Stalls in turbulence

[Steve Jeff]

I have some doubts which I don't master regarding the possibility of having a stall in turbulence. Considering I do most of my flights in good weather, I don't have much practical experience reagrding these issues.

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? I guess it will end up in a spin and being too low to recover...

2. On final, I encounter an updraft, I noticed that updrafts are +Gs, so is this scenario at risk for an accelerated stall?

3. How aircraft manufacturers make that all certified aircrafts are able to sustand the same amount of gusts? I mean all are certified to sustand 50 ft/s up gust (I think I remebered it correctly), but considering the fact a light sport aircraft will be more loaded due to low mass (inertia) than a heavier aicraft for the same gust?

[WLIU]

You have an imperfect visualization of how the atmosphere works. Imagine an invisible river where one layer is moving in one direction and the neighboring layer is moving in another. You climb to the top of the first layer and discover that you can not continue your climb into the second layer for a time. You ride the boundary until you accelerate enough to be able to continue your climb in the second layer. You do not fail out of the sky unless you do something really dumb that causes the airplane to stop flying in the first layer of air. If the two layers of air are are moving in really different directions or a greatly different speeds, you are likely to find turbulence at their boundary, but surprisingly, some days you do not.

A light wing loading means that you go up and down more as you move from one air mass to another. But your airplane is moving relatively slowly and it does not weigh a lot. You do NOT necessarily put higher loads on your aircraft. The G that results from a 50fps gust is different depending on aircraft speed and wing loading.

Momentary G is a different hazard than the sustained G of a steep turn. If an updraft momentarily bumps you to 2G, the inertia from the mass of your aircraft keeps the change in aircraft attitude relatively small. And then you are flying again, moving the controls to correct the undesired yaw or pitch. But that is why when you have a lot of turbulence on final you fly a slightly (5%) higher airspeed.

Go fly,

Wes
N78PS

[Steve Jeff]

If the two layers of air are are moving in really different directions or a greatly different speeds, you are likely to find turbulence at their boundary, but surprisingly, some days you do not.

And there, considering my example, 180 change in direction, that turbulence will cause your speed to drop, right? And thus having a stall, right?

A light wing loading means that you go up and down more as you move from one air mass to another. But your airplane is moving relatively slowly and it does not weigh a lot. You do NOT necessarily put higher loads on your aircraft. The G that results from a 50fps gust is different depending on aircraft speed and wing loading.

For an airplane, Va descreases as weight decreases.

But let's say same gust acts on 2 airplanes flying at the same speed (also they same the same Va), but they have different weight. Plane A is 600 kg, plane B is 300 kg. On plane B the gust will have a double acceleration effect. So, what differs that these 2 aircrafts still can have the same gust envelope as required by certification?

Momentary G is a different hazard than the sustained G of a steep turn. If an updraft momentarily bumps you to 2G, the inertia from the mass of your aircraft keeps the change in aircraft attitude relatively small. And then you are flying again, moving the controls to correct the undesired yaw or pitch. But that is why when you have a lot of turbulence on final you fly a slightly (5%) higher airspeed.

So, it might stall accelerated when it bumps at 2G or not?

[WLIU]

As you climb to the higher air mass, you will stop climbing but should not lose altitude. Your airspeed in relation to the lower air mass will not drop. Stall is in relation to the air mass around you.

Va decreases as weight decreases yes, but your attempted example is flawed for that very reason. If the airplanes have the same wings but are flying with different weights, they must have different Va's.

As for momentary gusts, you are trying to suggest that when a wing has a gust exceed its critical angle of attack for a second, total control of the aircraft is lost. That does not happen due to inertia. As you slow towards the stall speed, turbulence causes more AoA excursions and you will provide more and larger control inputs to correct in between the "gusts". As you slow, there will be a speed where the time that the wing is having AoA problems exceeds the time that the wing is flying. Somewhere around that time you will effectively lose control of the aircraft. That is why you fly the landing pattern at a higher speed on turbulent days.

