomes “09”)

3. The resulting number is the runway number.

The length of a given runway depends on the type of aircraft commonly used at the airport in question.

Airport operations, and specifically where aircraft fly in relation to the surrounding community, depend upon the layout of the runways at an airport as well as on wind direction. General runway identification is based on the compass heading the aircraft is facing as it is landing or taking off.

For example, Runway 17/35 is facing approximately 170° in one direction and 350° in the opposite direction. Even though a runway is a single strip of concrete, it is essentially treated as two separate runways by pilots and controllers. Parallel runways have the same compass readings, and therefore are further designated with L (Left) or R (Right) at the end of the runway number.

Runways are built to align with historical wind patterns specific to each airport because aircraft land and take off into the wind. Using MSP Airport in Minnesota as an example, at the Minneapolis-St. Paul International Airport (MSP), two parallel runways are oriented northwest to southeast to align with prevailing winds experienced in the Upper Midwest. These parallel runways serve as MSP’s primary runways.

MSP Airport in summer - View at Runway 22R

MSP has four runways: two parallel runways (12L/30R and 12R/30L), a north/south runway (17/35) and a crosswind runway (4/22). Each runway has unique characteristics.

• Due to runway layout and prevailing winds, the parallel runways are the most frequently used. They are used every day unless construction or unusual weather conditions disallow their use

• Due to the layout of the runways, Runway 17 is generally used only for departures in a southerly direction

• Similarly, Runway 35 is generally used only for arrivals from the south

• Because Runway 4/22, MSP’s longest runway, intersects the primary runways at MSP, it is used only when wind conditions demand it or for large international flights that require its extra length

Selecting which runways to use for aircraft departures and arrivals at MSP is a complex task. Decisions about airport configuration and runway use at MSP are made carefully on a continuous basis by the Federal Aviation Administration Air Traffic Control (ATC).

When selecting an airport configuration, ATC takes into consideration numerous factors including:

• Safety

• Efficiency

• Wind direction and wind speed (on the surface and aloft)

• Aircraft weight

• The number of inbound and scheduled outbound aircraft

• Noise abatement

• Where aircraft are going to and coming from (destination and origin airports)

This is the same procedure for any given airport.

You shouldn’t find any runway numbered either “0” or “00”. Any runway which points to magnetic north will normally be given the designator “36” (as in 360 degrees). This will also correspond with the headings read out by both ATC and the pilot.

Reciprocal vs Parallel Runways

Reciprocal Runways: A note.

Quite simply put, the reciprocal of any runway number varies by 180° as it is the opposite facing runway. But you’ll remember that we always remove the third and final digit when discussing runway numbers, so 180° becomes 18.

So, we can say that a reciprocal runway number is always 18 different from its opposing number.

Using a little clever math, we can come up with a ‘hack’ to easily work out runway numbers and their reciprocals.

Here’s a step-by-step guide and a working example.

If you can add and subtract 2 from any number, you are covered: –

1. Take any runway number as an example.

2. If it begins with a “0” or “1,” add 2 to the first number. If the runway number begins with a “2” or “3,” subtract 2 from the first number.

3. Do the opposite to step two to work out the second number. If you added 2 to the first runway number, subtract 2 from the second, or vice versa. Simple.

When an airplane is flying north, it is always said to be flying a heading of “three-six-zero”, and never “zero zero zero” degrees. The naming convention of the runway numbers reflects this.

This should tell you all you need to know about runways, at least for now!

Sources: https://pilotinstitute.com/runway-numbers/ - The Pilot Institute
https://metroairports.org/sites/default/files/2021-09/factsheet-4_how_are_runway_decisions_made.pdf - Metropolitan Airports Commission
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap2_section_3.html - FAA
The Bold Method pilot training

Regional aircraft runway requirements

Thank you for joining us at Brooke In The Air this week! Remember to like, comment, and subscribe on YouTube! Please join my Patreon for more content!