| In a forward slip, the pilot holds the airplane in a bank with aileron, and uses opposite rudder to keep it aligned with the runway. Drag increases dramatically, steepening glidepath and increasing the rate of descent. |
| When you turn final, pick a point on the runway where you want to touch down. Aim your glidepath at that point. If it's moving up in the windshield, you're too low--and if it's moving down in the windshield, you're too high. |
The quality of a landing is most often determined by how well we control the approach path or glidepath--affected primarily by our choice of descent angle and airspeed during the approach. This is another way of saying that the degree to which we understand the various techniques that can be used to modify the approach path will determine whether the airplane touches down exactly where we want it to.
At any point in the approach, the airplane has three kinds of energy available to it: kinetic energy, which is that given it by speed; potential energy that comes from its height above the ground; and the controlled energy, which can be given it by the throttle. The key to a successful approach, whether it is totally power off from downwind or flown onto final with power, is to first make sure the airplane has enough energy of all kinds--especially height and speed--that you're guaranteed to make the runway no matter what happens. Then, once we're on final, we can use the tools at our disposal to gradually get rid of excess energy so that we land on our chosen touchdown point.
The energy from speed is more or less fixed. For a normal approach we'll use the glide speed recommended in the pilot's operating handbook, for our weight and air temperature. Any speed that is either over or under the recommended speed will result in a steeper glidepath. However, depending on whether we're over or under that speed, the airplane will behave differently as we start to flare.
If we're above the optimum speed when the nose comes up to flare, the airplane will skate along in ground effect and float--which not only makes it more difficult to hit a given spot, but also gives any crosswind more time to work on us. If we're slow, the airplane will have less than the normal amount of hang time during flare and, if slow enough, can deposit us on the runway in a spectacularly abrupt manner.
The tools we have to safely steepen a glidepath are configuration changes (flaps and landing gear) and slips, all of which add more drag to the airplane so it comes down more steeply at a given speed. Configuration changes add permanent drag, meaning that when we put down flaps or landing gear we assume we're going to leave them down. Slips generate a variable type of drag that can be increased or decreased to add just a little rate of descent or a lot. When it comes to fine tuning glidepath, the slip is the most useful tool.
Extending the landing gear on a retractable-gear aircraft greatly increases drag and steepens the glidepath. This should be the first configuration change made while still on downwind, to avoid forgetting it and help to stabilize the approach. The only time gear extension would be delayed until final approach would be on an emergency landing where we don't want to commit to the permanent drag of the landing gear until we're positive we're going to make the field.
Flaps represent a way to increase the approach angle because the lift generated by flaps is accompanied by an increase in drag. Although it varies with the exact type of flap (Fowler, slotted, unslotted, simple hinged), it is generally assumed that as flaps are extended past 15 degrees, they begin generating more drag than lift. As the wing's lift increases, so does the drag, and the nose must go down to maintain speed--which increases the glidepath angle. The increase in lift from 15 degrees to 40 degrees of flap extension is small, but the drag skyrockets.
Those favoring power-on approaches generally extend their flaps by varying amounts on the different legs, so that when they turn final, they extend the last notch. For an emergency situation or a simple power-off landing, however, it's often wise to delay the last two notches until we're on final and we get a true reading of how much wind we're flying into. Until we know for sure that we have the runway made, it's never a good idea to put down the last notch of flaps.
And then there is the slip. Before training airplanes were equipped with flaps, the forward slip was a major part of a student's training. Today, it is included in training, but very little emphasis is placed on it. This is too bad because knowing when, and how, to use a forward slip gives a pilot almost total control over where his airplane will contact the runway.
