MAY'S "TEST PILOT" SHOWED the versatility and increased safety of using angle of attack (AOA) instead

the power set. (The power setting doesn't matter in this example, only that it does not change.) Point E in of airspeed indications for Figure 1 depicts this condicertain flight conditions. tion for m a n y airplanes, We concentrated on two and in our example Point E i m p o r t a n t cases—maxirepresents a 75-knot apmum engine-out glide and proach speed down a 2-delanding pattern final apgree glide path. proach. For the glide we Leaving the power alone, showed that a single AOA yields able assumption. It would be the if you pull the stick back a bit and your a i r p l a n e ' s m a x i m u m glide case during a forced landing follow- fly slightly slower than 75 knots, range. ing an engine failure. And it makes your flight-path angle becomes shalWe also touted the advantage of sense from the standpoint of the sta- lower or less negative. For example, AOA over airspeed as a f i n a l ap- bilized approach we all work so hard if you held enough back-stick to proach speed reference, but what if to establish—an approach where the maintain 70 knots, your flight path your airplane doesn't have an AOA pilot is not continually correcting would be about 1/2 degree below indicator? How do you know you're judgment errors with avgas. horizontal f l i g h t , as depicted by flying the proper final approach airAssuming constant power allows Point D. Fly slower and (assuming the unspeed? More importantly, do you us to legitimately explore how know what happens to your vertical changing just one variable (airspeed) changed power is sufficient) your flight path when you deviate from affects one other variable (vertical flight-path angle becomes shallower, flight path). We'll explain the effect reaching level f l i g h t at 66 knots the proper speed? A simple flight test answers all power changes have on flight-path (Point C). Slower still, and you'll be three questions. The data reduction stability later; for now the constant at Point B, and in a slight climb. So far your plane is behaving as is straightforward. The math is easy. power assumption will make the iniwe'd expect because it's been flying And the results for your a i r p l a n e tial explanation easier. on the front side of the flight-path may surprise you. Don't think it's Once Around the Curve stability curve, where all airspeeds are worth it? Read on. Flight-path stability describes how Let's start with your typical final ap- faster than the airspeed for the curve's changes in airspeed affect your air- proach: You're on glide slope at the peak at Point B. When operating on plane's vertical flight path (or flight recommended approach speed with the front side of the flight-path stability curve, pull the stick path or glide path or back to fly slower and descent angle). Genershallow your f l i g h t ally it applies to the fipath, and push the nal approach f l i g h t stick f o r w a r d to fly condition where your faster and steepen your wings are level and flight path. you're controlling your Airspeeds to the left glide slope (or descent of Point B are on the angle) by a d j u s t i n g back side of the curve, power and airspeed. where the slower you For now let's asfly the steeper your desume power is conscent angle or f l i g h t stant during your final path is, unless you add approach. This is not power. Here's w h y . an entirely unreason103

Suppose you establish your airplane on final approach at Point A, about 47 knots. Notice that your flightpath angle is the same as if you were flying at Point E, or 75 knots. Remember, the power setting is unchanged, and all the points in Figure 1 represent a single power setting. If you're not aware that you're flying on the back side at Point A and you want to make your flight-path angle shallower, you might pull the stick back and slow down a few knots. I n i t i a l l y , your f l i g h t path would become shallower because you've increased the wing's lift by pulling the stick back. In effect, you'd be trading airspeed for altitude. This result is only a balloon effect. With the increased lift comes increased induced drag. As your airplane stabilizes at its new, slower airspeed, with no change in power the increased drag results in a steeper descent angle than you had at the Point A airspeed. This is the insidious nature of the back side. At this point you have two options for achieving a shallower flight-path angle: You can lower the nose and accept a temporarily steeper descent angle (same balloon effect in reverse) until the speed increases above the Point A value. At your new, faster airspeed, the descent angle is shallower. One problem with this nose-lowering option is that it goes against pilot intuition, especially on final approach, where you don't have a lot of altitude to trade for airspeed. That leaves 1O4

is probably the best idea at this point. Returning to our constant power assumption, we made it so we could see how changing one variable—airspeed—affects the vertical f l i g h t path. If you add power, it shifts the entire flight-path stability curve in Figure 1 upward. Every speed slower than Point B would still be on the back side, and every speed faster would still be on the front side, but the corresponding descent angles would all be shallower. Add enough power, and you might even climb at the Point A airspeed. Reducing power shifts the curve downward. Pull enough power, and Point B would be a descent, just like it would be in an engine-out situation. In short, power changes move the curve up or down, but they don't significantly affect the curve's shape. Shape Matters

your only other option—add power. Either way, completing the approach after wandering the back side would be a salvage effort, and a go-around

