The accident record of overshoots and undershoots makes it clear that some people dont practice maximum performance landings enough, but you dont have to review the statistics to convince yourself its true.
Just sit out where you can watch the touchdown zone at an airport with a relatively long runway. Airplane after airplane will show that the pilot cares less about style points than somehow getting the thing on the ground.
Some will dive in at the speed of blazing heat and float halfway down the runway. Some will come in high and land 2,500 feet from the approach end. In either case, touchdown may be followed by the squeal of tires and a half-mile back taxi to the ramp.
A few will be low and the pilot will try to stretch the glide by pulling the nose up. Thats followed by the roar of power as the pilot realizes that a straight line to the runway now goes through rather than over the power lines.
Once in a while, one will come in on a stable glide path, flaps full down, pitched for a good approach speed, with the wings and nose making quick, smooth adjustments all the way through a gentle touchdown in a tail-low attitude 700 feet from the turnoff, and (like Bob Hoover) make that turnoff without touching the brakes or adding power.
The difference, of course, is precise control of airspeed, attitude and glide path.
The key is proper airspeed control – keeping the right airspeed during the approach and crossing the fence in a stable glide. The rule of thumb is to fly the pattern at about 1.3 to 1.4 times the stall speed. Though many pilots proclaim how important it is to hit the downwind at the right speed, sometimes what they think is the right speed is a bit off the mark. Two things pilots often forget when computing pattern and approach speeds are the difference between indicated and calibrated airspeeds, and the effect of weight on stall speed.
One of the gotchas that nails a lot of folks is the difference between calibrated airspeed and indicated airspeed – something thats particularly important in a Cessna 172. You probably learned that you should fly final at 1.3 Vso, slowing to about 1.15 to 1.2 Vso over the fence.
Check the book values for a 172, however, and youll see a recommended landing airspeed of 60 to 65 knots, while the airspeed indicator shows the stall speed, designated by the bottom of the green arc, at 33 knots. Does that mean the book is making the outrageous statement that you should land at twice Vso?
Look inside the numbers and youll see thats not what youre doing. The 1.3 Vso recommendation is for calibrated airspeed, not indicated. Calibrated airspeed is the true measure of the effect of the dynamic pressure of the airflow over the wing. Look in the 172 manual and youll see that the stall speed of 33 knots indicated corresponds to a calibrated airspeed of 46 knots. If you then multiply that 46 knots by 1.3, youll get a recommended CAS of 59.8 knots on final. Thats back in the range where IAS and CAS are pretty close together, and 60 knots is about as close as I can read to 59.8 on my ASI.
Try flying a 172 at 1.3 times the indicated stall speed of 33 knots and youll be indicating about 42 KIAS on final. Youre way on the backside of the power curve, in a high drag situation much too close to a stall. This means its important to check your airplanes book and make sure you know what CAS is at stall and in what portions of the airspeed range the IAS and CAS are more than a knot or two different.
The second airspeed consideration is the effect of gross weight on stall speed. You may correctly base the approach speed on multiples of Vso but neglect the fact that the approach speeds listed in the book are for maximum gross weight. How often do you land at max gross?
Take a Grumman Cheetah with fuel to the tabs and two people, and fly for two hours. The airplane is now down to about 1,850 pounds, only about 84 percent of its 2,200-pound max gross weight. Since stall speed changes by the square root of the gross weight, the stall speed is now 92 percent of the book value.
The book for a 1976 Cheetah lists the stall at 61 mph. Multiply that by 1.3 and youll get an approach speed of 79 mph. Take into account the reduced weight, however, and youll see that the stall speed is only 56 mph, which yields an approach speed of 73 mph. A change in weight of 350 pounds means a 5 to 6 mph difference in stall speed.
That may not sound like much, but what happens when youre 6 mph fast? Remember that to land properly, you have to slow in the flare until you reach a low enough speed that the proper landing pitch attitude is attained in level flight.
Flaring – essentially decelerating – is a matter of dissipating the airplanes kinetic energy. You have to slow from flare-entry speed to just above stall – say, 50 KCAS in a 172 with a 46 KCAS stall speed. Enter the flare in a 172 at 65 knots instead of 60 and you have 2.25 times as much kinetic energy to dissipate before the airplane will stop flying.
Since the aerodynamic drag available to slow you down hasnt changed significantly, youll float 125 percent farther before touchdown. Thats a lot of opportunity to overcontrol, porpoise or get impatient and try to plant it on. Thats what makes airplanes depart the runway, strike props or suffer bent firewalls. It can also turn your 1,500-foot obstacle landing distance into 2,000 feet, which is one of the reasons airplanes wind up in the trees at the end of the runway.
