How can you avoid the stall spin accident? Yaw is as much a culprit in causing these accidents as a stall. We all know that the improper use of rudder is the main cause of yaw. However, there are other causes that every pilot should be aware of. One of the least considered is what is known as adverse yaw or aileron drag. The down aileron causes this when a bank is established. For instance, if a pilot wants to bank to the right, he turns the yoke to the right, causing the left aileron to go down, producing more lift on the left wing. However, in the process of creating this lift, it also creates drag on the left wing and this tends to make the airplane yaw to the left.
The other cause of yaw is generally lumped into what we call torque, although it is actually a combination of factors which tend to yaw, and then roll, the airplane to the left.
Multiengine pilots are aware that the loss of an engine will result in considerable yaw towards the inoperative engine.
But about the spiral? I hope you remember from your student pilot days that lift is used to turn the airplane in a properly coordinated turn. When the airplane is banked, lift is divided into two components: the vertical component, which acts upward and opposes gravity; and the horizontal component, which acts sidewise and is the force that turns the airplane. The important thing to remember here is that the steeper the bank, the greater the horizontal component of lift, which means less vertical component to oppose gravity and keep us in the air.
There are various degrees of spiral, but to keep things simple, let’s just consider that a deep spiral occurs when the bank is so steep that any attempt to increase lift and maintain altitude results in more horizontal lift than vertical lift. Consequently, the nose will tend to drop, but any increase in back pressure will only result in a tighter turn and a greater descent rate.
So what happens in the dreaded stall spin accident in the turn from base to final? In the most often discussed scenario, a pilot overshoots final and in his haste to increase the rate of turn and get realigned with the final approach course, he, assuming a left hand pattern, applies left rudder, which does increase the rate of turn. It also creates yaw to the left. This error is usually caused by the fear of a steep bank low to the ground, when, in truth, a yawing or skidding turn is started, which is much more dangerous. Even if he maintains an indicated approach speed well above the published stall speed, the inside or low wing is retreating from the relative wind and may have a much lower speed than what is indicated. The wing will drop even more, creating a steeper bank.
The pilot is now faced with a dilemma. The nose is dropping, and the bank is increasing. The reaction to this by most pilots is the total opposite of what should be done. Since the nose is dropping, he applies back pressure to raise it, and since the bank is increasing, he applies opposite aileron in an attempt to raise it.
These actions are what lead to disaster. In the attempt to raise the wing, the down aileron produces more drag, and yaw actually increases. At this point the bank is very steep, so the increased back pressure results in an increase in the horizontal component of lift, which only tightens the turn. Because it is dropping in relation to the relative wind, the wing may reach the stall angle of attack or, because of the yaw, it could drop below the stall speed. The steeper the angle of bank or the tighter the turn, the greater the stall speed.
But what if it does not reach the stall speed or stall angle of attack? There would be no spin, right? This is true, but then you would be in a steep spiral close to the ground. It then becomes simply a matter of whether you hit the ground in a spin, which would be at a low speed, but high rate of rotation, or a spiral with a high speed and low rate of rotation. The results are the same. Many of the accidents blamed on stall spins are actually spiral accidents.
So what should the pilot do to avoid this? Above all, avoid the yaw. You can do this by proper use of the rudder. Remember, no yaw, no spin.
However, if the yaw does occur, your first action must be to stop the yaw. Attempting to raise the nose or level the wings before stopping the yaw will only aggravate the problem.