Airplanes don't share the same shape flightpath stability curve. Figure 2 shows the curves for two airplanes that have a 75-knot final approach speed. If the pilots of these airplanes pushed their respective sticks forward to establ i s h an 83-knot approach speed, they'd realize markedly different changes in their flightpath angles. Airspeed deviations in Airplane Y result in large changes in descent angle, making airspeed control a significantly more critical task than in Airplane X. Because small airspeed variations in Airplane Y result in comparatively large flightpath changes, its pilot must diligently control airspeed to remain on

you'd also be traveling forward slower. It will take longer to come down, but you won't travel as far as you would have at the faster speed. Let's take this scenario into the cockpit to see why it's an insidious perception deception. You're established on short final at 65 knots, but you see that you're not going to reach the runway. In an effort to stretch your glide, you nudge the stick back just a bit. As the plane's nose comes up, there's a reassuring balloon effect. Even after the balloon, the VSI needle settles onto a smaller descent rate as you continue flying at 55 knots. the desired glide slope. On the other behaved airplanes. The curve in Fig- From the information available to you hand, Pilot Y can make minor, short- ure 3 is smooth and predictable, in the cockpit, it looks like you've term glide slope corrections using too—unless you decelerate more solved the problem, and the VSI has just the stick and not have to adjust, than about 5 knots from the desired stabilized at a lower descent rate. then reset, the throttle. 75-knot approach speed. What you can't see is that your acAirplane X's descent angle is not Notice how sharply Figure 3's de- tions have steepened the airplane's as sensitive to airspeed variations, scent angle increases as the airspeed flight-path angle by 1/2 degree. and to make an effective flight path drops below about 70 knots. Trying Not in your airplane, you say? correction, Pilot X would have to de- to stretch a glide in this airplane can The curves in Figure 4 belong to a viate a lot farther from the proper result in the bottom falling out too Cessna 172. approach speed. The flatness of Air- close to the ground to recover, even It's the flight path angle that deplane X's curve can lead to sloppy with power. termines whether you'll reach the There's another insidious percep- runway or clear the trees into that airspeed control because Pilot X can maintain a near-proper glide slope at tion deception with some airplanes. forced-landing field. Unless your airAirplanes don't come with vertical- plane has a descent angle indicator, a number of different airspeeds. If Pilot X doesn't notice—and flight-path-angle indicators. They you need to understand your aircorrect—an airspeed deviation, he have airspeed and vertical speed in- plane's descent angle sensitivity to or she will likely see one of two out- dicators, and you adjust these to es- airspeed changes, i.e., its flight-path stability characteristics. comes. Airplane X will land hard be- tablish the desired descent angle. Some final approaches give you This month we introduced the cause it doesn't have enough airspeed left for a proper flare. Or it approximate glide slope indications flight-path stability concept, comwill float down the runway in the through VASI and PAPI systems, but pared different flight-path stability flare, dissipating its excess airspeed. at most VFR airports, pilots must vi- characteristics, played a couple of In A i r p l a n e X, you'd most likely sually assess their descent angle. We what-if games, and showed how difmake flight-path corrections by ad- do this by mentally combining the ferent flight-path stability characterjusting power rather than by chang- picture through the windscreen with istics determine your safety margins our flight instruments, but this in- and how your plane's flight-path staing airspeed. bility can affect the way you fly. The reality is most pilots don't fly formation can be deceptive. Figure 4 is a composite plot that Next month we'll present flight-test one-handed approaches. We manipulate the throttle and the stick to shows flight path and descent rate techniques and follow that up with control the vertical flight path. versus airspeed. Notice that slowing the data reduction that will enable Flight-path stability curves indicate from 62 knots to 55 knots results in you to create flight-path stability how much of each you'll have to use a slower descent rate but a steeper curves for your airplane. Please keep those comments and descent angle. This apparent contrato make glide slope adjustments. diction occurs because the change in suggestions coming to Test Pilot, Back Side or Dark Side? true airspeed has a bigger effect on EAA Publications, P.O. Box 3086, Because they are smooth and pre- the flight path than it does on the Oshkosh, WI 54903-3086 or edito dictable, the flight-path stability descent rate. Look at it this way. [email protected] with TEST PILOT as the ££•> curves in Figure 1 and 2 imply well- You'd be coming down slower, but subject of your e-mail. 105