So the first thing you can do to help yourself make good landings in the smallest spaces is to know the speed at which you really should be, and then be there.
Closely related to airspeed control is pitch attitude control. Although you need to select the proper approach airspeed based on gross weight, pitch attitude doesnt change. The aerodynamics of angle of attack at stall and optimum angle of attack for the approach are beyond our scope here, but those interested should read Aerodynamics for Naval Aviators or any basic aerodynamics text.
The upshot is that, no matter what your gross weight, the pitch attitude of the aircraft in level flight at the proper approach speed will always be the same. Airspeed will vary with the square root of gross weight, but pitch attitude will not.
This means you can use your attitude as a gauge of AOA when youre in a known flight path. Thus, in level flight on downwind, the attitude will be the same if youre flying at the proper speed regardless of your actual weight. Likewise, in a normal 5 to 6 degree visual glide path, your pitch attitude will be the same at the optimum speed for your weight no matter what that weight is on this approach.
Its important to learn what those attitudes are so you can focus your attention where it belongs during landing – outside the plane, not on the instrument panel. There are several reasons for wanting to do this. First, you cant see other traffic with your head in the cockpit. Second, your eyes have trouble accommodating the rapid changes in focus between an airport a mile away and an instrument 18 inches away. Finally, the more time you spend checking your position in relation to the runway and glide path to the touchdown point, the easier it is to pick up changes in those.
The point to be gathered here is that you should intimately know exactly where the nose should be with respect to the horizon at the proper speed for downwind, and proper speed and glide path on final so you can tell if youre fast (nose too low) or slow (nose too high) without looking inside. In a high wing plane, you can also use the angle the underside of the wing makes with the horizon.
I regularly cover the airspeed indicator with my pre-solo students in the pattern. They learn how little they really need it and that losing it wont kill them. The really good ones dont even seem to notice.
Glide path, which many pilots put at the top of their list, is actually one of the last things you need to consider. The fact is that if you have your airspeed and attitude under control, glide path control is easy. If you dont, its very hard and sort of pointless – who cares if you touched down on the numbers if you couldnt stop on the runway?
Good glide path control starts by being abeam the touchdown point, on speed, in the right configuration, and the right distance abeam. If youre off on any of these, youre playing catch-up ball before your team even gets to the plate. You should be very demanding of yourself as you enter the pattern, not letting small deviations from those desired conditions go uncorrected early on.
From this key position, construct in your mind a three-dimensional highway, sort of like an off-ramp from a highway overpass down to an underlying road. Start your imaginary off-ramp from downwind to the touchdown point at the end of the ramp – your desired touchdown point.
Extend the centerline out about 3,000 to 4,000 feet, using the runway as a ruler for your eyes. Thats the point at which you want to roll wings level on final. Picking the spot now allows you to select a landmark like a road, building or tree as a point to aim for. This works better than the touchdown point 45 degrees behind the wing technique because it makes you look at the point where you want to turn final. This makes it easier to spot and correct any drift away from or toward the airport on base, and ensures that you turn final the right distance from the runway.
With that final turn point pegged, start the descent from the abeam position at pattern altitude. Lower gear and flaps, reduce power, and begin a descent to be abeam the final point about 800 agl. Turn base and stabilize your descent on the base leg (adding the next notch of flaps if appropriate) so you hit the final turn about 400 agl, again on speed.
Make any needed drift corrections so your path leads just outside the final turn point (allowing for turn radius), and lead the turn to final so you roll out wings level pointed right at the runway. If youve properly pictured the glide path to the runway and flown yourself to this point, you should roll out on final on speed, ready to add the last bit of flaps, and looking down at the runway at the correct angle.
From that point on, the main key for glide path control is the movement of your desired touchdown point in your vision. If that point is moving up in your view or if you start seeing more grass between you and the end of the runway, you are going to land short. If its moving down or if the nose is moving to cover your touchdown spot, youll be landing long.
Since youre on speed, dont use the nose to help you. Make any corrections either with power or drag. Powers easy if you have it – add or decrease to adjust your descent rate.
Remember that when you add power to decrease your descent rate, you will have to pitch the nose up slightly to avoid accelerating. Youll still be at the same angle of attack, but because your flight path is now shallower, your pitch attitude must be the same amount shallower to keep the same angle between wing and flight path. The reverse is also true – power reduction requires a more nose-down attitude.
