so there's margin for roll correction in either direction. The yaw string in that moment, due to turbulence or normal awkwardness, will DANIEL L.JOHNSON wobble back and forth between a slip and a skid, "centered" on the average. But the wing does not care about its average air- flow; it cares about each moment's flow. How to "The middle of the air" (metaphori- cally) is the safe place to be, but the centered yaw string, in a steep turn, is at the border between a skid and a slip, Unintentionally so a (literally) centered yaw string is (metaphorically) "on the edge of the air" vector pointing straight down? (centered Try to stay in the middle of the air. in a slow turn. Humorously, an actually- Do not yo near the edges of it. ball) o o J middle yaw string is metaphorically at • Does it mean drag is balanced be- The edges of the air can be recognized the brink of the cliff. by the appearance of ground, tween the wings? Thus, we should prefer the yaw string buildings, sea, trees, • Does it mean that each wing is to flop between center and our high side, and interstellar space. about equally far from stalling? not crossing the center to the low side. It is much more difficult to fly there. These things are easy to synchronize "Attention," as we explained last - unknown wit when nearly level (up to a 30-degree month, acts through 'built-in brain net- bank). But as the turn becomes steeper, works that can be improved with training hesis: Most gliding fatal accidents it becomes impossible to meet all four of and practice. Instructors use words like occur due to stalls (Rich Carlson, these conditions simultaneously. Most T "protocol" and "discipline" to describe the SSF). Most of these occur close to the important, a centered yaw string no result of successfully training the atten- ground, usually in the turn from base to longer means that each wing is about tion networks. final, but they also occur, surprisingly, equally far from a stall. We must train attention toward the during free flight. In a steep turn, the yaw string and the most important things while turning, What are these pilots paying attention ball become less relevant to safety. The especially while landing. Training done to? What might have been done to avoid position and responsiveness of the con- well teaches the pilot to shift attention this? More to the point, what are pilots trols is more important. If the stick is doing who never stall inadvertently? against the high-side stop (due to over- among various key items during the The principle is simple: don't skid slow banking from differential lift), you have changes of flight, landing, and takeoff, turns, and; he who skiddeth a slow turn no control in the roll axis. This causes and teaches disciplined rigor to this at- stalleth, spinneth, and (quite possibly) anxiety when turning steeply, slowly, in tention protocol. dieth. Note carefully: Unless any turn a narrow, turbulent thermal. To teach, "keep the yaw string cen- is perfectly coordinated, we either are All you need to do is add enough top tered," is a good thing as a general skidding somewhat or slipping some- to balance the drag (reduce the principle, but religious obedience to what. The skid — bottom rudder - tends down-wingtip angle of attack) - this this pedantry is not a good thing, if by to create a spin. The slip - top rudder - restores roll control very nicely, and this we come to believe that it defines "improves" the sink rate, safely. you'll be able to comfortably make con- piloting virtue, or believe that a centered Caveat 1: this and all other discussions tinual steep turns in any or air- yaw string always indicates coordinated of control assume smooth air. In plane. When you do this in an , flight, or that it's most important to keep turbulence, whether in the pattern or in you may be a half-ball off center, and in it centered. a thermal, small turbulent vortices may a glider, the yaw string may drift 10 or 15 This would be inexact. betray your best intentions - a reason to degrees to the top side. • The yaw string indicates the direc- then carry extra speed. Therefore the safe pilot is maintain- tion of airflow in which it lives. Coor- Caveat 2: A discussion of the relation- ing balanced controls, "coordinated" dinated flight centers the yaw string in ship of the yaw string to coordinated or slipping but never skidding. The straight and level flight. In turns, this .flight becomes, frankly, somewhat com- pilot is in danger who is, while turning isn't always precisely true. plicated in a slow, steep turn in turbu- slowly and steeply, cluelessly loyal to • Coordinated flight, in a steep turn, lence, especially with long wings. In the instruction, "always center the yaw is not the safest condition - it's the most fact, what people mean by "coordinated string." It's not, just then, important to elegant condition. If the goal is to stay flight" becomes rather vague: center the yaw string. It's important to "in