Each aircraft has what is called a V-N diagram that charts the airspeed vs G caused by pilot maneuvering or "gusts". The V-N diagram has an outline drawn on it that shows whether a particular G at a specific air speed results in the wing stalling (exceeding its critical AoA) or the airplane actually breaking. But to effectively use the V-N diagram you need to have a G meter installed in addition to your airspeed indicator. My observation is that the majority of pilots grossly over-estimate the G's that they think they have experienced. Anyway, look at your V-N diagram, look at the airspeed that you fly final on a bumpy day, and see how large or small your speed and G margin is. That will answer the question that you seem to be asking.

Hope this makes sense.

Go fly,

Wes
N78PS

[martymayes]

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? I guess it will end up in a spin and being too low to recover...

No because a light plane will react to the 15 kt change in airspeed almost immediately. It's unlikely you would notice. Your speed over the ground would change by 30 kts, however.

2. On final, I encounter an updraft, I noticed that updrafts are +Gs, so is this scenario at risk for an accelerated stall?

Yes, an updraft would + load the airplane, for a second. Then the plane would respond to the gust by ballooning above the desired glidepath, not much else.

[Steve Jeff]

As you climb to the higher air mass, you will stop climbing but should not lose altitude. Your airspeed in relation to the lower air mass will not drop. Stall is in relation to the air mass around you.

Not necessary climbing, let's assume I'm flying straight and level at 150 ft AGL when the 15kts headwind suddenly changes its direction 180 degrees. My airspeed will drop 15kts or 30kts?

Va decreases as weight decreases yes, but your attempted example is flawed for that very reason. If the airplanes have the same wings but are flying with different weights, they must have different Va's.

As for momentary gusts, you are trying to suggest that when a wing has a gust exceed its critical angle of attack for a second, total control of the aircraft is lost. That does not happen due to inertia. As you slow towards the stall speed, turbulence causes more AoA excursions and you will provide more and larger control inputs to correct in between the "gusts". As you slow, there will be a speed where the time that the wing is having AoA problems exceeds the time that the wing is flying. Somewhere around that time you will effectively lose control of the aircraft. That is why you fly the landing pattern at a higher speed on turbulent days.

Few knots added gives just a small G margin in addition. What I thought was that when the critical angle of attack of one wing or both will be exceeded, the airplane will stall and possible to have also a wingdrop and considering we are talking about flying at a higher airspeed than 1G stall, the stall induced by a un upward gust will have a much greater effect, that's whi I thought it might induce a spin. The differential lift between the two wings will be really high assuming that only one wing stalled.

Each aircraft has what is called a V-N diagram that charts the airspeed vs G caused by pilot maneuvering or "gusts". The V-N diagram has an outline drawn on it that shows whether a particular G at a specific air speed results in the wing stalling (exceeding its critical AoA) or the airplane actually breaking. But to effectively use the V-N diagram you need to have a G meter installed in addition to your airspeed indicator. My observation is that the majority of pilots grossly over-estimate the G's that they think they have experienced. Anyway, look at your V-N diagram, look at the airspeed that you fly final on a bumpy day, and see how large or small your speed and G margin is. That will answer the question that you seem to be asking.

I guess it's not very large, that margin.

Yes, an updraft would + load the airplane, for a second.

Why that one second is not enough to induce a stall (with wing drop if it's stalled just one wing)?

[martymayes]

Not necessary climbing, let's assume I'm flying straight and level at 150 ft AGL when the 15kts headwind suddenly changes its direction 180 degrees. My airspeed will drop 15kts or 30kts?

If it were an instantaneous event, the change in airspeed would be 15 kts. Example: Flying straight and level at 115 KIAS. Doesn't matter if it's a 15 kt headwind or a 50 kt headwind, the plane's speed relative to the air is 115 kts. Wind shears to 15 kts of tailwind, the change in airspeed is 15 kts. 115 - 15 = new speed relative to the air of 100 kts. Now, thrust is > drag so the plane accelerates back to 115 KIAS. If the pilot did not change the angle of attack to compensate for the decrease in lift when the speed changes, plane would lose altitude but not 150 ft. If the pilot did increase angle of attack, his stall margin would decrease but not enough for the plane to stall.

