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Home » John E. McLain

Slips and Skids: An explanation of the terms is necessary

By John E. McLain (August 2001)

My two articles concerning yaw, its effects and its control seem to have aroused considerable interest among our readers. I have received quite a few e-mail messages and telephone calls regarding them. While I have responded to each inquiry, the questions indicate a need for a follow-up article on some of the concerns and desires to understand that were expressed by the readers. So here goes.

First, let’s discuss what slips and skids are. I often ask this question of pilot and flight instructor applicants. Here are some of the routine answers I get: A slip is when the ball is to the inside of the turn and a skid is when the ball is to the outside of the turn; a slip is when the tail is to the inside of the turn and a skid is when the tail goes to the outside of the turn. These answers may tell how to identify a slip or a skid, but they do not describe what they are.

A true understanding of slips and skids involves a lot more than where the ball is or what the tail is doing. You should remember, from your basic aerodynamics, that a standard rate turn is 3 degrees per second; i.e., it takes two minutes to complete a 360 degree turn at standard rate. But what is the real meaning of a standard rate turn? Simply, it is a turn in which the airplane is turning, while neither slipping nor skidding, at a three degrees per second rate. Why three and not four or two degrees per second? Well, somewhere, some place, years ago, the powers that be determined that it should take two minutes to complete a 360 degree turn. Voila, three degrees per second became standard rate by definition.

The amount of bank required to maintain a standard rate coordinated turn is dependent on the airspeed. The greater the airspeed, the greater the bank angle required to maintain a coordinated, or ball in the middle, standard rate turn. There is a mathematical formula for determining this, but it is not practical to use in flight and, frankly, I do not remember it.

There is a much easier method to determine the bank angle required for a coordinated standard rate turn. Take your airspeed, drop the last digit, and add five if your are using knots and seven if your are using miles per hour. Thus, if your speed is 130 knots, add five to 13 and you come up with 18 degrees of bank required to maintain a standard rate turn. Try 120 mph. You should come up with 19 degrees of bank. Believe me, up to about 250 knots, these values are accurate enough for all practical purposes.

Now that you understand the relationship between airspeed, rate of turn and angle of bank, it is easy to understand what a slip or skid is. But, let’s look at one other fact.

The best way to turn any vehicle, not just an airplane, is to bank it. Consider for a moment automobile races. How can those cars negotiate turns at speeds up to 200 mph? The simple answer is that the tracks have banked turns. Imagine how the lap speeds would drop if Daytona or Indianapolis had flat turns. Also, nobody has yet figured out a way to bank water. See the problem speed boats have in high speed turns? Even highways bank some curves to accommodate the high speeds of cars.

With all of this in mind, let’s really learn what a slip or skid is. Considering a coordinated turn, it is very simple. A skid is where the rate of turn is too great for the angle of bank. Conversely, a slip is where the angle of bank is too great for the rate of turn.

Simple, but accurate. Now let’s look at what this means.

Let’s first look at slips. In a perfect slip, a bank is established. This would normally result in a turn. However, rudder opposite the bank is applied to prevent a turn. In this case we have a bank that should result in a turn but there is no turn, therefore the angle of bank is too great for the rate of turn.

Is it possible to slip and turn at the same time? Of course. In the example just given, if the bank established should result in a standard rate turn of 3 degrees per second and your rate of turn is only 2 degrees per second, you would be slipping and the ball of the turn coordinator would not be centered.

In order to better understand this, try the following the next time you are flying: Establish a slip, and then look at your turn coordinator. Many pilots think that, because the airplane is banked, the little airplane in the turn coordinator will be banked. Surprise, that little airplane will be level because the big airplane is not turning. The ball, however, will be out of center. This brings up an important point. The only time you, the pilot, can be sure the little airplane in the turn coordinator is accurately depicting the position of the big airplane’s wings is when the ball is centered. The same is true of the needle if you have a needle-ball instead of a turn coordinator.