the middle of the air," midway be- • Does it mean straight flow of air maintain control responsiveness (with tween dangers, a centered yaw string over the canopy? (straight yaw string) speed) and to maintain "coordination" does not keep you there in a steep turn. • Does it mean the net "gravitational" that keeps the stick near the center, • Keeping our attention on centering May 2014 the yaw string distracts us from paying Here's how the perceptual bits work to attention to other much more important take us out of "the middle of the air." details like airspeed, aim point, angle of attack, apparent flight track, other traffic, The rising horizon and obstructions on the runway. This is not as serious an issue in the flat- SOARING Perceptions are crucial. There are three lands, where the effect is pretty subtle. But perceptual features that may delude a when there are hills or mountains nearby, SUPERSTORE landing pilot into flying too slowly or it is serious. While we're above every- skidding a slow turn: thing, we see the real horizon. As soon as • the horizon, we descend below any hump of earth, its • vection, or the sense of speed, and ridge line becomes a false horizon. • the movement of the down wingtip To understand that the ridge is a false across the ground. horizon is easy; to keep track of where Our surround changes during descent the real horizon is located takes actual GOODYEAR - MICHELIN - SPECIALTY in the pattern, creating three mispercep- attention. Look down the valley. Find tions that trick landing pilots: the true horizon. Swivel your head back • The horizon begins to rise. If we and image that horizon line onto the keep the nose at the same point on the mountain. Do this several times while horizon as we did when high, we will you descend - if you don't, you will tend steadily slow down. (Fail!) to raise the nose. OXYGEN SYSTEMS • The ground moves faster and faster. If you don't keep track of the actual We may slow down to make this feel horizon, for example by "transferring" it AEROX - MOUNTAIN HIGH right. (Fail!) to the mountain, your subconscious per- • The down wingtip moves forwards ceptions may cause you to feel as though across the ground. This may cause us to a strong hand is pulling the stick back feel that: while the airspeed decays mysteriously, • We are failing to turn with the over and over again, (which a pilot at the intended small radius. This may inspire Reno convention asked me to explain). AVIO us to step on the inside rudder (Fail!) But why are you paying attention to the • We are moving too fast. This may horizon, anyway? It's airspeed we want, TRIG - BECKER cause us to induce back pressure to slow and we put the nose at -whatever attitude down. (Fail!) keeps the airspeed where it's meant to How is this relevant to the yaw string? be: 1.3 VSO + % the gust velocity (more, Simply this: There's no one thing that if it's really gusty). If you're trying to fly we can fixate our attention to, and remain attitude in the pattern instead o/~manag- safe. The loyal yaw string, the wingtip, ing airspeed, it's time to do some seri- the horizon, the sense of motion: none ous practice to revise unhelpful habits of these is enough. We should also pay of attention. (Correct practice leads to NATIONAL - STRONG attention to the wind noise, the control correct decisions.) responsiveness, and - most important — where we're headed, the apparent and the Vection intended touchdown points (these are Vection is apparent movement, in dis- different). We need to scan these, we need tinction to actual movement. The clas- to integrate the information they provide sical vection illusion is you sitting in a INSTRUMENTS with our knowledge and experience. stopped train, and beginning to move WINTER -UMA If we are watching only the horizon, or smoothly backwards - until you realize the ground under us, or our wingtip, we from the lack of vibration that it's the train on the other track moving forward. I SSSS58 may cause the yaw string to point down FREE toward hell, or cause the airflow to lose There's a similar vection illusion when its affection for our wing, which may take we are up high — it seems as though CATALOG! us there. everything on the ground merely creeps So, for a left turn: past. And a complementary illusion when we are low - the ground seems to Yaw string Yaw string Yaw string race past. Both are "wrong" - neither is left (down) centered right (up) wedded to the . WWW.AIRCRAFTSPRUCE.COM Death, fear Excitement, Life, safety, We may get in trouble because we 1-877-4SPRUCE glory slop, descent spend too much of our time soaring www.ssa.org • May 2014 • Soaring and not enough doing pattern tows. the pivotal altitude, the airplane seems to Don't Center the Yaw String! Our brains then build perceptual pat- accelerate when it may be slowing. Here's the deal. In an imperfect world, terns that don't include the sensations of Ground reference maneuvers are in a steep turn (45-60 