Wind gusts on final

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mxalix258

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mxalix258
I have a question that I keep thinking about in regard to gusts on final. It confuses me when someone says they will fly a higher approach speed to hedge against a strong gust that may cause a stall - my question is why a gust would matter?

If all the wing knows is the mass of air it is flying in, then it really shouldn't matter if it's gusting or steady wind...right? Your ground speed may definitely fluctuate, but am I missing something when someone flies a faster approach speed in gusty conditions? I can also understand that you may have additional rudder/elevator authority, but still don't see a relation to stalling the airplane.

Thanks for the input!
 
The higher your airspeed, the greater the less net effect the gust will have and the greater effectiveness of your rudder. In other words, the faster you are moving through the air, the less lateral movement will be created when the gust comes.
 
What you're missing is that it takes some time for the ground speed to change. If a headwind gust of 10 knots suddenly comes about (so, now, it's a tailwind gust of 10 knots), at the next instant, your ground speed is unchanged, but your airspeed is down by 20 knots. If you're approaching in a 172 at 60 knots, you now have a big problem.

Given time, the aircraft will react to the wind and the ground speed will change. If you're in the flare, you aren't going to have that time.
 
The problem is that you still have to slow down to land so you actually accomplish nothing imo. Just another example of where an airliner crashed for a barely related reason and a bunch of wisdom and training materials were developed that were then misapplied to the small ga fleet.
 
Your ground speed doesn't react as fast as the wind gust. Your headwind component could drop 10-15 kts in a second or so. A second or so after the headwind component drops 15, your plane will have a pretty close ground speed to what it was before the headwind dropped.

I'll try to type an example. Coming on final with a 10G20 headwind:
Wind is 20 kts and airspeed is 65 kts. Therefore 45 kts ground speed.
Wind drops to 10 kts.
Wind is 10 kts and, at least briefly, groundspeed is still 45kts. That means your airspeed is now 55 kts.

Keep in mind the plane weighs a bunch and has a lot of momentum.
 
I guess I'm confused on why my airspeed would change. If I'm flying in a 20 knot headwind, my airspeed isn't affected...my ground speed would be. Why would a gust be any different? Maybe something is getting mixed up in my mind.
 
Maybe I'm getting indicated and true airspeed turned around. True airspeed would decrease, but indicated would not, correct?
 
mxalix258 said:
I guess I'm confused on why my airspeed would change. If I'm flying in a 20 knot headwind, my airspeed isn't affected...my ground speed would be. Why would a gust be any different? Maybe something is getting mixed up in my mind.
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In the brief window that the wind changes, the plane will have the same ground speed.

Think of riding a bike. In normal operations, your body will react to changes in the speed of the bike. Bike speeds up, body on top speeds up, bike slows down, body on top slows down. However, if that front tire hits a stump, the bike speed drops fast. However, the speed of the body on top doesn't react so fast and over the bars you go.
 
mxalix258 said:
Maybe I'm getting indicated and true airspeed turned around. True airspeed would decrease, but indicated would not, correct?
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Unrelated:wink2:

Think of it this way. Plane is flying into a 50 kts headwind at 100 kts airspeed. Ground speed is then 50 kts. If the wind suddenly drops to 0 kts, does the groundspeed instantly go to 100 kts or will there be a lag?
 
mxalix258 said:
I have a question that I keep thinking about in regard to gusts on final. It confuses me when someone says they will fly a higher approach speed to hedge against a strong gust that may cause a stall - my question is why a gust would matter?

If all the wing knows is the mass of air it is flying in, then it really shouldn't matter if it's gusting or steady wind...right? Your ground speed may definitely fluctuate, but am I missing something when someone flies a faster approach speed in gusty conditions? I can also understand that you may have additional rudder/elevator authority, but still don't see a relation to stalling the airplane.

Thanks for the input!
Click to expand...

Jet technique taught for flying high inertia aircraft. It does effect light aircraft as well, but one is better to apply extra throttle for better tail control from a 'drag in' full flap throttle to arrest sink and add some rudder force to the scene; either that or know the plane very well. Even with the throttle catch it takes some skill.
 