Lest I be sharply criticized by the astute pilot, I should point out one other thing about the turn coordinator. Initially, it will indicate a roll, not a rate of turn. Thus, as you enter a slip, even if you use opposite rudder to prevent the turn, the little airplane will deflect in the direction of the bank until the bank is stabilized. Then it will center if there is no turn. This is not true of the needle of the needle-ball.

Now let’s examine the skid. As explained above, this is a situation where the rate of turn is too great for the angle of bank. It also means the airplane is yawing. In other words, skid equals yaw. From the previous articles in the June and July issues of The Southern Aviator, I discussed the evils of yaw and the possible problems created by it. I will not repeat this here.

To understand the skid better, use the turn coordinator again. While flying with the wings level, apply some rudder, in our example here, left rudder. The airplane will turn left, and eventually bank left. What you now want to do is prevent the bank by applying opposite aileron to keep the wings level. Now you will be turning with the wings level, but surprise, the little airplane in the turn coordinator will be deflected to the left, the direction of the turn, even though the wings are level. This again shows that the turn coordinator does not accurately depict the position of the wings of the big airplane unless the ball is centered.

I hope the reader now has a better understanding of slips and skids. My previous article on yaw and spins delved into the problems encountered in skids, i.e. yaw. However, from reader comments there seemed to be some concern about the adverse effects of slips.

Some of this concern seems to come from an incorrect definition of, or should I say explanation of, spins. One very common explanation of a spin is that it results from an uncoordinated stall. This is only a half-truth. A spin requires two things to be maintained. They are stall and yaw. So, yes, if the airplane is yawing as it stalls, a spin could result. However, only a skid, not a slip, involves yaw. Therefore, a stall while in a slip will not result in a spin.

I hope this explanation will alleviate the fears of some readers who thought that a spin could result from a properly executed slip, such as to lose altitude or land in a crosswind. It is just not true. There are, however, some problems that a slip could produce, although in my opinion they are not nearly as significant as what a skid or yaw could produce. So let’s look at these.

Without going into a lengthy explanation, let me just say that a stall out of a slip will normally result in the high wing stalling first, which is what most pilots refer to as an over-the-top stall.

Let me walk you through one. You are slipping to the right. The right wing is down, and you are holding left rudder to keep the nose going straight. For some unexplained reason, you increase the pitch angle sufficiently to induce a stall. In this case the high wing will stall first and the low wing will start to rise. You will then be returning to a wings level position. The application of proper stall recovery technique will result in a minimum loss of altitude. Compare this to a stall out of a skid, where the low wing stalls first and you are pointed directly at the ground. You will lose much more altitude in the recovery.

One word of caution here: Remember, you were holding left rudder in the slip. As the stall occurs, the airplane will tend to return toward the wings-level position. When it reaches that position, if you continue to hold left rudder, the slip now becomes a skid and you could be in trouble. Your recovery must involve proper rudder application, which in this case means getting off the left rudder and probably applying right rudder. Just keep in mind that, at the time of the stall, a roll to the left is induced even though the right wing is down. Think of it this way: If you stalled out of level flight and the left wing dropped, you would coordinate right aileron and rudder to level the wings and stop the roll. In the case of a stall out of a right slip, the same theory applies. You are starting a roll to the left, and must use coordinated right aileron to stop the roll.

Most experienced pilots will tell you that it is very difficult to stall out of a slip. You really have to work at it, and I concur with this idea. For those of you who are worried about slipping stalls, I encourage you to get together with an experienced instructor and do some. I think you will learn quickly that slips performed to lose altitude, or to land in a crosswind, present little or no danger to inducing an inadvertent stall, much less a spin.

Understanding is an important part of the learning process, and I hope this article has increased your understanding of slips and skids. Now, take it to the next level of learning, which is Application. Find an experienced instructor who understands slips, skids, and cross controlled stalls, and learn to apply what I hope you have learned here. It will make you a better, more confidant, and above all a safer, pilot.