degrees), when low-level flight. A reason to go to the totally relevant to flying safely in the going slow, "the middle of the air" is airfield and do pattern work on poor pattern. Whether in gliders or , bounded on one side by the center of the soaring days is to rebuild these low- if we are in a turn as we descend through canopy. If we touch top rudder and the level gestalts, so that our brains easily the pivotal altitude, we experience a yaw string drifts high, we may feel (and shift from the high-altitude to the low- vection illusion as the ground appears to possibly look) sloppy, but we are safe. altitude vection sensations as both being speed by more rapidly as we descend, (And who knows, maybe we could stand normal and expected. even if our ground speed is constant: we to lose a little extra altitude in a slip.) The speed at which the ground seems feel or "observe" acceleration. But if the yaw string drifts to the low to move is also related to the objects - This will cause reflexive slowing, in side of center, we may not only look texture. When we fly from a novel air- response to subconscious processing of sloppy, we may spin, and may look very port, the texture of ground objects may these misperceptions, unless we disci- broken, on the hard ground. be much different than we are used to, pline ourselves to keep glancing at the If we have a ball in the panel, de- and unless we discipline ourselves to be airspeed indicator. pending on the shape of our glider and conscious of this difference, the novel A second vection illusion as we de- the position of the yaw string, we may vection sensations at the new airport scend in a turn through the pivotal alti- discover at times that the ball is about may be grossly deceptive. tude is the sense of a decreasing rate of turn. centered while the yaw string is going The answer to this deception is the Why is this? While above the pivotal adrift. same as for the false horizon: include altitude, in a constant-radius turn we To repeat: all you need to do is add the airspeed indicator in our scan - fly observe the radius to be relatively tight enough top rudder to balance whatever attitude keeps the needle on because the wingtip is creeping back- drag - this restores roll control very the best hash mark for the conditions. wards across the ground. After we pass nicely, and you'll be able to comfortably below the pivotal altitude, we observe control continual steep turns - in other The pivotal altitude the radius to have decreased, the turn to words, to thermal. The pivotal altitude is the altitude at have widened out, because the wingtip is which the apparent wingtip movement now creeping forward across the ground. Stall-Spinning in Free Thermaling across the ground in a turn changes, The combination of these two vection Flight and finding this altitude is the key to illusions - of increased speed and de- This analysis is all well and good close performing ground reference maneuvers creased turn radius, will naturally cause to the ground, but pilots are also stall- well. a strong inclination to step on the in- spinning up high. There aren't the same If you have an airplane rating, you have side rudder to correct what seems like illusions up there. What could possibly had to do ground reference maneuvers, a decreased rate of turn and to raise the be happening? Besides, there's room to classically turns about a point and pylon nose to correct what seems like exces- recover! 8s. These aren't really possible in glid- sive speed (compared to a few seconds We all understand what's happening ers, because we can't sustain a constant before). The false sense of high airspeed aerodynamically - the longer the wings, altitude reliably, especially at the pivotal is especially important flying the down- the steeper the turn, the slower the air- altitude at which these must be flown. wind leg on a windy day, when airspeed speed, the more likely that the down To take another view, the pivotal al- discipline in the turn to base extremely wing will stall. The airplane rotates titude is the altitude at which the down important or a stall-spin can end your faster, the nose drops. wing can be kept on one apparent day in the worst way. Perhaps a problem is that the pilot's ground point. (Y'know, if you've never This combination is all too likely to brain may not be fully in gear. Sitting done ground reference maneuvers, or take us out of "the middle of the air." here in our armchair, it's all so obvious. don't clearly recall doing them, go get It may make the controls feel less re- And when the stall or spin is done in three or four hours of airplane instruc- sponsive (sitting here in our armchair, training, we're trying to make it happen, tion until you can do them comfortably, it's obvious that this is due to mushing, and it's expected. in some wind.) an imminent stall - but in the glider we My own instinctive perception with Above the pivotal altitude, the down are not so