Stalls in turbulence

[SteveJeff]

1. Let’s say I took off, climbing at 65kts and there is a 15 kts headwind. If at 100ft, SUDDENLY the wind direction changes and becomes a 15 kts tailwind, my airspeed will suddenly drop to 65-30=35 kts, right?

2. On final, I encounter an updraft, it increases the AOA, it induces 2G, the airplane stalls even flying at approach speed? Smth like an accelerated stall, right?

 

[JordanD]

1. I'm not sure you'd have a 30 knot loss, but this is possible, it's the danger of wind shear. The other danger being that if the opposite happens, and the airspeed increases, a lot of pilots will instinctively slow down not realizing they're going to lose all that speed they just gained when the headwind goes away or shifts to a tailwind and then some since they slowed the airplane down on top of that. 2. This is possible. This is the reason why in very turbulent air it's wise to slow the aircraft below maneuvering speed, so the wing will stall before it has a chance to pull too many Gs and overload itself, potentially causing structural damage or failure.

 

[SteveJeff]

Ok, thanks!

Regarding question 1, I hope someone here can clarify the actual loss of airspeed in that scenario.

In question 2, unfortunately it seems that my concerns are true, I guess a strong gust on low final might spoil the day if you get a wing drop/incipient spin.

 

[drunkenbeagle]

Ok, thanks!

Regarding question 1, I hope someone here can clarify the actual loss of airspeed in that scenario.

The instantaneous loss of airspeed will be 30 kts, correct. However, assuming you make no trim or control changes, there is no risk of stalling. There will be a loss of lift and your vertical speed with drop for a bit, and then quickly accelerate to what the aircraft was trimmed for.

In question 2, unfortunately it seems that my concerns are true, I guess a strong gust on low final might spoil the day if you get a wing drop/incipient spin.

An updraft is going to make your descent rate less, that's it. May cause a bank if only one wing is in it. Just remember - downdrafts are generally found right next to updrafts (whatever you gain, you are going to lose in a few seconds)

 

[SteveJeff]

The instantaneous loss of airspeed will be 30 kts, correct. However, assuming you make no trim or control changes, there is no risk of stalling. There will be a loss of lift and your vertical speed with drop for a bit, and then quickly accelerate to what the aircraft was trimmed for.

I thought the same, however, it was somehow confusing because I though at the same time your AOA may change due to the fact that the vertical speed drops and the flightpath will be a little more abrupt, so thus AOA change, resulting in an AOA increase even, of course if you maintain the same attitude during the whole event and you usually do at the best, even if some ppl tend to pull the yoke and so making it worse than it is.

An updraft is going to make your descent rate less, that's it. May cause a bank if only one wing is in it. Just remember - downdrafts are generally found right next to updrafts (whatever you gain, you are going to lose in a few seconds)

What confuses me here is: we know any updraft/downdraft modifies your AOA. An updraft usually increases it. So, on final, beyng low and slow, high AOA, a strong updraft might stall you. Put that on an airplane which has the tendency to drop hard a wing at stall and it seems the perfect scenario for an incipient spin at low altitude. I may be very wrong here, so that's why I'm asking.

Thanks so much and I really appreciate your help!

 

[drunkenbeagle]

What confuses me here is: we know any updraft/downdraft modifies your AOA. An updraft usually increases it. So, on final, beyng low and slow, high AOA, a strong updraft might stall you. Put that on an airplane which has the tendency to drop hard a wing at stall and it seems the perfect scenario for an incipient spin at low altitude. I may be very wrong here, so that's why I'm asking.

An updraft is not going to make a material change to AoA. A typical strong updraft associated with convective heating and an unstable atmosphere is about 6 to 8 kts. Your climb will increase by that much while in the updraft, but you will continue to fly through the updraft at essentially the same airspeed and AoA. If you could find an updraft so strong that it would disrupt airflow over the wing that much, the altitude gain will make the stall irrelevant anyway.

The thermal updrafts causing turbulence tend to have smaller cores the stronger they are though, so you are likely to enter and exit the core before the impact on A/C performance makes much difference. Under those conditions, altitude control will be very difficult though.

