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?

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

SteveJeff wrote: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?

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

Wind direction changes to a 15knt tail wind, your ground speed changes to 80knts, speed through the air, which is what matters in a stall, does not change.

up draft wont cause a stall on final, push the stick forward to maintain your target touchdown point.

A sudden change in wind speed/direction aka windshear certainly induces a momentary airspeed change. So the question is what's the airspeed in that example, how much does it drop, because it certainly does.

Regarding the second question, I assume that if the gust is strong enough and it induces enough G to make the current airspeed=stall speed, the airplane will stall. Somehow like an accelerated stall. Wing is loaded, stall speed increases. Actually the wing is loaded at gust because the angle of attack increases. So, if you are already flying at high angle of attacks (i.e. low speed), the gust may momentary increase the angle of attack beyond the critical one and thus the airplane stalls.

SteveJeff,

One time I had a 20kt headwind change to a tailwind instantly while I was landing. I was probably 50 to 75 feet off the ground at the time flying one of my Stearmans. The plane didn't stall, but the sink rate increased dramatically. Since I always takeoff/land with one hand on the throttle I just pushed the throttle forward to stop the sink. I have my private instructor from all those years ago to thank for that. He brow beat me constantly with "pitch controls airspeed, power controls altitude".

At the time I didn't know what happened, but as soon as I finished my landing roll and turned her around I could see the wind sock. I hadn't really thought about it till you posed your questions but I guess she didn't stall because I had a low angle of attack.

With the Stearmans I spend a lot of my flying time low and not to far above stall speed. I pound into my head over and over and over that a wing can't stall if it's not creating lift.

http://youtu.be/xwrfEsCiltc

One of my favorite instructional videos.

Sorry, this gets longwinded:
An aircraft's inertia keeps it from reacting instantly to a change in wind speed or direction. The higher mass X inertia the longer the response time.

So why is low level wind shear more dangerous to a jet than a typical single engine airplane? Several reasons.
An aircraft with a low wing loading, say your J-3, will react much faster to a 10 knot change in winds compared to a 747. Of course the low wing loaded airplane will get bounced around much worse too.

The "book" approach speed in a light aircraft is 1.3 Vso. In a jet it is 1.2. Your LSA superbird has a stall speed of 50 so the book approach speed is 65. A margin of 15. (knots, mph, kph, fathoms per week, it doesn't matter what scale you use). A jet with a stall speed of 100 has a book approach speed of 120, a margin of 20. Better, right? Well, sort of.

Porterjets rule: (This will seem obvious when you read it) In any airplane a loss of inertia (airspeed) without changing the mass (which is hard to do over a 2 second period) will give you a loss of a combination of airspeed and or rate of climb. Airspeed and vertical speed are always related.

A light weight (low mass) propeller driven airplane will change inertia quickly, so by the time the pilot realizes he has lost 10 fpw the airplane has already started to make some of that up all by itself. Add in the virtually instantaneous response time of the prop and all that happens is a bit of turbulence and a good story. Although as barnstormer said in a larger wind change you will also see an increased rate of descent and a higher pucker factor. A good mountain wave at a high density altitude is best avoided.
In a jet the higher inertial mass means the instant loss of the same 10 fpw and, since the inertial mass has decreased, an increased rate of descent which on a normal approach is already 6-800 fpm in most jets. Add in a 2 to 3 second engine response time and things get critical pretty quickly.
Yes, turbulence changes stall speed because of the changing angle of attack. It can be an increase or decrease. A lot of today's jets (and yesterdays Learjet) have enough extra power to climb well above the wings optimal altitude for the weight of the airplane. Airlines actually use high altitude turbulence forecasts to sometimes limit the cruise altitudes of their flights because of this.

SteveJeff wrote: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?

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

Remember that wind doesn't suddenly change direction, what's actually happening is that you're flying the airplane from a body of air moving in one direction into a different body of air moving in a different direction. I have never seen a scenairo In any light plane where that transition from one airmass to another could happen fast enough to cause a stall in and of itself.

When an accident occures, far more likely what happens is some version of the rate of climb decreasing (or the aircraft starting to sink) because of said tailwind or downdraft, and it's the pilot that then pulls the aircraft into a stall trying to maintain the same attitude or rate of climb after the transition from the first airmass to the second.

Cub271 wrote:I have never seen a scenairo In any light plane where that transition from one airmass to another could happen fast enough to cause a stall in and of itself.

In windshear that might happen. Windshear is sudden change in speed/direction.

Cub271 wrote:When an accident occures, far more likely what happens is some version of the rate of climb decreasing (or the aircraft starting to sink) because of said tailwind or downdraft, and it's the pilot that then pulls the aircraft into a stall trying to maintain the same attitude or rate of climb after the transition from the first airmass to the second.

