Wet is dry

wet runwayAs you move into bigger and faster airplanes—especially jets—you need to become aware of things that didn’t necessarily matter as much before. There are new definitions that you need to learn. Besides the various V speeds—V1, V2, V-REF, et cetera—there are terms that probably didn’t really catch your attention before.

Take the runway, for instance. In the United States, most every airport that provides airline service has runways that are grooved. The grooves run perpendicular to the runway direction (that is, across the runway) and are evenly spaced from one end to the other. Further, whenever possible, there is a bit of a crown to the runway. The purpose of the grooves is to provide drainage and runoff for rain, snow, and deicing fluid that flows off aircraft upon takeoff.

This is need-to-know information for pilots, because performance data takes into account whether or not a runway is wet or dry, or if it has standing water. To add to the confusion as well as to the paychecks of the engineers, the standing water (and snow) categories are broken down into various depths, each succeeding level of which will further degrade the performance (read: payload) of the airplane.

What initially might catch you off guard is the seriousness with which these terms are defined. For instance, a runway that is grooved but has water on it is not necessarily wet. Depending on the airline and the aircraft manufacturer definitions, a wet runway may be defined as dry if it is grooved, and a runway that is physically dry is dry (you can’t make this stuff up). Generally speaking, if a grooved runway has water on it, it is considered wet only if the surface is reflective or if a certain percentage of the surface has standing water. Otherwise, it is considered damp or dry because the grooves carry away the water that might induce hydroplaning.

When it is raining hard enough that there is clearly standing water, performance numbers begin to suffer. There are three major concerns. The first is rejecting a takeoff without skidding or hydroplaning. The second is continuing a takeoff after an engine failure on a slick runway that is not only slick, but produces drag thanks to the puddles. The third is the use of reduced thrust. It is common for jet aircraft to take off at well less than full power, but in certain circumstances, full power is required. Contaminated surfaces are one of those circumstances.

As I mentioned, in the United States, this is rarely a problem. However, if you go to Canada or Mexico, most runways will not be grooved. This is also a common problem overseas. Don’t be lulled into a trap. Pay attention to the wet versus dry issue, and know when—and when not—to apply the various penalties.

It may not be the dictionary definition of the words as you know them, but you will learn that the industry and the FAA can be very specific in how words are used or defined.

In fact, I think they have a specific definition of “used…”—Chip Wright

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One Response to “Wet is dry”

  1. Tom Chromy says:

    Your idea of grooved/wet runway is incorrect and it contradicts itself in the fifth paragraph. Where are your references?

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