Features  / 06.13 /

Stall+yaw=spin


It’s probable that no single subject in aviation has been discussed more than stall/spin accidents and how to prevent them. That’s understandable, considering that stall/spin accidents still account for nearly 15 percent of general aviation fatalities. However, the stall/spin accident is easily the most preventable in aviation. Although they are embarrassingly easy to avoid, pilots keep making the same two basic mistakes that no properly trained pilot should make.

Stall+yaw=spin

The first mistake is that pilots don’t keep visual track of what the aircraft’s nose is doing and let airspeed decay. The second mistake is that pilots don’t keep the turn coordinator’s ball in the middle. Seems simple, doesn’t it? Let’s look at these two factors separately, then figure out how to make tight control of them a part of our aeronautical thought patterns.

The stall. First is the “stall” part of a stall/spin accident. Why do stalls happen so often? Considering that stall recognition and recovery is a major part of everyone’s flight training, there has to be something about the way that we’re teaching or practicing stalls that doesn’t apply to spin prevention.

And there is. The way we teach and/or practice stalls is not at all realistic in terms of the way stalls actually happen. When most of us think of a stall, our mind’s eye sees the windshield full of blue and we’re tugging the yoke to our chest. But that’s not a real-world scenario. No one accidentally stalls an airplane with the nose blanketing the horizon and the yoke full aft. They stall it while making a turn onto final when a number of factors combine to make the stall occur in an entirely different way.

In a real-world stall situation the nose isn’t pointing noticeably up because the airplane is in a glide and probably has at least half flaps deployed. In most aircraft—especially Cessnas, with their wonderfully effective Fowler flaps—when the flaps are down, the nose is down, too. The flaps are generating enough lift and drag that the nose has to be well down to maintain the approach speed specified by the pilot’s operating handbook. The more the flaps are extended, the further down the nose must be—so a stall can be induced with the nose level or even slightly down.

The next time you’re in the practice area with plenty of altitude, slow the airplane to approach speed in a power-off glide. Note where the nose is in relationship to the horizon. Then, put the flaps down and notice how much farther down the nose has to be to maintain approach speed. Now, very gradually, bring the nose up just a few degrees—don’t yank it up. Wait a few seconds for it to slow down and stabilize, then do it again. Very slowly, keep the nose coming up. Nothing dramatic, just a little too much back-pressure, moving it a little at a time. Also, notice that in this kind of gentle scenario, once the airplane starts to decelerate, it seems to get progressively easier to make it decelerate even more.

Keep increasing back-pressure and notice how the nose isn’t really coming up much at all. The speed, however, keeps going down. In all probability, you’ll get a stall buffet before the nose comes up to the horizon. Now, if you’re feeling adventurous, make a coordinated roll (rudder and aileron) into a 30-degree bank, and add back-pressure to hold the nose attitude. Notice how, in that condition, most airplanes quietly decide to quit flying. It happens very gently without a lot of fanfare.

The point to this rather benign exercise is to see that in the landing configuration, it is easy to creep up on the stall and still have the airplane in what appears to be a more-or-less “normal” attitude—which is to say, it’s nearly level. Because of the high-angle manner in which we usually practice stalls, we’ve become so used to stalls being associated with extreme attitudes that we don’t recognize when we’re about to stall in what appears to be a very normal nose attitude.

The way to keep these sneaky little stalls from happening is twofold: First, develop a conscious habit of monitoring the relationship of the nose to the horizon in a normal approach situation, which is flaps down and power off (if in a power-off approach). Each time we change the configuration and the power setting in the approach, we should note how much space there is between the nose and the horizon; it will vary with each speed and configuration change. Then, once stabilized on a speed, resolve not to let that nose attitude change; further fine-tune it by making the airspeed indicator part of the visual scan.

A never-ending scan is vitally important on approach: Our eyes will be constantly on the move, taking in a series of factors all at the same time. These include ground track; glideslope, as represented by the position and trend of the runway numbers in the windshield (up, down, or stationary); the attitude of the nose in relationship to the horizon; the airspeed; the power setting (assuming a power approach); and finally, monitoring the pressure on our rear end (left or right) and what the skid ball is saying about what we are feeling. The primary focus is in the windshield, because speed changes happen there first, as do glideslope changes.

Stall+yaw=spin

Focusing on yaw. So, if we eliminate the “stall” part of stall/spin, we’ve essentially eliminated the spin, and only the yaw factor remains to be mastered. Yaw, as represented by what the skid ball is telling us, means that the airplane is crooked. In a glide, a part of that yaw will be nothing more than the ever-present P-factor (assuming your airplane has a propeller). So, while power-off, a little left rudder (in U.S. airplanes) usually is called for to keep the ball centered. But that’s not a big problem. What can be a big problem is that people forget the ball can be moved around by the ailerons just as easily as it can by the rudders.

When in flight in the practice area, step on the rudder. Predictably, the ball runs away from the foot that’s down. Now, with feet flat on the floor, crank in a bunch of aileron and you’ll notice you can push the ball off center with aileron just as easily as you can the rudder. Then, slow down the airplane to approach speed and repeat the last two movements.