I don't see any reason why a tailwind will increase your rate of descent if there's no change in airspeed. All it changes it the distance travelled over ground. Actually you'll overshoot your intended point if there will be an increase in tailwind without airspeed change, am I right?

SteveJeff wrote:

Cub271 wrote:I have never seen a scenairo In any light plane where that transition from one airmass to another could happen fast enough to cause a stall in and of itself.

In windshear that might happen. Windshear is sudden change in speed/direction.

Cub271 wrote:When an accident occures, far more likely what happens is some version of the rate of climb decreasing (or the aircraft starting to sink) because of said tailwind or downdraft, and it's the pilot that then pulls the aircraft into a stall trying to maintain the same attitude or rate of climb after the transition from the first airmass to the second.

I don't see any reason why a tailwind will increase your rate of descent if there's no change in airspeed. All it changes it the distance travelled over ground. Actually you'll overshoot your intended point if there will be an increase in tailwind without airspeed change, am I right?

If you fly an approach with a 10 knot tailwind exactly like you fly an approach with a 10 knot headwind then yes you are correct.
Don't forget the air we fly in is a three dimensional fluid. Turbulence and wind shear can be caused by a sudden change in speed or direction (horizontally or vertically) or usually a combination of those. Where there is an updraft a downdraft isn't far away. On very short final, i.e. heights less than 50 ft., I would guess that mechanical turbulence is the man culprit. In other words winds swirling around hangars or trees. If you are 2 feet above the runway nicely slowing down and hit a 5 knot headwind reduction followed immediately by a less than admirable landing did you stall or just hit a downdraft? A. who cares, it is academic. I would be more worried about hitting moderate turbulence turning from base to final. In any type of airplane.

And the reason for the last statement? Spin induced by windshear?

SteveJeff wrote:And the reason for the last statement? Spin induced by windshear?

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

Not necessarily, control your airspeed, your nose may pitch up slightly, correct it, an updraft means an increase in altitude or vertical speed, pitch down to correct it will mean increase in speed. You might encounter sink next.

In moderate turbulence I felt Gs high enough that may stall you at 65 kts on final, I guess.

Okay SteveJeff now you are making me think, which is never a good thing.

Perhaps what I experienced was due to what Porterjet mentioned, mechanical turbulence. The north end of the runway has a bunch of Oak Trees (which an overloaded Luscombe flew right into in ground affect after doing an hour of T&Gs, but that's another story). Depending on the temperature, direction, and velocity of the wind you get lifted, dropped, or put in the wash cycle.

Doesn't really matter why the sink rate changed during the 180 degree wind shift, just that it did, and that I reacted correctly and didn't pull the stick back to arrest the sink.

This guy has carried on a long, long thread on the EAA forums, same topic, same questions. Here's what I wrote there, this morning, after one of his posts claimed that he is a private pilot:

FWIW, I'm beginning to think we're all being played, which may explain the unwillingness to accept the good advice that has been given by so many, as well as the lack of basic aerodynamic knowledge of the OP. There is no Steve Jeff in the FAA's pilot data base. There is only one person with the last name of Jeff, and he's a commercial pilot of very long standing, some 55 years. Reversing the name, there is no Jeff Steve in the FAA's data base. There are 3 persons with the last name of Steve in the data base who have addresses showing, only one is a fairly recently certificated pilot, and his name would not easily include Jeff in it. Of the remaining 7, one has no certificate, one is a mechanic, one was certificated some 30 years ago, one was certificated 59 years ago, one is a commercial helicopter pilot of more than 20 years, one is a commercial pilot certificated 24 years ago, one is a private pilot certificated almost 2 years ago.

Now granted, many people create handles for posting on forums, so this tiny bit of research doesn't prove anything, but it may explain things a little.

Then he responded:

Ha ha, really funny offtopic Cary, for you FAA is the whole world? What about JAA? You heard of it? Anyway, no further offtopic, if you can help on the topic issue, please do it, thanks.

Just FWIW.

Cary

SteveJeff wrote:

Cub271 wrote:I have never seen a scenairo In any light plane where that transition from one airmass to another could happen fast enough to cause a stall in and of itself.

In windshear that might happen. Windshear is sudden change in speed/direction.

Sorry, but I don't buy it. You're defining wind shear incorrectly. Wind shear is the variation of wind over either horizontal or vertical distances. It is not an instantanious phenomena, and wind shear would not cause a stall in my expirience. It would be the reaction of the pilot to the wind shear that would stall the aircraft.

