QUESTION: Hello, nice to meet you and I'm sorry for disturbing!
I would like some help from someone with a great experience like you. I'm a private pilot, heading towards my commercial license and I realized that I have some doubts regarding some flight regimes which I don't master on my own. I will try to summarize them below, I hope you could follow and help me.
The first issue I have is about stall and spin in bad weather/turbulence. For example, if suddenly the 15 kts headwind shifts its direction and becomes a tailwind, I will lose 30 kts of airspeed, right? If so, I don't get it if the result would be a stall or just losing lift and momentary have an increased rate of descent. Actually, I can't figure out on my own if the AOA would change even if the pilot maintains the same attitude during the windshear event.
The same problem for updrafts/downdrafts. As far as I know, these gusts momentary change the AOA and thus it would results a stall, right? What if hypothetically just one wing is affected by the gust? The result would be a spin? Or more likely a snap roll? All these scenarios are in the low and slowflight regimes i.e. takeoff/landing, because I guess this would be the most probable scenario to be problematical.
The next problem is about crosswind landings. We use to make stall landings in small airplanes, so, considering this and a kicking-the-crab in flare technique, you are uncoordinated close to stall when you kick the crab and use opposite aileron to prevent banking due to rudder use. How safe is this?
Also regarding stall, is there any difference between an accelerated stall and a dynamic one?
Thanks so much!
ANSWER: Thank you for your questions, the best pilots are those who are curious. I'll try to answer your questions in order. Yes, if your 15 kt headwind instantaneously reverses, you'll lose 30 kts of airspeed until the airplane's thrust builds the airspeed back up. Such changes in wind direction are almost always more gradual, and it's likely that you'll never encounter a situation like you described. If it does happen, the airplane will descend because the wings will produce less lift at the slower airspeed. If you keep the same attitude while in a descent, the AOA could decrease enough to induce a stall. In reality, you would see the decrease in the airspeed indicator and the descent in the altimeter and vertical speed indicator, and lower the nose to prevent a stall.
Your second scenario is a much more likely occurrence; updrafts and downdrafts are often sudden. In instrument conditions in turbulent air, you need to be particularly attentive to your airspeed so you can prevent a stall caused by gusts. With experience, you'll learn to feel these conditions and take the needed actions even if you don't see the indications on your instruments. The likelihood of a gust affecting only one wing is extremely small; it's another situation you'll never encounter. Theoretically, it would cause a spin or snap roll. Remember that a spin and a snap roll are exactly the same thing; we call it a spin if it occurs at low airspeed and the nose points down, and we call it a snap roll if it happens at a higher speed and the airplane rotates on essentially a horizontal flight path.
The crosswind landing technique you were taught is safe. You're flying down the runway at low airspeed, banked into the wind, with the nose pointed straight down the runway. That condition doesn't greatly increase the stall speed, and even if the airplane does stall, if you've done it properly, you're right above the runway surface, just like in a normal landing.
Finally, the types of stalls. Most pilots understand accelerated stalls; they occur when the aircraft is under a g-load greater than one, and thus the stalling speed is increased. They occur most often when turning and/or climbing. A dynamic stall is an entirely different concept; it involves rapid changes in AOA of an airfoil. It is most often associated with helicopters, as the spinning rotor creates a condition of changing AOA.
I hope this has been helpful, please feel free to ask a follow-up question if my explanations haven't been adequate.
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QUESTION: Thanks so much for your precise answers!
Regarding the crosswind landing technique, I do not know if I expressed myself clear, I was referring to crabbing technique, not sideslip, actually in more detail, I thought about kicking out the crab and when you do this you are somehow uncoordinated and not in a slip cause you are wings level and using rudder and opposite aileron, maybe I'm wrong here and this is also a slip and it is not risky to do it.
Sorry for misunderstanding your question. You are correct, when you transition from the crab ("kick out") you are briefly in an uncoordinated flight situation. Remember that stalls are caused by angle of attack, not by airspeed. Therefore, as long as you keep your wings level (by using a little bit of opposite aileron) the angle of attack of both wings will be the same, even though their airspeeds are momentarily different. As long as your approach and landing speeds are correct, you won't have to worry about stalling the airplane before you're ready to finish flying. If you have a long runway, it may make you more comfortable to add a few knots to your approach speed on crosswind or gusty landings. I hope this answers your question, and thank you for asking a follow-up question rather than settling for an incomplete answer.