At one point in time, when you'd ask the correct approach speed to be used, you would be given the best-glide speed, and that would be that. Pilot's operating handbooks gave a single number, say 62 knots. It was clearly understood by all that the factory had tested that particular model and found that the airplane would be most efficient--moving the maximum amount forward for the minimum amount down--at this speed. Environmental factors such as wind and heat could require adjusting the speed slightly, but this was the one we'd use as our focal point.
It is this last point, the understanding that there is a single most efficient approach speed, around which we build a series of judgments to fit changing situations that doesn't seem to be discussed enough anymore. Or, if you look at many POHs, you'll find that the approach speed is no longer given as a single number. Rather, it is given as a range, say 65 to 75 knots, with no discussion as to the reasons for the range. The reasons for the range are the foundation upon which the judgments required for changing conditions are based.
The best glide speed is usually the speed that offers the best lift/drag compromise: the lift being generated is maximum, and the drag being created by that lift is minimum, so the airplane covers the most distance across the ground from a given altitude. What is important to know about this relationship is that there is nothing you can do to improve upon it. If you increase the speed, you're going to lose more altitude. If you decrease the speed, you're going to lose more altitude. The number stated is the optimum, and changing it in either direction is going to cost you altitude. And keep your flaps up, too--lowering them will cost you altitude.
The need to conserve altitude runs head-on into the prevalent mindset that fast is good. Besides giving up altitude unnecessarily, excess speed invariably causes you heartburn because, as you work your way into ground effect, you find that the airplane is still moving along, refusing to touch down. The impact of ground effect is changed greatly by speed. If you come blasting across the threshold 10 knots over the recommended approach speed, the airplane is going to float there at three or four feet and refuse to come down until all of that excess speed has gone away. While this is happening, runway is whizzing past under you, and the wind has that much more time to make your life miserable.
One of the most common reasons for excess speed is an assumption that because there's a lot of wind out there it's necessary to tack a lot of speed onto the approach "...just in case." This is where judgment and understanding come into play. Whether you need increased speed in a wind depends on the character of the wind, not its strength or direction.
If the wind is steady with no gust factor, for all intents and purposes the airplane doesn't know the wind is out there. Your groundspeed will be lower at a given glide speed if you're flying into the wind, so the distance you're covering across the ground each second will be less, and the angle of descent will be steeper. Note, we said the angle you'll be coming down is steeper, not the rate. If your airplane descends at 500 fpm in a given configuration, weight, and temperature, it'll still descend at 500 fpm no matter how much the wind is blowing.
What fools some pilots into flying faster is the fact that wind on the nose slows the forward motion but doesn't change the downward motion. The illusion is that the airplane is falling out of the air, when it is actually coming down at a normal rate. At a ground speed of 60 mph, you're covering 5,280 feet per minute and you're coming down 500 feet during the same minute. If there's 20 mph on the nose, you're only moving forward 3,520 feet, but you still come down 500 feet, and the perceived difference of the approach angle is huge. However, the way you fly the airplane is the same, and the timing of the flare is the same. Everything is the same except that you appear to slow to a stop as you bring the nose up and you cover very little ground during the flare.
If you tack another 10 mph on your approach speed in this situation, you'll only hang in the air longer during the flare and give the wind more time to play with you. So, in a reasonably steady wind, increased speed is a disadvantage--even if it is a crosswind.
Incidentally, don't be persuaded to add speed just because there's a crosswind. Yes, the faster the airplane is moving, the better it is able to handle the wind. But regardless of what happens in the flare, touchdown is still going to happen at exactly the same indicated airspeed as if there was no wind at all. So, you may feel more comfortable with the extra speed at the beginning, but you still have to slow to touchdown speed, and excess speed complicates what should be a simple process.
When you're carrying excess speed, every single movement you make in pitch becomes critical, because the excess speed means excess lift, and just the slightest increase in pitch forces the airplane to balloon. You make what would have been a normal pitch adjustment, but suddenly, there you are, much higher than you want to be with speed rapidly deteriorating.
There are situations in which adding speed is the right thing to do, but it is almost always to protect yourself from gusts. If there's a good-sized gust spread, say 10 or 15 knots, it's prudent to add half of that gust spread to the approach speed to act as a buffer during flare.
The reason for adding the speed isn't the increased control it gives in a gust. Extra speed is used because a gust is temporary and can change in an instant. For a moment the wind jumps up 15 kt, then just as suddenly drops that much. Even though the airplane is floating on the air just like a boat in a river and doesn't sense the flow of the current, it will sense instantaneous changes because the inertia of the airplane won't let it slow or speed up as quickly as the wind changes. So, if the wind leaps up, the indicated airspeed goes up, the lift goes up and the airplane goes up--it's ballooning. Worse, if the wind suddenly dies, the airspeed falls, and lift is reduced. However, if you're carrying extra speed in the form of half the gust spread, you're protected and won't get dropped onto the runway.
The really fun winds are the ones that hammer you with a gust, you balloon, and just as you begin to settle again, the wind dies. That's what a throttle is for.
There are times when excess speed is a good thing. However, there are also times when a less-than-normal speed is good.
Let's make something clear. When we talk about speeds, and especially when talking about lower-than-normal speeds, we're talking about a "stable" speed, where the airplane is trucking along at that speed and it's not likely to change because you have the nose attitude under control. We are not talking about a speed that is part of an accelerating or, much worse, a decelerating trend. An airplane that is decelerating on short final had better be doing so because you asked it to--not because you didn't notice it. A sure way to damage an airplane on landing is to be oblivious to uncommanded deceleration. It's important to be aware of the nose attitude and constantly cross-check the airspeed.
When an airplane is very far below normal approach speed, it generates very little ground effect, so the hang-time in flare amounts to nothing. In fact, if you're above stall speed by five mph, when you flare, the airplane will just grin a little and squat right onto the runway without floating. When you're performing a short-field landing, you purposely design the approach so that you arrive over the threshold carrying a little power and are indicating stall speed plus around five mph. Then, when the power is slowly bled off and the nose brought up, the airplane plants itself right on your mark.
The one and only time you let the speed get below the recommended approach speed is the aforementioned short-field landing. Even then, short runway or not, if there's a gust spread, you tack on plenty because you'll already be slow and a gust could actually stall the airplane and drop you short of the runway. There's an old bush pilot's adage, "It's better to roll off the other end of the runway at five mph than it is to land 10 feet short."
Running throughout this entire conversation is the assumption that you know what the actual approach speed is supposed to be and are working hard to hold it precisely. When a POH says 65 to 75 kt, the manufacturer is assuming that you know when to be at 65 and when to be at 75. This is not institutionalized approximation. What we are aiming for is intelligent approximation based on the knowledge of what the speed is supposed to be and why. Then you know when to adjust that speed up or down a few knots to achieve a desired effect.
In normal flying, you should do your level best to hold the desired speed plus or minus two knots and shift it up or down, when situations call for it. There are bound to be those who say this is asking too much and it will discourage those who can't hold such a tight margin. The logical answer to that is that there is no such thing as too tight a margin when flying an airplane, and the effort involved in trying to be precise is what makes a pilot improve.
If you're satisfied with approximate, then you'll be an approximate pilot and someday in your career, the ability to hold an exact speed could spell the difference between a safe landing and an accident. Flying is a skill, and our success depends on our ability to do it as correctly and as accurately as possible.
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.
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