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I had a discussion about air speeds with one of my instructors recently. He is claiming that there is a sudden drop in the wind velocity as you get closer to the ground (which is absolutely true, due to the wind friction with the ground). However, he also says that this will affect the airspeed by making it lower and this could be dangerous because it could lead to stall conditions.

For example: he says that, if my approach is with 90km/h airspeed, the headwind is 30km/h at a certain height and by the ground the headwind goes down to 10km/h, and the stall speed is 80 km/h, I will stall because the air speed will drop down to 70km/h.

Here is where I am confused, since I have read before that the wind does not affect the air speed in any way and it will only affect the ground speed by making it 20km/h higher.

Could you please try to make this clear for me?

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    $\begingroup$ Welcome to Aviation.SE! Nice first question, clear and concise. Hope to see you more in the future (-; $\endgroup$ – kevin Oct 17 '17 at 9:37
  • $\begingroup$ This is a great question. My question is, how can a pilot compensate for this effect? $\endgroup$ – Bassinator Oct 18 '17 at 1:49
  • $\begingroup$ in some way you should be aware of the wind speed and direction before the approach. This information you can get from the ATC, or if there is no ATC and you are doing circuits at your home airfield you could estimate the wind speed and direction before the flight. So recently I found a general equation for the airspeed that you need to set during approach: $\endgroup$ – The Fly Oct 18 '17 at 9:44
  • $\begingroup$ Approach Airspeed=1,5xStallspeed(1,5 is safety factor)+0,5x headwind velocity. So, you should be prepared for this effect before your approach.If not prepared adding thrust will help. If flying glider(no engines) nose down to gain airspeed. $\endgroup$ – The Fly Oct 18 '17 at 10:07
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Your airspeed does not remain constant because of inertia: it takes more time for the airplane to adapt to the new relative wind, compared to the time it takes for the wind to change.

Example One: you're flying 80 knots and the headwind is 20 knots. Over a time of 3 minutes, the headwind gradually reduces from 20 knots to 10 knots. Since the change is gradual, you will notice the ground speed smoothly increases from 60 knots to 70 knots.

Example Two: you're flying 80 knots and the headwind is 20 knots. In 2 seconds, the wind drops from 20 knots to 10 knots. At the start (t = 0), you are moving 60 knots over the ground. However, 2 seconds later, the plane has not yet accelerated 10 knots faster (maybe only 61.5 knots), but the wind is now 10 knots. Therefore you experience an airspeed drop in 2 seconds from 80 knots to 71.5 knots.

Given sufficient altitude, the plane will eventually speed up to 70 knots ground speed (airspeed 80 knots again). When you are landing, you may end up touching down short of the runway if corrective actions are not taken in a timely manner.

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You always fly with respect to the mass of air, but when flying in a wind gradient, (as is the case in the example) you can suddenly find yourself with an insufficient headwind, hence not enough lift, and descending rapidly. If you try to compensate by rising the nose, you may stall...

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  • $\begingroup$ Yes, this is exactly what he told me, but I still don't get why your airspeed will drop down (in wind gradient) instead of remaining constant and making the ground speed higher. $\endgroup$ – The Fly Oct 17 '17 at 9:07
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    $\begingroup$ @The Fly If the mass of the plane were negligible, with zero inertia, it will adapt immediately to any wind gradient-motivated change in the airspeed, but having a mass, it has an inertia too, making it relatively slow to adapt. $\endgroup$ – xxavier Oct 17 '17 at 9:12
  • $\begingroup$ So, as I understand the wind does not change the airspeed only in case when there is no sudden change, so the aircraft could accelerate to its initial airspeed ? $\endgroup$ – The Fly Oct 17 '17 at 9:25
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    $\begingroup$ @The Fly When landing with a headwind having an important gradient, one should pay attention to have enough airspeed (and thus potential reserve lift) to compensate for any sudden, transient change in airspeed. $\endgroup$ – xxavier Oct 17 '17 at 9:44
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The extreme example can sometimes make it plain. Say you are flying at 50 knots with a 50 knot headwind. What happens if the headwind goes to 0 knots? The airplane has no real velocity, but it will gain velocity quickly in a downward direction. Ground speed is not so important when high, but when low, ground speed becomes very more important.