60 is about right for a 172. Anything faster is just speed that must be dissipated prior to flare.

MAKG1 said:
What you're missing is that it takes some time for the ground speed to change. If a headwind gust of 10 knots suddenly comes about (so, now, it's a tailwind gust of 10 knots), at the next instant, your ground speed is unchanged, but your airspeed is down by 20 knots. If you're approaching in a 172 at 60 knots, you now have a big problem.

Given time, the aircraft will react to the wind and the ground speed will change. If you're in the flare, you aren't going to have that time.
Click to expand...
 
wabower said:
60 is about right for a 172. Anything faster is just speed that must be dissipated prior to flare.
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Yep, there just isn't enough inertia to make a difference. The key is to get it to transition from flying to not fying in the shortest time.
 
I understand that there would be an affect on the airplane moving through the air, but don't understand how this relates to the aircraft stalling. If I'm flying my approach in a 172 at 65kts indicated, and it's calm winds, and out of nowhere I get a 10kt straight on gust...I would still be flying at 65kts but my ground speed would be 55kts.

Now, if we are talking about the gust causing a slower ground speed which causes us land before our intended landing point so we take corrective action (eg. pull back on the yoke). etc etc. I can follow that train of logic, is that what we are talking about?
 
Traket92X said:
Your ground speed doesn't react as fast as the wind gust. Your headwind component could drop 10-15 kts in a second or so. A second or so after the headwind component drops 15, your plane will have a pretty close ground speed to what it was before the headwind dropped.

I'll try to type an example. Coming on final with a 10G20 headwind:
Wind is 20 kts and airspeed is 65 kts. Therefore 45 kts ground speed.
Wind drops to 10 kts.
Wind is 10 kts and, at least briefly, groundspeed is still 45kts. That means your airspeed is now 55 kts.

Keep in mind the plane weighs a bunch and has a lot of momentum.
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The word you're looking for here is inertia.....
{edit} now I see that Henning has beat me to it.

To the OP, gusts aren't the source of the trouble it's the lulls in between the gusts. I think that's the mental confusion you may be having.
 
mxalix258 said:
I understand that there would be an affect on the airplane moving through the air, but don't understand how this relates to the aircraft stalling. If I'm flying my approach in a 172 at 65kts indicated, and it's calm winds, and out of nowhere I get a 10kt straight on gust...I would still be flying at 65kts but my ground speed would be 55kts.
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Inertia. F=m*A.

A change in fluid velocity from V1 to V2 does not cause a body at rest or in motion relative to that fluid to instantly match the velocity change.

The danger is mostly in a tail-wind gust. For example flying in a steady head wind of 10 kts and the headwind suddenly vanishes. Not a tail wind as such but until your plane accelerates back up to the equilibrium velocity it had with the air, the wings suddenly will be 10 kts slower relative to the air.

The typical mass and drag coefficient of a C-172 are available on the net and when I find some spare minutes I may try to estimate how long it takes a C-172 to accelerate back to its equilibrium velocity after an instantaneous relative wind change. (I'll have to do an integration since the force is proportional to the relative velocity, which is changing.)
 
I might be on the verge of understanding...thanks everyone!
 
I believe you are over-thinking.

First of all, ground speed has nothing at all to do with it. Secondly you should stop looking at gusts as either headwinds or tailwinds, it's simply turbulent air that can be swirling in any direction including up and down. It's quite possible and common to hit downward moving air with one wing while encountering upward moving air with the opposite wing. The higher approach speed gives you more positive control authority, especially with ailerons. Because when you are low and slow on final, having to input full aileron to arrest an upset like this is not the ideal position to be in.
 
At 1.3Vs0 the airplane should be fully controllable up to (and possibly beyond) max demonstrated X-Wind. No reason to fly faster when winds are within demonstrated. Flying 1.3Vs0 + (1/2 gust factor) may be prudent, but in some cases may even also be unnecessary.
 