You can also use speed/pitch attitude to control glide path without changing power (or in a power-off approach). In this case, you have to know where your approach speed is with respect to your best angle of glide speed. Usually, folks know what this is in the minimum drag configuration, but not in landing configuration where best glide speed is lower with all that extra drag from flaps and perhaps gear.
But in most cases, final approach speed is at or slightly below best glide. Therefore, decreasing your airspeed by pitching up will result in going a shorter distance for the altitude lost. Slowing down will increase your glide angle and land you at a point close to where you are now. Thats why you cant stretch a glide by slowing below optimum approach speed.
On the other hand, lowering the nose a bit will, if youre below maximum glide speed, lengthen your glide and send you farther down the runway. You will also arrive at the flare with more speed to dissipate, which will lengthen the flare and send you farther still toward the ditch at the far end.
Thats why diving for the runway when youre high is futile. The actual effect is generally that slowing lands you shorter, and diving sends you longer – a point somewhat counter-intuitive but potentially quite dangerous if misunderstood.
Resistance Is Not Futile
The other way you can help yourself is by slipping – something of a lost art now that just about all light planes have flaps. You can decrease the amount of lift that is countering gravity, allowing Mother Earth to pull you down more quickly, and increase your total drag, slowing your progress over the ground. Both steepen your descent path.
Slips to a landing are pretty simple – hold the rudder one way, and bank the other. If you need a steeper descent, hold in more rudder and play the bank angle accordingly. Youre doing two things. First, with the wing banked, youve put the wing lift vector off vertical with respect to the ground, reducing its vertical component, which accelerates you downward until a new equilibrium is attained at a higher descent rate. Second, the side of the airplane is turned into the relative wind, and thats a lot of flat plate area. More drag means you can lower the nose (thus lowering angle of attack and thus reducing lift) without increasing airspeed.
Be aware of a couple of points about what your instruments do in slips. First, the airflow is a bit across rather than straight down the pitot tube, reducing the sensed pressure, and causing your airspeed indicator to read low. If you know what your pitch attitude should be, you can stay with it and not chase the airspeed indicator into a fast approach.
In addition, the static source on the side of the fuselage will be either somewhat into the relative wind or in a reduced pressure area away from that relative wind, so the sensed static pressure will be higher or lower than actual. Your instruments show lower airspeed and altitude, or higher airspeed and altitude, respectively. Focus on the visual cues of attitude and visual picture of the runway, and dont let temporarily unreliable instrument readings lure you into wandering off the true path to a good landing.
The last glide path consideration is obstacles. When scoping out a field, make sure your planned touchdown point accounts for trees, terrain, power lines, buildings or any other obstacles at the approach end of the runway. Most public airports have been surveyed for these and displaced thresholds established. When you look in the A/FD before flight, pay attention to obstructions noted for a runway, especially if theres a displaced threshold.
Once on final, its pretty easy to see whats going to work or not. If your touchdown point is obscured by a tree top, you are too low, of course, but also look at the space between the obstacle and the touchdown point. If that space is getting shorter, you are descending at too steep an angle to stay above the obstacle even if youre still on the glide path right now. If the space is getting longer, you are heading above the glide path, and you may land long if you dont increase your descent rate. The relationship between the obstacle and the touchdown point should be pretty stable, with the obstacle heading under your nose so youll pass above it.
Some folks say the best way to approach an obstructed runway is to get low, level off and then cut power as you clear the obstacle. This approach has a couple of flaws. It puts you in a position where you cant see the runway until youre on top of it, which can result in an unpleasant surprise if theres a cow standing on the touchdown point when it comes into view. Or worse, at one of the many one-way airports, you may see an airplane starting its takeoff roll.
You also may find that you dont have a good picture of your drift and alignment until the last minute. It also puts you in the position of having to make major changes in pitch and power very close to landing. Thats a far cry from the stabilized approach, in which you establish speed, power, attitude and configuration as far out on final as practical and ride the rails to the flare point. Making big changes in close is an invitation to make too big or too small a change – and find yourself in a position from which you cannot recover.
The fundamentals of landing apply on every flight, but become even more important when only your best will put the airplane where it needs to be on the ground. The trick to being good at managing airspeed, attitude and glide path is demanding of yourself that you control them all on every approach. That way, when the field is short and obstructed, its just another landing. And maybe youll even impress those other pilots sitting around the shack watching everyone land.
Also With This Article
Click here to view “Indicated Airspeed vs. Calibrated Airspeed.”
-by Ron Levy
Ron Levy, an ATP and CFI, is an assistant chief flight instructor at American Eagle Flight Academy.