Common approach formula for gusty conditions: gust factor / 2 + approach speed = new approach speed

20 gusting to 30, a 10 gust factor, 10/2 = 5, add 5 to your approach speed

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A lot of answers (all corect) come from a physics/mecanics point of view,so I’ll try to come up with a more-aviation-themed answer: Trim speed

Your airplane is trimmed for a specific speed. With no movement on the control stick the plane is designed to keep whatever airspeed the plot has trimmed for, by pitching up and down.

When your wind changes, or rather when you fly into a different mass of air that moves at a different speed/direction (wich we call wind on the ground), your indicated airspeed will change, and the plane will nicely struggle to regain the originally trimmed speed, yes, by pitching up and down.

Naturally this process takes a few moments, to accelerate/decelerate the ship. The larger the aircraft, the longer it takes. On a large jet it can take 10s of seconds indeed.

In most cases the pilot will usually compensate with thrust to regain the desired speed. Why? Because waiting for the ship to return to trim-speed will take too long and, more importanly, it willalso make the airplane climb/descend , usually messing up your nice 3 degrees approach path on final.

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Yes a sudden horizontal wind gust suddenly changes relative air speed.

Adding speeds must be done at any moment in time. One moment you have a headwind, the next moment you don't: that's a drop in air speed. If you don't change the power setting, the aircraft will accelerate until the original airspeed is reached again, but until that time you have lower airspeed.

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In the spirit of blake's answer, lets look at a real world case:

You're flying along and your airspeed is getting too high. You respond by lowering your airspeed to what it's supposed to be. The wind suddenly reverses, your airspeed is too low. Delta Airlines flight 191 is splattered across Dallas with 136 dead and the airline industry got a big wake-up call of the dangers of microbursts.

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    $\begingroup$ There's more to it than that. They had an episode where they gained headwind, started to shed airspeed, thought twice about it, had it reverse on them and saw how that could go... experienced some up/downdrafts... and continued the approach nonetheless and got into more of the same. So it's also a lesson of knowing when to quit. $\endgroup$ – Harper - Reinstate Monica Oct 18 '17 at 4:42
  • $\begingroup$ @Harper Yeah, I was only looking at what actually swatted them from the sky, not what lead up to it. $\endgroup$ – Loren Pechtel Oct 18 '17 at 18:42
  • $\begingroup$ I don’t think this actually answers the question asked and should probably be a comment. $\endgroup$ – Notts90 Oct 20 '17 at 8:57
  • $\begingroup$ @Notts90 I'm showing that in the real world it does happen, that's showing that the answer to the original question is true. $\endgroup$ – Loren Pechtel Oct 20 '17 at 19:27
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Absolutely a sudden drop in headwind speed will temporarily lower your airspeed, due to the airplane's inertia, as many answers have noted. That's why it's wise to carry extra airspeed near the ground in gusty conditions.

Another point not noted in most other answers is that if you in are in a prolonged descent through a CONSTANT wind gradient-- i.e. the rate of decrease in headwind is constant with altitude-- your aircraft will stabilize at the normal trimmed airspeed, but a nose-lower-than-usual pitch attitude, and a higher-than-normal sink rate. This is another reason why it's wise to carry extra airspeed when landing in strong wind-- if there's a strong gradient, which there often will be, then it takes extra energy (airspeed) to round out from the steeper-than-usual glide path. (Alternatively, in a powered plane, you could carry some extra power to preserve your normal glide path.)

So, take your pick-- in sunny turbulent conditions, strong winds usually are associated with strong gusts, and in smooth overcast conditions or in stable marine air, strong winds usually cause a pronounced wind gradient. Either way, you'll likely want to carry some extra airspeed for landing. There is nearly always SOME amount of wind gradient near the ground and this has a very real effect on the dynamics of flight.

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There are two things going on here. 1 - Your speed relative to the ground. 2 - Your lift.

Lets assume you leave your engine rpms the same and the engine puts out the same energy at all times.

1 - speed relative to the ground will increase as your relative speed is your aircraft speed moving forward, minus any head wind. Now a head wind of 20knots wont slow you down exactly 20 knots as there is the matter of wind resistence / aerodyamics of the craft to take into account but in general my above equation is true. 2 - Lift. If there was no head wind, or any wind, then lift is directly controlled by your forward speed and the angle of attack of the wing. if the wing angle stays the same then lift increases as a head wind increases as there is more air flowing backwards over the wing. A tail wind decreases the lift.

Ground speed may decrease as head winds decrease as you may have to lift your nose to increase the angle of attack to maintain lift. if the engine remains the same revs then more energy is spent in an upwards direction and less on a forwards direction.

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