You're just one small step of re-inventing Vref (which usually is 1.3*Vs0 but not always).
 
mxalix258 said:
I might be on the verge of understanding...thanks everyone!
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It's simply a problem of inertia. Inertia, as you probably know, is a body's resistance to change in motion. For instance, a car's resistance to accelerating when you press the gas pedal, or its desire to continue going forward when you press the brakes.

If the the headwind gradually drops from 25kts to 10kts, the airplane will stay at the same IAS, and the groundspeed will gradually increase by 15 kts.

If you headwind suddenly and dramatically drops from 25kts to 10kts, the inertia of the airplane prevents it from suddently and dramatically accelerating to a 15kt faster ground speed. So, for a moment, the aircraft is flying at a lower IAS.

How much this affects light aircraft, and how much we should worry about it, is up for debate.
 
mxalix258 said:
I understand that there would be an affect on the airplane moving through the air, but don't understand how this relates to the aircraft stalling. If I'm flying my approach in a 172 at 65kts indicated, and it's calm winds, and out of nowhere I get a 10kt straight on gust...I would still be flying at 65kts
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No. Your airspeed would momentarily be 75Kts.
 
jesse said:
The problem is that you still have to slow down to land so you actually accomplish nothing imo. Just another example of where an airliner crashed for a barely related reason and a bunch of wisdom and training materials were developed that were then misapplied to the small ga fleet.
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Devil's Advocate here. So reducing your risk exposure by narrowing the time window wherein you are within a few mph of VSO in gusty or turbulence on final is useless because...?

I don't think anyone is arguing for attempting to plant your wheels on the tarmac at VREF+half the gust spread in most spam cans. But dealing with an inopportune gust near VSO is much more inconvenient at ten feet than ten inches. IMO.

Unless you are in a taildragger lol.
 
mxalix258 said:
I guess I'm confused on why my airspeed would change.
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The airplane not reacting in steady-state wind is based on the aircraft's momentum through the air. If the wind increases or dies down at a rate faster than the aircraft's momentum is able to change, you'll have a change in airspeed.

You mean you've never taken off in gusty conditions and had the stall warning kick on and off? Never approached to land with wind shear and saw the ASI fluctuate? :confused:
 
In addition to all the great info above, don't forget that in that 10 kt direct gust you will also get increased lift, (increased airspeed = increased lift), which can throw a wrench in a steady approach, especially when you are a few feet off the runway.
 
Threefingeredjack said:
In addition to all the great info above, don't forget that in that 10 kt direct gust you will also get increased lift, (increased airspeed = increased lift), which can throw a wrench in a steady approach, especially when you are a few feet off the runway.
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Followed by the BIG SMACK. I remember my instructor trying to demonstrate a "nice soft landing" when a wind gust picked the plane up and then it smacked into the runway. He looked at me and said "ok, I was demonstrating one of YOUR landings."
 
Sometimes gusts or even thermals on the runway will challenge the best approach. I will fly a little faster, only for a faster response rate from the control surfaces, just in case.
There is definitely that moment when the airplane has to stop flying, but usually you can "fly it on" just that little bit more... it's just a question of how good you are with that airplane at higher speeds with all 3 on the ground.
 
So the airplane gets bumped around? What does it know? Its all how humans respond.

I had an instructor tell me once that if you fly the airplane in the envelope it simply does not matter. Its all about the relative wind . . .
 
Lets not forget the "target fixation" that happens when it gets gusty. Often the correct reaction to a strong gust is to freakin' pour the coals to it and GO AROUND, but people want to land so bad they think there's a magic bullet in "fly a little faster".

Fly normal approach speed and if it gets squirrelly, go around and set it up again. If you're not comfortably holding the desired glidepath or attitude at 1.3 Vso (or whatever your normal VRef is), then it's time to put some airspeed on, climb, and come back and try it again and see if the gusts and timing will cooperate.

And if they really won't, it's time to exercise that PIC brain and go somewhere else more suitable for a landing in such conditions.

Flying faster just makes the crunch of the aluminum louder at the stopping point.
 
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