SteveJeff wrote:

Cub271 wrote:When an accident occures, far more likely what happens is some version of the rate of climb decreasing (or the aircraft starting to sink) because of said tailwind or downdraft, and it's the pilot that then pulls the aircraft into a stall trying to maintain the same attitude or rate of climb after the transition from the first airmass to the second.

I don't see any reason why a tailwind will increase your rate of descent if there's no change in airspeed. All it changes it the distance travelled over ground. Actually you'll overshoot your intended point if there will be an increase in tailwind without airspeed change, am I right?

I didn't say a tailwind would increase your rate of descent. However, a downdraft would, and transitioning from a headwind to a tailwind on a departure (your scenairo) would decrease the rate of climb relative to terrain. Once again, the real world stall threat is not from the movement of the aircraft through differing velocity airmasses, but rather from a pilot in a loosing battle trying to avoid terrain because he/she can't climb fast enough to avoid it.

That's my opinion for whatever it's worth, your milage may vary.

SteveJeff wrote:And the reason for the last statement? Spin induced by windshear?

glidergeek pretty much covered it, I would add that turbulence can create stall conditions today when yesterday in exactly the same place with smooth air you managed to turn the corner without stalling. Turbulence is just one of the changing factors in aerodynamics. Whether you spin or not is another story, although an inadvertent stall even without spinning at 500 feet will get your attention.

Off the top of my head 99% of Windshear encounters result in less than a 10 knot change in airspeed which, 99% of the time, will not directly cause you to stall. However the relatively quick accompanying change in your rate of climb/descent will cause an unstabilized approach which has it's own problems.

In the questioner's first issue you may well find that it is not the change from head wind to tail wind that gets ya, if you are unlucky you may well get caught by the rotational forces as the wind changes. This will most definitely upset the apple cart as you will have asymmetrical lift where one wing stalls and falls while the other wing still has airflow and creates lift.

On takeoff on runway 08 we started with a fair headwind, 10 to 12 knots. At just about rotation speed the wind shifted to about 15 knots on the tail. The right wing went up hard, the left wing went down hard, the tail feathers had no effect at all. I needed to turn into the wind to get the right wing down but the only control that might work was the right brake but that wheel was about four feet off the ground while the left wing tip light went away in a shower of grimes glass. We plodded along until ground speed was down to a very slow walk waiting for the plane to roll over and or break a wing off when the wind let off for a second and allowed the right wing to come down level. Needless to say we returned to the hanger for a complete inspection of the left wing and to replace the tip light. Now this was undoubtedly mechanical turbulence but the wind direction change is what was asked about.

SteveJeff wrote:In moderate turbulence I felt Gs high enough that may stall you at 65 kts on final, I guess.

Here read up: http://en.wikipedia.org/wiki/Stall_%28flight%29 this all depends on what you are flying and your experience level. As far as "moderate turbulence" depends, what one guy considers moderate another might consider light and a third considers severe. I've had the stall warning horn go off at 80 mph and not go off at 50 landing.

I suggest if you want real answers push the throttle in, pull back on the yoke and go get answers, fly safe have fun and learn from hands on.

Cub271 wrote:

SteveJeff wrote:

Cub271 wrote:I have never seen a scenairo In any light plane where that transition from one airmass to another could happen fast enough to cause a stall in and of itself.

In windshear that might happen. Windshear is sudden change in speed/direction.

Sorry, but I don't buy it. You're defining wind shear incorrectly. Wind shear is the variation of wind over either horizontal or vertical distances. It is not an instantanious phenomena, and wind shear would not cause a stall in my expirience. It would be the reaction of the pilot to the wind shear that would stall the aircraft.

Ok, it is variation over either horizontal or vertical distances, but you are moving at a considerable speed over those distances.

Glidergeek wrote:

SteveJeff wrote:In moderate turbulence I felt Gs high enough that may stall you at 65 kts on final, I guess.

Here read up: http://en.wikipedia.org/wiki/Stall_%28flight%29 this all depends on what you are flying and your experience level. As far as "moderate turbulence" depends, what one guy considers moderate another might consider light and a third considers severe. I've had the stall warning horn go off at 80 mph and not go off at 50 landing.

What to read there? I'm familiar with 2Gs feel from coordinated 60 turns. I can easy say when a gust induced 2G on the airplane. When you encounter an upgust, your angle of attack increases, if it 2Gs, your AOA becomes critical even if you were flying at 65 kts and the 1G stall speed is let's say 47. It's like an accelerated stall. The airplane might stall even if it's above 1G stall speed due to the load imposed by the upgust.

Sounds like flying is too scary. Time to find another way to get around.

Gump