Running the Runways: A High-Level Overview of Airports’ Most Mysterious Components

Whether you’re a first-time traveler or a seasoned pilot, you know what a runway is. What you might not know is how they’re chosen for takeoff and landing.

I’ve talked to a number of friends who fly frequently. Despite the fact that some of them are in the air more often than I am, they wonder aloud at why they take off going one way one time and another way the next. And while it’s tempting to assume that the decision is made arbitrarily, reality is that it’s anything but. Indeed, there are multiple considerations that have to be taken into account before a plane can either leave or land on the tarmac.

The Basics

If you look below, you’ll see that I’ve displayed the Boston Logan airport chart. The runways are the long, black lines, while the taxiways are the grey lines that intersect them.

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Boston Logan International Airport and its runways. (FAA.gov)

As you can see, there are six runways at Logan. They can be divided into the following categories: long runways, medium-length runways, and short runways.

Long Runways: These are used by aircraft of all sizes, from the Dehavilland Dash 8 to the Boeing 747. Barring an anomaly, one of these two runways is used at all times – normally alongside a medium-length runway.

  • 15R/33L (10,083 feet)
  • 4R/22L (10,006 feet)

Medium-Length Runways: As previously stated, one of these runways is normally used alongside a long runway at all times. Under the right conditions, both runways can take aircraft as large as the 747.

  • 4L/22R (7,864 feet) – 4L is generally used for (visual) arrivals alongside 4R’s ILS approach, while 22R is used for departures alongside 22L’s ILS approach.
  • 9/27 (7,000 feet) – 9 is generally used for departures when 4R and 4L are being used for arrivals, while 27 is generally used for arrivals when 33L is used for departures.

Short Runways: These are traditionally used alongside the 27/33L configuration by smaller commercial aircraft (14/32) and general aviation aircraft (15L/33R).

  • 14/32 (5,000 feet)
  • 15L/33R (2,557 feet)

Generally speaking, major international airports have at least one long runway (10,000+ feet) to accommodate large aircraft. Of course, there are exceptions, such as San Diego having a single 9,400 runway that sees heavies on a daily basis, but such is the exception rather than the norm.

So how are runways numbered?

You may think that the signage around and numbering for a given runway is mysterious, but the answer is relatively simple: they’re numbered 1-36 based on their magnetic heading. Basically, you take the magnetic heading of a runway and divide it by 10, rounding up if it ends in 5 or above and down if it ends in 4 or below. For example, a runway whose headings are 42 and 222 would be Runway 4/22. Parallel runways are generally differentiated by labels of “Left, “Right,” and even “Center,” as Seattle-Tacoma International Airport has runways 16L/34R, 16C/34C, and 16R/34L.

Due to the natural movement of the North Magnetic Pole, a runway may have to have its numbers reassigned should its magnetic heading deviate from its assigned numeric value. For example, a runway may have been numbered 15/33 when its magnetic headings were 333 and 153, but the runway must be renumbered 14/32 now that it has magnetic headings of 144 and 324.

How is it decided which runway is used for a given flight?

While there are a number of variables that can affect the above answer, it ultimately boils down to a couple of things:

  1. Direction of the wind: By taking off and landing “into the wind,” meaning that the wind is blowing at the front of the plane, the plane is able to generate lift at a lower speed than it is otherwise. As a result, the active runway at a given airport is usually the (closest to the) direction of the prevailing wind.
  2. Length of the runway vs. distance required by the aircraft: If there are two runways that are parallel, one being 12,000 feet and the other 6,000 feet, then a fully-loaded 747 will take off using the former, while a small 737 making a half-hour shuttle flight would likely use the latter.

While the above criteria are usually the most common considerations, there are a number of other factors that can play a role in the decisions made by pilots and ATC:

  1. There are certain situations where 747s/777s/etc. can use a 7,000 foot runway instead of a 10,000 foot runway, as I’ve seen 747s in Boston take off from Runway 9 rather than Runway 4R, but that’s because they’re making “short” (as compared to other 747 flights) trips to London and Frankfurt rather than Shanghai or Tokyo, which would necessitate much more fuel, a longer takeoff distance, and a longer runway.
  2. Altitude and atmospheric conditions can also require a longer takeoff distance and necessitate the use of a longer runway. Generally speaking, “hot and high” conditions are present when the air density is decreased, thus requiring a higher speed to achieve adequate lift.
  3. Pilots can request a runway other than the active runway(s) when taxiing out or coming into land. However, there are occasions where ATC will reject a pilot’s request for their preferred runway: I heard Boston Ground tell a 747 that he couldn’t have Runway 15R instead of Runway 33L because there were a number of inbound arrivals that were approaching the latter. Though 15R was closer to where the aircraft was parked, and is indeed Logan’s default “calm winds” runway, ATC turned down the request because they would be forced to divert inbound traffic to accommodate it. The pilots understood, and proceeded to taxi to the active runway without incident, but it just goes to show that the decision is up to the controller’s discretion.

This is just the beginning

This is very much a high-level overview of how runways work, and I’m sure that some will point out that I missed a few things here and there. That said, feel free to reach out if you have a clarification or are curious to learn more about something explained here.