Understanding airport runway design is probably unnecessary if you aren’t in the business or industry of caring about it, but it is actually pretty cool stuff. (It’s not likely that you’ll be running around on the tarmac anyway.)

What’s a runway for? It is for airplanes to take off and land. That one’s easy. But what are they made of, and how do people know how to position them in an airport? Considering the frequency of take-offs and landings at any major airport, it’s important to know that people have thought of these things (for the most part) to keep everything flowing smoothly and safely.

Airport runways are carefully designed to make air travel as efficient and robust as possible.

Some Specifications

As for the specifications of an average runway, they can run from anywhere between 6000 feet to 10,000 feet (about two miles) long. Most of them are able to accomodate landings of aircraft that are less than 200,000 pounds, but others can take much more weight. A typical Boeing 747-400 is about 800,000 pounds upon take-off, but it can only land safely if it is less than about 700,000 pounds. (This explains why we sometimes hear of airplanes circling an airport for some time before landing; it needs to burn up fuel to lose weight in order to land.) Similar situations are apparent in all aircraft (albeit not necessarily those particular weights). And of course, under no circumstance can the maximum landing weight be higher than the maximum take-off weight.

As for the runway length, we would need to consider the density of the air. Remember that, at higher altitudes, the density of air is less, which decreases the amount of lift and thrust. Just know that an aircraft landing at a higher altitude would require a longer runway. And along the same lines of density, fluids (like air) are less dense at warmer temperatures. As such, an aircraft will require a longer runway in hotter or more humid conditions than in cooler, dryer ones.

Design Considerations

Main runways are usually oriented so that during take-offs and landings, the airplane is flying into the wind. Flying into the wind upon take off is a good thing because it allows for more airflow across the wings (see last week’s post on lift: “Lift Force From Aircraft Wings“) while using less runway length. But once we’re in the air, however, going against the wind can massively reduce our efficiency and increase our flight time. We’ll want to avoid this as best we can, but there isn’t a way to control wind speed in nature just yet.

From HowStuffWorks: “Main runways are usually oriented to line up with the prevailing wind patterns so that airplanes can take-off into the wind and land with it. Local and ground air traffic controllers determine which runways are used for take-off and which for landing, taking into account weather, wind and air-traffic conditions. In some airports, main runways cross each other, so the controllers have to pay even closer attention.”

Markings

Each runway is labeled with seemingly random combination of alphanumerics. But of course, nothing in engineering is random (hopefully). Following navigation and surveying convention, picture a north-south-west-east compass with angles broken down from 0 to 360 degrees. In this configuration, 0 and 360 are pointed north (not east, as we are wont to expect!). So, 90 degrees points east, 180 points south, and 270 points west.

At the end of each runway is a number that indicates the angle that the runway points to, rounded to the nearest 10-degrees, and then divided by 10. So, if an airplane were to land by flying west, the “right” end of that runway would show a big 27. The “left” would show the opposite, which would be a 9.

Isn’t that cool!?

That’s all for now. We’ll pick up on this next time, with runway configurations.

(Image from NASA.)

Categories: In-Depth Articles
Tags: Airplane, Airport, Runway
3 Comments →