A slip is nothing more than holding an airplane in a bank with aileron and keeping it from turning with opposite rudder. The resulting cross-controlled, wing-down, nose-out-of-line attitude dramatically increases drag, although the maximum amount depends very much on the make and model of airplane. Most modern airplanes of the Cessna/Piper variety don't have the rudder or aileron authority of their ancestors, so they can't generate a very steep slip angle. This means the drag they produce will make for a steeper glidepath, but they can't be made to almost fall out of the air as older airplanes could. Also, some aircraft carry either prohibitions or cautions about slipping the airplane with the flaps fully extended; check your POH and/or placards.
The amount of drag produced and the resulting increase in approach angle are very much functions of how much the airplane is slipped--in other words, how steep is the bank angle and how far the nose is off to the side. Also, if the airplane is to travel down the extended runway centerline on final, each specific amount of bank angle has a specific amount of offsetting rudder-induced nose angle that is required to cancel out the turning tendency. Too little rudder and the airplane will travel toward the down wing. Too much rudder and it will travel toward the down foot.
In what seems to be the accepted method of slipping in something like a Cessna, the airplane is set up in the final approach configuration (generally not with full flaps, but that depends on the model). Then, because the rudder authority is the limiting factor, the rudder goes to the floor and opposite aileron keeps the airplane traveling straight. As the airplane approaches ground effect, the controls are released and the airplane physically aligned with the runway--unless a crosswind requires that you transition into a sideslip, of course.
By changing the bank angle and matching it with correctly balanced rudder input, the increase in the rate of descent can be varied from just a few additional feet per minute to the maximum that particular airplane can generate. That being the case, the all-or-nothing method of slipping is a pretty crude way of controlling the glide path. In addition, slamming into the slip, then popping back out all at once assumes the pilot is capable of making both entry and exit in exactly the right place, which is almost never the case.
There is a better way. Before we dissect the slip and add a couple of nuances, let's mention the single most important aspect of controlling the glidepath, whether slipping or not--that's seeing exactly where the airplane is going to go and controlling it. We can't control what we can't see, so here's the key step: Do not look at the runway--look at a specific point on the runway. This can't be emphasized strongly enough. Picking a specific point as a reference is like the front sight on a rifle; it gives us our accuracy. Looking at the runway in its entirety is an approximation and is a shotgun approach. If we use the numbers, or threshold, as a reference point, we won't land on them because of float in ground effect, but we will certainly land 500 to 600 feet beyond them, which is optimum.
When looking over the nose, the point at which the airplane will contact the runway will appear to be stationary in the windshield. The points beyond that will appear to be moving up in the windshield (or moving away), and the points closer to you--which you'll fly over--will appear to be moving down in the windshield (or moving toward you). So, if we use the runway numbers as the reference point, we want to hold them stationary in our vision by aiming the glidepath right at them. And this is where a slip really shines.
Before we start slipping, our reference point/numbers will be moving toward us (or down in the windshield), indicating that we're high and our glidepath will take us past the reference point. When we ease into the slip, our approach angle will change immediately, so look at our reference point (the numbers) and judge whether it is still moving toward us (we're still going to be high), has stabilized, or is moving away from us (up the windshield and we're going to be low). The steeper our bank, the steeper our rate of descent will be, so we'll change bank to hold the reference point steady in our vision. At the same time, we'll change the rudder deflection to match the bank angle so the airplane continues right down the extended runway centerline.
For recovery, rather than suddenly neutralizing the controls at once, we're going to slowly release the inputs and exit the slip gradually, which gives us more opportunity to accurately judge the wind. At the same time, it lets us do a much better job of getting into ground effect at the altitude we want, rather than just accepting whatever we get if we pop out of the slip.
The beauty of coming out of the slip slowly is that, if we see that we're higher than we want to be, we just hang on to a bit of the slip a little longer and lose those last few feet of unwanted altitude.
Controlling the glidepath requires understanding the techniques available and the dynamics of the airplane, as well as the effects of the air through which it is moving. Still, number one on the list of skills we need to develop is the ability to visually fix on our reference point on the runway and understand what its movement is telling us. As we have said before: We can't correct what we can't see.
Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