Note that when you’re slower and you step on the rudder, the airplane yaws more or less the same as before—but the ailerons seem to be able to move the ball much more easily at approach speed than in cruise. That’s because at the higher angles of attack associated with a slower speed, the adverse yaw is greater. It takes a lot of yaw to spin an airplane, and it is the unknowing crossed-up application of both rudder and aileron that can generate much more yaw than either control can generate by itself. And that’s what happens in a stall/spin accident. Both controls contribute to the required yaw.

Set-up for a stall/spin. Stall/spin accidents
generally happen when the airplane is on base and the pilot is a little late initiating the base-to-final turn. Or the wind is behind him, pushing him. Either way, the pilot overshoots final and cranks in a lot of bank in an effort to make the turn. Things are OK to this point.

Then a light bulb goes on in the pilot’s head that says he shouldn’t have a high bank angle that close to the ground. So, he tries to hold a moderate bank angle, but finds it’s not enough to make it around the corner onto final. As a result, the pilot applies a little inside rudder to encourage the nose around onto final. But as he does that, the bank tries to increase. In fact, to hold the bank stable, the pilot must apply aileron to the outside of the turn. So there he is with left (inside) rudder and right (outside) aileron, in a fairly steep bank.

He’s now in a bad situation. The rudder he’s holding to the inside of the turn is driving the ball to the right (assuming a left turn). The aileron he is holding opposite to the turn, to keep the bank from increasing, also is pushing the ball to the outside of the turn. The yaw from either the rudder or the aileron isn’t great, but when they are combined, the ball is all too happy to slide almost all the way to the end of the tube. Because this makes for a very dirty configuration with a lot of drag, the airplane is all too happy to slow down, if asked.

In addition to all of this, the displaced ailerons are altering the lift on the individual wings. The airplane is being dragged through a left turn by left rudder and the bank angle, while the outside ailerons are trying to lift the left wing and lower the right wing. The left aileron is down, asking that wing to generate more lift. Hold that thought while we talk about the nose attitude.

Our friend has his airplane in a tight left bank with the ball solidly glued to the right end of the tube. The bank angle has raised his stall speed and he makes the situation much worse by not noticing that he’s pulled the nose up a little and his airspeed is deteriorating.

At this point the yaw isn’t really hurting him, but the second the airspeed drops to a critical point, the inside wing will stall first because of the down aileron—then the bank suddenly increases as the airplane tries to roll over the top. At the same time, the yaw forces the airplane into the beginning of a spin. Right there, if the pilot immediately lowers the nose to break the stall and applies opposite rudder, he’ll at least avert the spin and may even have enough altitude to recover from the stall. However, if he hesitates a nanosecond, the stalled airplane will have rolled into a nearly vertical nose-down attitude. When you’re in the traffic pattern at about 500 feet, the statistics are not encouraging. If he’s even a second late in releasing the back-pressure and applying opposite rudder, the statistics will tally another accident.

A student doesn’t need spin training to avoid a spin. All he needs are good, basic flight techniques. With those, he’ll never accidentally spin an airplane. An easier, guaranteed fix to the base-to-final stall/spin is to make a firm decision that any time we overshoot the turn to final, we just go around and avoid the temptation to force what we’ve shown is a dangerously tight turn.

As with everything else in life, it’s often the simple things that get you. Remember these simple cures for this not-so-simple problem.

Budd Davisson is an aviation writer/photographer and magazine editor. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his website.

Accidents can happen

Practicing various types of stalls at altitude can sometimes seem disconnected from the realities of flying, which is often the case. Searching through the accident record, it’s easy to find any number of different scenarios that result in stalls and spins close to the ground.

In February 2012 a Cirrus SR22 crashed while on approach to Melbourne International Airport, Florida. The pilot had intended to land long on the 10,000-foot-plus runway, and came in tight on a right base. The controller called out traffic for the first time on a short final. He then instructed the Cirrus pilot to “extend to follow the [airplane] out there on a mile final, cut it in tight now, cut it in tight for nine right.” Witnesses saw the airplane pitch up abruptly, then drop a wing, roll inverted, and hit the ground.

The abrupt pitch up, followed by the wing dropping, indicates that the pilot put the airplane into an accelerated stall. Most pilots don’t learn to practice and recover from accelerated stalls, instead limiting their exposure to approach and departure stalls. Accelerated stalls demonstrate that airplanes can stall at any airspeed and any attitude. By simply pulling up too hard while flying at a normal approach speed, the pilot exceeded the critical angle of attack and stalled. Avoid this by making smooth, coordinated control movements.

A TBM 700 that crashed on July 15, 2008, exhibited more classic stall/spin characteristics close to the ground. The airplane was on a long final approach when air traffic control asked the pilot to execute S-turns for spacing. The pilot did so, each time getting progressively slower. Finally, he got the airplane too slow, and—in a bank where the stall speed increases—stalled the big turboprop. By keeping the nose down or the power up, he could have avoided the situation.

ATC was involved in both cases, which leads to a great lesson—above all, fly the airplane. If you don’t feel 100 percent comfortable following a direction from ATC, don’t. You are the pilot in command, and it’s your job to keep the wings flying.—Ian J. Twombly


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