You may have heard that a skid during a stall is more dangerous than a slip, and it's true. But, why?
Stall-spin accidents have been a problem since the first days of flight. Most of us are simply taught to keep an aircraft coordinated when stalling. But, the problem is, most stall-spin accidents don't happen during an intentional stall. They usually happen unintentionally and down low - like when you're turning base to final.
Here's a common scenario: You're turning left base to final, but you're going to overshoot the runway. What do you do? Here's what you absolutely shouldn't do: You add left rudder to tighten the turn, but you don't keep the bank and rudder coordinated - putting the airplane into a skid.
What can happen next is pure disaster. The skid causes an over banking tendency, which you counter by adding opposite aileron (often subconsciously). That also pulls the nose down, which you oppose with elevator. Suddenly the aircraft stalls and snaps to the left in an incipient spin. At 700' AGL, you make it through about a turn before you crater into the ground.
OK - that's bad. But why can a skid lead to a spin? The Airplane Flying Handbook offers a little guidance here. It says:
If the airplane is slipping toward the inside of the turn at the time the stall occurs, it tends to roll rapidly toward the outside of the turn as the nose pitches down because the outside wing stalls before the inside wing. If the airplane is skidding toward the outside of the turn, it will have a tendency to roll to the inside of the turn because the inside wing stalls first.
OK, but why does the inside wing drop first in a skidding turn? There are quite a few aerodynamic factors that play out here, but the key principles are actually pretty simple. During a skid, the aircraft is turning too fast for the bank angle, and yaws into the turn. (Most likely, you're pushing too much rudder and causing the skid.) That causes the outside wing to move faster, increasing its lift, and causing the aircraft to roll into the turn. You compensate by adding opposite aileron - increasing the angle of attack on the inside, low wing.
As the inside wing exceeds the critical angle of attack, it stalls and drops. The downward deflected aileron on the low wing is still generating drag, which pulls the aircraft's nose further into the turn. And, the aircraft is still yawing into the turn from the rudder, which accelerates the roll. The result is a quick roll into the turn, and your entry into an incipient spin.
There are a few other factors at play, as well. During a skid, the relative wind isn't coming straight down the airplane's nose, it blows crosswise at an angle from the outside of the turn. That causes the relative wind to flow over the wing at an angle, creating "spanwise" flow - a component of the air flows perpendicular to the wing's leading edge, traveling laterally down the wing.
As you move towards the wingtip, you get more and more spanwise flow. And here's the problem - spanwise flow doesn't generate lift. It effectively reduces the airspeed over that portion of the wing. That causes the wing to stall earlier than normal - so the wing with all of the spanwise flow stalls first.
Finally, the fuselage may block some of the airflow over the wing during a skid, further decreasing the airflow over the inside wing and causing the wing to drop during a stall.
All of these factors play out differently on various aircraft designs - but when combined, they make a skid a deadly condition during a stall.
The Slip
During a slip, the opposite scenario happens. The nose of the aircraft yaws to the outside of the turn, and the aircraft's banked too much for the rate of turn. The outside wing has a higher angle of attack, and you're most likely lowering the aileron on that wing to keep it up.
The outside wing has a higher angle of attack and stalls first, dropping and leveling the aircraft. In fact, the aircraft becomes more coordinated during the stall, because the bank angle is now appropriate for the rate of turn. As opposed to rolling into a spin, an aircraft in a slip rolls towards level flight and away from a spin.
The Traffic Pattern
You'll hear many people say you should limit your traffic pattern bank angle to 30 degrees, and others will say that's dangerous because it can lead to a skidding turn-to-final.
The answer is really simple - don't use rudder to tighten a turn. Limit the bank angle if you want, but simply go-around if you can't make that turn from base to final. A go-around gives you the chance to set up again, and line up the landing like a pro. And who wouldn't mind a little extra time in the logbook, anyway?
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