The two places where updraft and downdrafts are most severe are the leeward side of a mountain in high winds, and in a thunderstorm. In both cases, the downdrafts will easily exceed the climb performance of most aircraft.

 

[SteveJeff]

http://flysafe.raa.asn.au/safety/wind_shear.html At paragraph Vertical gust shear effects there is a table and the result is that a gust about 10 knots makes your AOA critical.

Actually I'm more concerned about stall behaviour in those situations, not really about height loss, that is I'm thinking about having a wingdrop, nose drop and thus an attitude hard to recover being low and slow. What do you think?

 

[shdw]

What do you think?

You're worrying about it too much. Question one, yes you'd lose 30 knots. You're assuming an instantaneous 180 degree shift with a steady wind, however. I think you'd be hard pressed to find this except in some pretty extreme conditions. Even if you did, you're in a climb. Just pitch nose down.

Question two: Let me ask you this, if you feel an updraft how do you react? I know I instantly pitch nose down. The end result is you aren't going to stall. You'd be surprised to know that in most gusty conditions with a well trained pilot AOA will vary very little. Gusts are instantaneous, not sustained. Even if they were sustained the pilots reaction and the aircraft's stability are both combating the bad things you fear might happen.

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As a practical experiment, go up with someone who is trained in aerobatics or if you can go on an aerobatic flight. Ask the pilot to fly a 60 degree upline, when the plane is just about to stall hold enough forward stick to feel 0G., it may be nearly full forward. Check out the ASI, chances are it's showing zero. Feel how the plane is flying, because she's not over critical AOA in this maneuver.

The point is to show you that with hard forward stick you can combat virtually any degree of updraft you'll see in your piloting career. At least in our little puddle jumpers: I can't speak for transport aircraft.

 

[SteveJeff]

Question two: Let me ask you this, if you feel an updraft how do you react? I know I instantly pitch nose down. The end result is you aren't going to stall. You'd be surprised to know that in most gusty conditions with a well trained pilot AOA will vary very little. Gusts are instantaneous, not sustained.

Of course I relax back pressure. Just that Gusts are instantaneous makes me wonder about the potential stall effect. At the moment I'm flying a C172 which is pretty stable even fully stalled unless you give her a full boot of rudder or so, but I'm not going to fly a C172 for the rest of my life. There are planes which have the abrupt tendency to drop a wing/nose, thus such a stall might lead to an attitude hard to recover close to ground. Unfortunately not all airplanes are so stable to maintain their initial attitude during an accelerated stall.

 

[drunkenbeagle]

Even if you did, you're in a climb. Just pitch nose down.

Even if you did manage to stall somehow, the updraft would be bringing you up faster than gravity would pull you down... (That killer 1700fpm thermal is 29fps, the force of gravity is 32f/s^2, and your stalled airplane is not perfectly clean. It will lose altitude somewhat slower than that.)

 

[shdw]

Unfortunately not all airplanes are so stable to maintain their initial attitude during an accelerated stall.

If you're not sleeping at the controls you don't have anything to worry about. Push forward, go on business as usual.

Remember, all Part 23 and Part 25 airplanes must be positively stable in pitch when in published CG limits. In other words, in the situations you speak of, the airplane will always seek recovery. That recovery action is nearly as instantaneous as the gust itself. You're recovery actions only assist it.

Next time you're out on a bumpy day, with an instructor on board, trim a hands off approach speed with enough power for a shallow final approach far out starting at or above pattern altitude. Fly as hands off as you can till 500' and do a go around. Notice every pitch change will be accompanied almost instantly by a countering action by the aircraft. Seeing that inherent stability might help set you at east.

 

[drunkenbeagle]

Worth pointing out - strong updrafts happen at altitude, not close to the ground

 

[shdw]

Worth pointing out - strong updrafts happen at altitude, not close to the ground

Unless you're landing on a runway with a nice ridge ~half mile away on the leeward side of a stiff wind. Then god only knows which way the wind is going to throw you. Otherwise, yes I'd buy that.