prone to disbelieve our senses, an unintended stall-spin is, "Oh, the wingtip seems to move backward over as we are built to give visual cues higher controls stopped working! Is something the ground as we turn; below this alti- priority than tactile ones). broken?" This is because an abrupt stall, tude, the down wingtip seems to sweep Again, the answer is to discipline our- in a turn, usually precipitated by a ther- forward across the ground as we turn. In selves to pay attention to the important mal gust, occurs without a preceding addition, while performing these turns, items: the airspeed indicator, turn coor- change of bank angle, attitude, or air- when we rise above the pivotal altitude, dination, and our aim point for landing. speed. It simply happens. the airplane seems to slow more than it If we take care of these, we will be res- Then there's a distinct thought shift, actually does. When we descend below cued from our own senses. a moment of clarity. "Doh. It's a spin." Soaring May 2014 • www.ssa.org Relax back pressure, center the , our ignorant ambition of having the yaw • During checkout and check rides, top rudder. Level off, turn back to re- string centered), and the controls truly routinely do stall-spin maneuvers dur- center the thermal we just lost. don'fwork for a moment. ing thermaling turns, so they're famil- Second, we're taught to fear stalls, • Two, we might panic. This tends to iar. It's the entry that's important, and a by many influences, such as this essay. cause brain freeze, focusing on "Why prompt recovery, ideally while the spin is If we've got altitude, there's no reason did the fall off?" Which if it incipient. to fear them at all. I've entered many, had, the nose would fall and the stall • During any instruction, the instruc- many inadvertent stalls and incipient end. Yet if the brain is frozen, the result tor should sometimes gradually enter a spins while thermaling (back in the is the same, either way. thermaling stall and say, "your ship" just days when I ignorantly thought the Bill Daniels related to me an accident as it's about to begin. The debriefing yaw string must always be centered). I'd like this, decades ago at Torrey Pines, in should cover recognition, recovery rate, been urgently warned: "Be careful not which a 2-seat glider was seen to stall- and control technique. to spin! Those glass ships really pick up spin from free flight, doing 24 turns; the • Instruction about stalls and spins speed fast!" witnesses near the impact site heard the should be focused on resolving fear. My experience is the opposite. Abrupt, pilot and passenger screaming all the Make the incipient spin feel familiar, yes. Scary, no. Pick up speed fast? Well, way into the ground. so it becomes a normal-seeming part of sorta — and I'm glad, because the airspeed A horrible, needless tragedy like this flying at altitude, and the pilot response quickly ends the embarrassment. After a happens every so often. But there's automatic. while, I came to enjoy these. They don't more: it's not merely the pilot. Spin-avoidance training has its place help the average climb rate any, but in The March-April, 2014, issue of and is important - we don't want ever my glider they are gentle; in a way, a nice Gliding International described a fatal to feel comfortable with an incipient break from the tedious struggle to keep stall-spin in an ASW-27 while therma- spin in the pattern, for example. Nor is a thermal centered for the half-hour'it ling. The subsequent lawsuit involved it particularly important to push along may take to figure out where the top is whether the deceased pilot had been in- to fully developed, multi-turn spins. But located in a narrow, turbulent but lazy, structed that this glider could enter an the incipient spin, in a thermaling turn, midwestern thermal. unrecoverable spin. (This is not the only in turbulent air, is common enough that (And please don't play with that if aircraft that may in some conditions we should train for it as a normal part anyone's sharing the thermal!) enter an unrecoverable spin.) of flight. Again, there are really two problems: The fact that unrecoverable spins can • One, we weren't planning on a stall- occur is a reason to keep the yaw string Acknowledgments spin. We haven't changed the aircraft on the safe side of center, to slip turns Thanks to Bill Daniels and Cindy attitude, the controls simply quit work- a bit to maintain full control authority, Brickner, two of the finest instructors ing. Since a spin wasn't on the agenda, and to practice recovery from incipient in soaring, for reading this in draft and our mind may be completely elsewhere, spins. offering comments. And thanks to Rich and we may reflexively pull back the To reduce the risk of spins from Carlson for his analysis of accidents. stick (the aileron will already be full- thermals, I can think of three things to deflection toward the top wing due to practice:

May 2014 • Soaring