Beyond the burble: once the airplane's wing exceeds its critical angle of attack, you've got to recover. It can be as simple as relaxing back pressure.
We're going to do what?" Okay; I don't usually get quite that reaction when I discuss full aerodynamic stalls during a preflight brief with my students receiving checkouts, or flight reviews in high-performance aircraft. But you can see it in their eyes. Many pilots haven't practiced full stalls since their private pilot checkride, and a large number of my students in 300-horsepower retractable singles--and especially light twins--have never stalled the airplane they currently fly. No wonder there's trepidation about a maneuver some have not flown in many years. No wonder stalls continue to take lift, and life, away.
To reduce the chance a stall might go unrecognized or uncorrected, let's go back to what we learned in our early training, and build upon that knowledge to deal comfortably with what occurs beyond the first stall burble.
LOSS OF LIFT
We're taught early on to watch airspeed to avoid a stall. We recite "the stalling speed" from the pilot's operating handbook as if it is a single, fixed value, or perhaps two values--one for flaps up, one for flaps fully down. Stalling speed, however, varies with airplane weight, sometimes significantly; it changes with g-loading resulting from sharp pull-ups and level, steep turns. Stalls can occur at just about any indicated airspeed. Further, indicated airspeed is only a secondhand indication of when the wing is about to stall. The true value to know is one we can't measure in most airplanes--the wing's angle of attack.
Also, it's a misnomer to say the wing stalls when it "no longer produces lift." A stalled wing may still be producing some lifting force, it's just that the magnitude of lift is insufficient to support the current weight of the airplane.
A typical wing generates greater and greater amounts of lift as its angle of attack (AOA) increases. Of course, AOA is the angle formed between the chordline of the wing and the relative wind into which it flies. Pitch the wing upward to the relative wind and increased lift results. Maximum lift occurs at about 16 to 20 degrees angle of attack for most general aviation wing designs.
Beyond this angle of maximum efficiency, lift generation drops off sharply. Consequently, this is often called the "critical" angle of attack. Most instruction leads us to believe the wing immediately stops generating lift at angles greater than critical, the airplane plummeting to earth uncontrollably until and unless the pilot recovers. This isn't entirely true. If maximum lift occurs at about 18 deg. AOA, beyond which the wing stalls, the pilot still has control and merely needs to reduce the wing's AOA back below critical angle to recover.
I frequently see insufficient knowledge of AOA among rated pilots during my flight instruction in high-performance airplanes. Most pilots I fly with have not practiced stalls in such airplanes; the only memory they have of stall recovery comes from flight in low-power trainers with draggy struts. When they experienced stalls, it took a fair push forward on the yoke to both reduce AOA and overcome drag in order to recover into a climb.
In cleaner, higher horsepower airplanes, it's really only necessary to establish a normal pitch attitude and add power to recover--force the nose below the horizon as you're adding power and you'll only accelerate toward the ground in a power dive. With just a little practice now and then, any pilot can be proficient in recovery with a minimal loss of altitude; it may be simply a matter of relaxing back pressure instead of pushing hard to get angle of attack back toward maximum. In most cases, you can recover into a climb with almost no loss of altitude at all. Done promptly, you can do this same thing in a low-powered, strut-braced trainer. Level the wings, establish climb pitch and power, and it'll "resume flying"--to use a common misnomer--with very little altitude loss.
BY ANY OTHER NAME
Sometimes the combination of airplane weight, center of gravity, and other conditions cause a stall wherein the nose does not drop, but instead the airplane "mushes" downward without a decided stall break. You should treat any unexpected descent as a stall, and use pitch and power to recover.
But what about control? An aerodynamic stall induces a downward pitching motion in most airplanes--built-in stability naturally makes the airplane try to reduce angle of attack on its own. But a slight uncoordinated condition, and even the torque of propellers in perfectly rudder-coordinated flight, will induce roll and, to a lesser extent, yaw as well.
Our trained-in reaction to roll is to correct with aileron. Normally this isn't a problem, but when the wing is stalled, roll is very sensitive to minor variations in lift and drag on one wing relative to the other. If the airplane is rolling to the left (a common result of propeller forces in a stall) we naturally want to apply right aileron to raise the dropped wing. In response, the right aileron deflects upward, further spoiling lift on that side. At the same time, the left aileron deflects down, adding drag. The net result may cause the airplane to roll even faster, or to deepen the stall on one wing.
The correct response to stall-induced roll (and yaw) is to "pick up the wing" with rudder. If the left wing drops, level the wings with a healthy application of right rudder, and vice versa. Once the wing is level, reduce rudder input as needed to maintain rudder coordination. Unless there's something in the pilot's operating handbook for the airplane you're flying that specifically recommends using aileron to level the wings in stall, it's best to assume you should hold the ailerons neutral until you recover to keep from aggravating the stall.
BANK AND STALLS
In certain situations, wing bank angle can alter angle of attack, and therefore affect stalls. In a level turn, as the bank angle increases, so does the stalling speed. Banking 60 deg. while maintaining level flight causes a 2G load factor and a roughly 40 percent increase in stalling speed. This is because pulling an increased G-load increases the angle of attack as the airplane pitches against the relative wind--the airplane is trying to turn faster than inertia will let it, so instead of hitting the wing nearly head-on, the relative wind is approaching from an angle "below" the wing's chordline.
