The slip is still part of the teaching and testing curriculum, but sometimes it appears as if many generation-X instructors teach it because they have to, not because they believe in it. In fact, this author sat in a CFI refresher clinic and heard the class instructor flatly state that slips are dangerous and there is absolutely no reason for anyone to ever slip an airplane. Of such comments are old wives' tales born.
If every one of your approaches was perfect and always deposited you right on your selected touchdown point, then that instructor would be right; slips would be unnecessary. But, how often are our approaches that dead-on?
Also, how many of your approaches are biased purposely to the low side so that you can use a little bit of power to put you where you want to be? What happens if the power isn't there? What happens if this is an emergency approach because the engine quit? Will you automatically build a little extra height into the approach to ensure making the runway, knowing that you can slip it off at the last second?
There are lots of reasons we don't see slips used very often, but between the old wives' tales and a lack of understanding, a lot of pilots simply don't feel comfortable with them. Let's take a look at some of the rumors floating around about slips and see how much truth there is to them - if any.
Is the slip "too dangerous to be used"? First of all, if slips were dangerous the FAA wouldn't have them in the Private Pilot Practical Test Standards (PTS). In fact, a sizeable portion of the landing phase is dedicated to the slip, and the PTS says in no uncertain terms that the pilot should be able to forward slip the airplane and put it down no more than 400 feet past a preselected point.
The "it's dangerous" school of thought is more wrong than right, but nonetheless it is based on the fact that you are flying in a cross-controlled configuration that at stall speeds could precipitate a spin. It's no secret that if you stall most airplanes while cross-controlled they will try to depart into a spin. So, how do you prevent that? You don't stall the airplane. That seems pretty obvious, and it all comes back to a couple of basic aviating techniques called attitude and speed control. More on that later.
Slips are also labeled dangerous because they supposedly build up too high a descent rate close to the ground. Of course, that idea is open to definition. Compared to older machines like Piper Cubs and their peer group, most modern airplanes don't slip very well. Most don't have enough rudder travel to hold a severe slip angle, and a few don't have enough aileron. This is true of just about every airplane certified in the past 50 years. The result is that it's actually hard to induce much of a sink rate in a modern light airplane. The best that you can hope for is to steepen the glide enough to make a more accurate touchdown.
The rate of descent that can be generated by an airplane like a Cessna 172 at partial flaps and fully crossed up will barely match that of a Piper Cherokee in a normal full-flap approach. So, it's not exactly falling out of the air.
Another old wives' tale is that you can't slip an airplane with the flaps down. It depends 100 percent on the airplane being discussed. For most airplanes, slipping with the flaps down is no different than slipping it clean, although a little higher rate of descent will be generated with the flaps down. In addition, if you let the nose come up while in the slip, it will bleed off speed faster because the drag of the slip is added to the drag of the flaps. A few airplanes - some Cessnas in particular - carry an admonition that "slips not recommended at full flap deflection." First, that doesn't say, "slips prohibited." It just says not recommended. Also, note that it says at full flaps, not partial flaps. Although the exact logic behind the notation/placard isn't known, it may be based on one of two factors: the possibility of speed bleed-off at full flaps while in a slip, and turbulence over the tail in that configuration.
In airplanes whose manufacturers don't recommend slipping with full flaps, the full-flap configuration generally requires such a sharp nose-down attitude to maintain speed that the factory may be concerned that pilots will inadvertently let the nose up and allow speed to dissipate to a dangerous level. If that happens, the flaps, coupled with the crossed-up configuration, could cause a stall/spin departure.
The other argument against slipping with the flaps down is blanking the tail or blocking off airflow across the tail. There is some validity to this. Where the POH for a given airplane strictly forbids slips with flaps, this is the reason why. However, what people often mistake for tail blanking in full-flap slips is turbulence over the tail. In some airplanes, Cessnas being the most obvious, the flaps go down far enough that a high-angle slip causes airflow to tumble off the edge of the flaps in such a way that it is felt as turbulence over the tail. The elevator rumbles and feels as if an elf is tap-dancing on it. In some cases, the nose may also hunt up and down, but the pilot is still in full control. It just feels scary.
In extreme cases of disturbed airflow, there is the possibility that the tail may lose effectiveness and the nose will try to pitch down. If a safety issue is involved, there will be a clear restriction posted in the POH and/or in a placard on the panel that says, "Slips with full flaps prohibited." Such an instruction, like any other placard, must be obeyed.
Here again, when you want to know something about your airplane, got to the POH. If there's no restriction against slips with full flaps, it's OK. If you have any concerns, discuss the practice with your flight instructor - and if you've already earned your certificate, ask a CFI to go with you to explore forward slips in the airplane you fly.
There are a lot of folks in the small-airplane community (as opposed to bigger airplanes that aren't normally slipped) who may not slip on every approach, but they always have it in the top drawer of their aviation toolbox, ready when the occasion arises. And what occasions are those?