Why that one second is not enough to induce a stall (with wing drop if it's stalled just one wing)?

When the plane is unloaded (i.e. the gust is past), it resumes flying like nothing happened. A gust is a momentary event. It would happen so fast the pilot would go "duh" and the event would be over.

I think your imagination is getting the best of you. Perhaps a flight lesson or two will clarify some of these misconceptions.

Under the right conditions, you can impress your friends by flying the plane backwards! All you need to do is fly at 40 kts into a 50 kt headwind.

[Steve Jeff]

If it were an instantaneous event, the change in airspeed would be 15 kts. Example: Flying straight and level at 115 KIAS. Doesn't matter if it's a 15 kt headwind or a 50 kt headwind, the plane's speed relative to the air is 115 kts. Wind shears to 15 kts of tailwind, the change in airspeed is 15 kts. 115 - 15 = new speed relative to the air of 100 kts. Now, thrust is > drag so the plane accelerates back to 115 KIAS. If the pilot did not change the angle of attack to compensate for the decrease in lift when the speed changes, plane would lose altitude but not 150 ft. If the pilot did increase angle of attack, his stall margin would decrease but not enough for the plane to stall.

If not 150ft, how much?

If the pilot did increase the angle of attack, his stall margin might decrease to critical if it was already high before he pull the yoke.

When the plane is unloaded (i.e. the gust is past), it resumes flying like nothing happened. A gust is a momentary event. It would happen so fast the pilot would go "duh" and the event would be over.

I think your imagination is getting the best of you. Perhaps a flight lesson or two will clarify some of these misconceptions.

Not necessary to take flight lessons. I am a private pilot. I usually fly in good weather. However, I know that these scenarios don't happen. I just know to understand why. There were moments when I felt the G for a few seconds, I think a few seconds would be enough to stall the airplane. If I suddenly pull the yoke, I can stall accelerated the plane in less than 1 seconds. Why the gust doesn't?

[martymayes]

If not 150ft, how much?

Not much. In the real world, a light airplane will start responding and never see the 15 kt speed change. In a heavier airplane, windshear is more of a problem because of higher inertia. In airline operations, during a critical phase of flight, like takeoff and landing, a -20 kt windshear can trigger a windshear alert which requires the pilot to perform an escape maneuver.

If the pilot did increase the angle of attack, his stall margin might decrease to critical if it was already high before he pull the yoke.

Perhaps if he was practicing minimum controllable airspeed when the shear occurred....even then recovery would be a non-event.

There were moments when I felt the G for a few seconds, I think a few seconds would be enough to stall the airplane. If I suddenly pull the yoke, I can stall accelerated the plane in less than 1 seconds. Why the gust doesn't?

A 2g load increases the stall speed by ~40%. It's harder to apply a sustained 2g load than you realize, a momentary event doesn't count. So you can fly a 172 at 75 kts, which is pretty slow, apply a 2g load and it won't stall because it's still within the flying envelope.

[Steve Jeff]

Not much. In the real world, a light airplane will start responding and never see the 15 kt speed change. In a heavier airplane, windshear is more of a problem because of higher inertia. In airline operations, during a critical phase of flight, like takeoff and landing, a -20 kt windshear can trigger a windshear alert which requires the pilot to perform an escape maneuver.


Perhaps if he was practicing minimum controllable airspeed when the shear occurred....even then recovery would be a non-event.



A 2g load increases the stall speed by ~40%. It's harder to apply a sustained 2g load than you realize, a momentary event doesn't count. So you can fly a 172 at 75 kts, which is pretty slow, apply a 2g load and it won't stall because it's still within the flying envelope.

On final you are 65 kts, I would say you'll stall if 2G load is applied. Sustained? I don't know here, but sometimes it feels like the updraft load is sustained 2-3 seconds when you encounter it suddenly.