We're often taught to keep bank angle shallow in the traffic pattern because of the increase in stall speed that results from bank angle. This isn't entirely correct, however. Unless the pilot attempts to hold altitude in a bank, the G-load does not increase--hold a constant-airspeed descent in a bank and the G-load, and therefore the stalling speed, is unchanged. If you're flying at the recommended 1.3 times Vso, you'll have to pull nearly 2G to stall the wing. Combine too low a speed with pulling against the descent in a bank to extend the glide, however, and even a slight increase in G-load will narrow the margin between where you are and where the wing will stall.
There's another hazard of excessive bank in the traffic pattern or in the visual phase of a circling instrument approach, that of the incipient spiral.
WHERE PILOTS STALL
This raises the question: How and why do stall mishaps occur? Accidents resulting from stalls happen in these situations:
* Turn to final, and final approach: This is the stall we usually train for, the power-off or approach-to-landing stall. It's not the stall that happens most frequently in accidents (maybe because it's the one we train the most). Usually aggravated by rudder coordination issues while trying to compensate for overshooting the extended runway centerline on the turn to final, the approach-to-landing stall is prevented with practiced rudder coordination and smooth Sturns, if needed, to align with the runway. Remember: As long as you have power, a go-around is always an option (unless you're at the rare, one-way backcountry airport).
* Departure, go-around or missed approach: We try to simulate a power-on, departure stall in training and checkrides, but at typical training weights it's hard to get an airplane to stall at realistic pitch attitudes. The Practical Test Standards acknowledge this, and call for demonstration at reduced power settings to make the stall occur sooner. Trouble is, load an airplane so its weight is greater than usual for training and the center of gravity is further aft, and you set yourself up for a stall unlike any you've ever seen. Perhaps this is why despite all our concern for stalls on landing, it's stalling on takeoff, go-around or missed approach that historically is the more frequent killer. Aft CG increases angle of attack while reducing aircraft stability, so the wing is more likely to stall. Too, control is less precise; higher weight means the wing needs to work harder to overcome gravity, so stall speed is higher. How can you prepare for recognizing and recovering from departure stalls? With an instructor highly experienced in your airplane type, secure water jugs or other ballast in the back of the airplane to arrive at a center of gravity carefully calculated to be inside, but near the aft limit, of the CG envelope. Then cautiously practice power-on stalls at a safe altitude.
* Density altitude: Whether on approach or departure, there is a correlation between high density altitude (DA) and stalls. Density altitude affects airplane performance, but everything performs "by the book" if the indicated airspeeds are flown. The issue with density altitude is more likely that conditions will not permit a safe margin of performance under existing conditions, and the pilot tries to "force" the airplane to fly at high angles of attack to compensate.
Avoid density altitude stalls by flying at the lowest safe weight, calculating aircraft performance under existing conditions, and delaying or diverting when high DA erodes safe operating margins.
* Trim: Most training airplanes take off and land at about the same elevator trim setting. Larger airplanes, however, often have radically different trim settings between takeoff and landing. This is most detrimental, in terms of stalls, in nose-heavy aircraft if the pilot trims off all the pressure on final approach. If the situation calls for a go-around or missed approach, or if the pilot lands and forgets to re-trim the airplane for the next takeoff, applying power can cause the nose to pitch up excessively and contribute to a departure stall. It may take significant forward pressure during a go-around to keep the nose in a climb attitude.
Train to recognize and avoid this scenario by practicing stall recoveries with an experienced instructor, with the airplane trimmed where you'd normally have it on short final. Be religious about a Before Takeoff check of trim position even if practicing multiple takeoffs and landings on a single flight. And consider using a little less trim on final approach, overcoming some of the forces manually so there's less of a nose-up tendency if you need to power up and climb away.
* Engine failures: If there's a scenario where we see stalls more than anywhere else, it's accompanying an engine failure. If an engine quits on climbout the airplane will begin to follow a parabolic path and, if the pilot resists descent, the angle of attack increases phenomenally fast.
Gliding in to land, your rate of descent will be higher than usual and the angle of attack at a "normal" pitch attitude will be greater than when at even idle power. Train for engine failure attitudes by experiencing glide (or in twins, single-engine flight) with an eye toward memorizing the sight picture out the windscreen and the pitch attitude on the panel that results in the appropriate engine-out speed.
* Frost and airframe ice: According to the FAA, "as little as 0.8 millimeter of ice on the upper wing surface increases drag and reduces aircraft lift by 25 percent." All built-in stall warning systems go out the window when a wing is contaminated. Worse yet, ice does not always build symmetrically, so one wing may be likely to stall before the other, and all at unpredictable speeds. There's no way to train for stall avoidance with frost or ice on the wings; your only option is to remove all contamination before attempting takeoff, and to use deicing equipment to clean off wings in flight before attempting to land.
