Usual Compared to What?

FAA guidance warns about illusions caused by runways that are narrower or wider than usual. But what does a “usual” runway even mean?

FAA guidance has cautioned pilots about runway width and slope illusions seemingly forever, usually describing them in terms of runways that are narrower or wider than usual. That leaves it to pilots to supply their own definition of “usual.”

This is most clear when you can see it for yourself, which you might have if you flew Looks About Right. In that Sim Challenge, we showed how the normal vertical path to California City (L71) Runway 24 may make it seem like you’re too high due to its long, narrow, upsloping, runway sitting out in the desert at night, and how it could tempt you to fly a lower approach that looked more normal. Back at Santa Monica (KSMO), Runway 21 did the opposite: its short, wide, downsloping runway could make a higher approach look more normal and possibly lead to a long landing.

But you don’t need to fly these in a sim or real airplane to consider how these illusions could ambush you on a future flight.

A short, wide, downsloping runway could make a higher approach look more normal and possibly lead to a long landing.

A Different Kind of Runway Model

“Usual” is mostly a product of where and what you’ve flown. If you fly out of an international airport with a 10,000 x 150-foot runway, that probably sets your baseline for usual. Fly out of a 3000 x 50-foot strip and your usual is quite different. Either way, it’s what seems normal to you because you’re landing on it every time you go flying.

Over time and with a variety of destinations, you develop a working sense of how the runway should look at a given distance and from a typical vertical path. Change the runway dimensions and the same vertical path presents a slightly different picture. This is especially the case at night, when there are fewer surrounding visual references. In those cases, the normal vertical path gets tougher to determine since there’s less data for your brain to determine what’s usual.

A bit of research can hint your destination runway might present a sight picture that will try to trick your brain. Consider the runway dimensions. Divide length by width, and you get a simple ratio description of the runway’s overall shape and how well it compares to your sense of usual. A 5500 x 100-foot runway presents a length-to-width ratio of 55:1. That would look pretty ordinary to many pilots. It turns out that 55:1 is the median length-to-width ratio across all of the paved, lighted, public-use runways in the United States. About four out of five fall between 40:1 and 70:1.

That can be a handy planning tool. If the runway you’re approaching falls significantly outside of that range, the sight picture waiting for you may present a higher risk of misperception. Here is an example of the application of the runway dimension ratio applied to different runway widths to determine typical “usual” lengths.

L71’s 6027 x 60-foot runway gives us about 100:1, and KSMO’s 3500 x 150-foot runway gives us about 23:1. Both sit well outside the normal middle, in opposite directions. If you flew Looks About Right, you hopefully picked this up intuitively without needing the math. Even so, considering the dimension ratio can put a number on what you’ll be looking at, allow it to be compared, and can help you anticipate a potential problem during your flight planning—instead of discovering it on short final.

Reading the Runway

On final, your brain is trying to make sense of the whole runway image.

The dimensions of the runway are only the starting point. On final, your brain is trying to make sense of the whole runway image. This includes the relationship between the edges of the runway, the apparent distance between the near and far runway thresholds, and their relative motion.

Pilots usually use those relationships to judge vertical path, but runway slope can change them even when the airplane is flying the intended path. An upsloping runway can give you the picture you’d expect from a steeper approach to a level runway, and a downsloping runway can give you the picture you’d expect from a shallower approach.

Upsloping or downsloping terrain near the runway can work the same way even when the runway itself is level. Is the runway flat and the terrain sloped, or the other way around? The picture alone doesn’t necessarily tell you whether it came from the vertical path, the runway, or the terrain around it.

During the day, the horizon, terrain, foreground motion, roads, buildings, and surrounding lights can all help you judge height and distance. When those references are missing at night, the runway can seem to exist in a featureless black-hole. It’s easy to spot but not so easy to understand. Runway lights can even compound that problem because edge lights may be nearly all you have for judging position, while bright runway or approach lights can make the runway appear closer than it is. AIM 8-1-5’s illusion warnings can compound here because runway width, slope, featureless terrain, and lighting are all independent different factors that contribute to your ability to accurately judge your height and distance to the runway.

Cross-Checking the Picture

Considering runway proportion during flight planning can warn you that the sight picture might end up working against you, but it doesn’t tell you what to do about it. Once you’re approaching a runway, a good strategy is to lean on whatever vertical reference is available as a check against your perception of the runway. That vertical path could come from a Visual Glide Slope Indicator (such as a PAPI), an instrument approach, or a visual approach generated by your navigator.

In the absence of that, you can make your own check by computing an expected altitude at a known distance from the runway, along with the descent rate needed by your aircraft to stay on that path. This gives you a practical “how-goes-it” about what the airplane is actually doing regardless of whether your brain is perceiving the runway sight picture correctly or not.

Add all this up, and it’s clear that a “usual” runway doesn’t really exist. Every runway brings its own combination of length, width, slope, terrain, lighting, and surroundings. And, the same runway will present different challenges, day versus night, sunny versus cloudy, or even different times of year.

The more useful questions during planning are whether the planned runway or its environment has a reason to look different, and what you can use to check the picture when it does. On an unusual runway, remember that the correct approach may be one that doesn’t look quite right.

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Lee Smith

Lee Smith is an ATP-rated pilot, flight instructor, and software engineer in Maryland who enjoys bridging the gap between flight operations and technology. With a background in corporate aviation and over a decade of experience supporting flight procedure design and analysis, he brings both cockpit experience and engineering insights to complex IFR topics. Lee's work has appeared in IFR Magazine, IFR Refresher, Aviation Safety, and AOPA. He also writes The Logical Skies available at LogicalSkies.Substack.com exploring why the aviation system works the way it does.

Latest posts by Lee Smith (see all)

• Usual Compared to What? - August 27, 2026
• Sim Challenge: Looks About Right (VFR) - August 26, 2026
• Sim Challenge: Pitch Imperfect - April 24, 2026