First of all, there is a common misconception that a slip is used only to make up for big mistakes, and it always involves lots of rudder and aileron. That's not the case. In fact, if an approach is so off-profile that you think you need a full-deflection slip to get it back in the groove, maybe you ought to think about going around.
Those who habitually use slips seldom have to resort to really big, high-angle slips because they see the off-profile situations coming early enough that they can start a moderate slip earlier and keep the airplane in the groove from farther out. They see the slip as an altitude eraser, and they are just as likely to erase 10 feet at the last second as they are to scrub away a hundred feet farther back.
So, how does that process work?
The beauty of the slip is that it's variable. Once you have a given amount of flaps down, you're pretty much stuck with that rate of descent and you can't decrease that rate of descent without using throttle. Not so the slip. It is infinitely variable from zero to maximum. You can slide in and out of the slip, varying your rate of descent.
It helps if you look at the slip the same way a glider pilot looks at spoilers. If you need to come down a lot, you slip a lot. If you're just a few feet high, then you gently squeeze in a hint of cross-control, the airplane settles those few feet, and you're back in business. It is the variable aspect of the slip that makes it such a good tool.
The traditional method of slipping is to treat it as if you're throwing a switch: You slide hard into the slip, you wait until you're about where you want to be above the runway, and then you quickly kick it straight. You're either in it, or you're not in it. That puts square ends on your slip. What we'd like to propose, however, is viewing the slip as having tapered ends, not square ones.
Let's say the second you start the slip another 400 feet per minute is added to your rate of descent. When you kick it straight, 400 fpm is subtracted from your rate of descent. . This kind of approach to the slip works, but it assumes that you know how much slip you need going in and, more important, it assumes you will kick it out at exactly the right altitude. This is assuming a lot, and it tosses a couple of unnecessary variables into the approach.
What we're suggesting is that we slide into the slip a little more gently and establish a rate of descent. Then, if we see that the point of intended touchdown is still moving toward us, indicating that we're above our desired glideslope, we crank in a little more aileron and balance it with rudder. This way we gradually match the slip angle and its rate of descent with exactly what's needed to hold the point of reference stationary in our field of view.
If during the slipping part of the approach we see the numbers start to move away from us, we just come out of the slip a little, rather than adding power. Using the slip in this manner allows us to nail the exact glideslope required to touch down at our chosen point. This is only common sense and not a particularly new approach to the slip. Our form of recovery is what's a little nontraditional and is what makes the slip really work.
Rather than roaring down to the perfect position above the runway in a full slip with the hope we will kick it out in the right spot, we're going to slowly decrease the slip angle and creep into that position. While we're on final and still a little higher than we want to be, we're going to release just a little slip angle. That will start to decrease our rate of descent. Then, we look ahead of the airplane and visualize the exact height we want the airplane to be when our wings are level and we stop coming down. Since we don't want to be wings-level too high or too low, we're going to continue gradually out of the slip at a rate that puts us exactly at the height we want.
If we look ahead and see we're likely to be low, we come out of the slip a little faster. If we see we're still high, we hang on to just a little of the slip until we've burned off a few extra feet. At that point we're barely cross-controlled and in a very slight slip. By using this kind of tapered or gradual recovery, we're reducing our rate of descent a little at a time until it zeros out right where we want to be and we're neither high nor low.
Another advantage of coming out of the slip slowly is that it gives us more time to figure out what any crosswind is doing. Incidentally, while most of us are more comfortable slipping left wing down, that complicates a right crosswind landing, so always try to slip with the down wing into the wind for the obvious reasons.
The question of speed control during a slip is an important one because you never know how accurate your airspeed is in that situation. Also, some airspeed indicators will read slightly different slipping right versus slipping left because of the slipstream effect on the static ports. The key is to hold the nose in exactly the same position in relation to the horizon that it was in before entering the slip. Most of the time the speed will be within a few knots of what the airplane originally had. Some people think stuffing the nose down and slipping at a higher-than-normal airspeed is safer, which isn't always true. Yes, that will prevent any dangerous speed bleed-off, but it also puts you in ground effect with too much speed. Hit ground effect fast and you'll waste runway by floating, which works against the logic of slipping in the first place. Also, most airplanes don't slip well when they are fast.
What you don't want to happen during a gradual recovery is for the nose to come up inadvertently while still in the slip. In some airplanes it takes back pressure to hold the right nose attitude during the slip; because the decreased projected area of the elevator makes it less effective, you have to pull to keep the nose from falling. When you start to recover and the slip angle is decreased, if the elevator is held in the same position, the nose will gradually start up and it's possible to start a decelerating airspeed trend. This is not good. Just watch the nose during recovery and don't let it come up, which will usually require relaxing the elevator pressure just a little as the wings level out.
The slow, tapered recovery will forever change the way you look at slips. Also, the first time you're on short final and think to yourself, "I'd really like to be about 10 feet lower," and you gently cross-control for a second or two and watch the airplane obediently settle down to the right altitude, you'll know why some folks think the slip is the final, and possibly best, tool for making good approaches perfect.
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 for 36 years, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site ( www.airbum.com ).
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