Pass your sport, recreational or private pilot checkride, and you may never have to practice a "full" stall again. Many of the scenarios that cause real accidents are difficult to reproduce accurately in an airplane. It's important, then, to include stall recognition and recovery in your flight review and other regular training, and to consider how real-world conditions are affecting the wing.
Try to visualize how the relative wind is meeting the wing in all phases of flight, but especially at slower speeds or when increasing G-load. Be ready with rudder, pitch and power if you unexpectedly feel the effects of excessive angle of attack.
Angle Of Attack
The generic graph above is used to demonstrate how much difference a slight relaxation of back pressure on the yoke or stick can make as the critical angle of attack for a given wing is exceeded Consider Point A on each curve. The wing remains flying at this AOA, and generates close to its peak lift coefficient. Meanwhile, at Point B, the wing is stalled, or nearly so. When flying at Point B, reducing the AOA slightly to Point A is all that's needed for the wing to "resume flying."
RECOVERY--ANGLE, CONTROL AND CONFIGURATION
To recover from a stall:
* Simultaneously use rudder to level the wings and elevator to lower the angle of attack.
* Keep the ailerons neutral unless your airplane handbook specifically recommends using aileron in stall recovery.
* Establish climb attitude, power and (slowly, as needed) configuration.
Much has been written in the last few years about a fairly recently identified condition, the tailplane stall. Ice contamination on an airplane's horizontal tail can cause it to stall before the wing itself, usually resulting in a violent nose-down pitch. Tailplane stalls are most likely when flaps are extended near the high end of their operating speed range--this is why identified cases of tailplane stall usually happen near the outer marker of an ILS approach, because that's where pilots often extend flaps. The real trick of tailplane stalls is that the correct recovery technique is exactly opposite that of a wing stall--to recover from a tailplane stall, the pilot needs to reduce power and pull back on the elevator controls.
When Continental Express 3407 crashed at Buffalo, New York, in February 2009, tailplane icing was considered a possible factor, initially because the crew had been discussing ice accumulation, and the crash occurred just after they extended flaps for landing. Subsequent flight recorder data showing the pilot reduced power and pulled back abruptly when the airplane departed controlled flight seemed to validate this assumption.
Although as of this writing the investigation is far from complete, it appears the NTSB has ruled out tailplane ice as a significant factor in this tragedy. It may be possible, however, that the "emergency de jour" nature of tailplane icing in pilot training literature may have at least contributed to the crew's response when things began to go terribly wrong. The lesson, among many other things: Don't use flaps when landing an ice-contaminated airplane. This dramatically lessens the chance of a tailplane stall, and drives away the notion of pulling power and pulling the controls if you notice the indications of a stall.
ANGLE OF ATTACK
If stall avoidance is all about angle of attack, not airspeed, why don't we have angle of attack gauges in our airplanes? Frankly, that's a good question. Naval aviators literally live and die by cockpit angle of attack indicators. Many business jets and large turbine aircraft have them. But with the exception of some aftermarket "Lift Reserve Indicators" or other AOA devices, angle of attack indication hasn't made inroads to most of the general aviation market.
It would be easy enough to simply keep the AOA in the green on takeoff and approach, and peg it to a specific value for maximum-performance, short-field maneuvers. Sadly, AOA indicators haven't proven their worth to most pilots and airplane manufacturers, and we're stuck using indirect airspeed references for stall avoidance.
FAA PRACTICAL TESTS: WHAT IS A STALL?
The FAA's view of demonstrating stall recognition and recovery differs, depending on the level of pilot certificate sought. The Practical Test Standards (PTS) for the private, recreational and sport certificates call for configuring the airplane for a power-off or power-on stall, slowing until inducing an actual stall, and recovering promptly "after the stall occurs." The commercial pilot PTS has a subtle but important distinction: It calls for demonstrating stalls with recovery promptly" as the stall occurs." This is usually interpreted as recovery at the first aerodynamic indication of a stall, or the onset of stall buffet. The airline transport PTS requires demonstration of "approaches to stalls," with recovery begun "at the first indication of an impending stall," be that "buffeting, stick shaker, decay of control effectiveness [or] any other cues." Hearing a stall warning horn typically set to go off at an angle of attack five to seven knots above stall speed is enough to command recovery for the ATP candidate.
The goal of stall training and evaluation is to recognize when an aircraft begins to stall unexpectedly, and recover quickly and correctly. The progressively earlier recovery of checkride stalls as a pilot progresses through advanced certificates assumes the pilot has a growing knowledge that prompts recovery with a greater margin above the stall. The unintended result, however, is that a pilot might demonstrate acceptable stall recognition and recovery once, for an initial pilot certificate, and never again take an airplane past the first aerodynamic indication of a stall or, for that matter, past sounding of the stall warning horn. It's a good philosophy for recovery before the airplane begins to stall, but the lessons of aircraft control once the wing actually stalls go unrepeated for a pilot who might unexpectedly find him- or herself in the nether regions of lift.
Tom Turner is a CFII-MEI who frequently writes and lectures on aviation safety.
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|Title Annotation:||STICK AND RUDDER|
|Author:||Turner, Thomas P.|
|Article Type:||Cover story|
|Date:||Sep 1, 2009|
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