Wind Measurements

jnmeade

jnmeade

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Jim Meade
If you are flying a small GA airplane such as a C172, Piper Cherokee 6, or Piper Cub, how do you apply the wind information you receive at an airport to the wind guidance found in your POH?

Many of us are most concerned about wind when we are on the ground or transitioning between ground and air. We learn how to taxi in wind and we learn how to crab or slip to compensate for wind on landing or take-off.

What about the wind reports? Do they accurately reflect wind conditions as they influence operations of the Citabria, the Apache and the RV7?

"Before ASOS, airport wind sensors were generally
exposed 20 feet above ground level. With modern, high performance
aircraft, this standard no longer applies. Now,
current federal standards for siting meteorological equipment
specify (with some variance permitted) a height
of 10 meters (32.8 feet). Typical ASOS wind sensor heights
are 33 feet or 27 feet, depending on local site-specific
restrictions or requirements."
http://www.nws.noaa.gov/asos/aum-toc.pdf

You're flying your Super Cub out of Billings, MT. What does the wind 33' in the air have to do with you, who are maybe 7-8' in the air?

" In flat grassy areas (with low grass height) at standard temperature this change in wind speed as a function of height can be approximated using the formula: V= Vref(H/Href)0.142"
http://belfortinstrument.com/height-wind-measurements-ground/
"As can be seen on this chart wind speed of 12 knots measured at 5 meters above the ground will be 13.3 knots 10 meters above the ground and 8.2 knots 1 meter above the ground due to surface friction creating a boundary layer of air near the surface of the earth." [I'm not smart enough to know how to show the chart - see the URL.]

This looks to me like the ASOS wind of 26 knots is more likely to be 16-17 knots at wing height on your Piper Warrior and maybe a little more on your high wing C182.

Does this mean that your buddy who bragged that he could consistently and safely land in a cross-wind higher than the POH was actually perhaps within the POH if one corrected for the height of the anemometer the wind measurement was based on?

Some of you experienced weather guys jump in here and extrapolate or correct this question.

NB. I'm a farmer and many herbicide labels prohibit application in winds over 10 mph. The wind speed is commonly obtained from the nearest ASOS so a 12 mph ASOS reading taken from 33 feet above ground may well be within the legal limit at boom height. You can see why this is an item of interest.
 
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You need to ask your instructor to explain crosswind component and then teach you that an airplane's demonstrated crosswind numbers are in no way a limitation but are a statement of what the manufacturer demonstrated during flight trials.
 
The POH shows 'demonstrated' x-wind from the manufacturer. That is not a 'limit'.
I look for higher wind days to do x-wind work with students (I'm one of those masochistic instructors :eek:)
Actual winds at the runway can vary a LOT from the 'reported' winds, especially if there are buildings surrounding the runway. A windsock on the ground near the runway is very helpful - but not always/often available.
Note that the formula works for 'flat grassy areas'. If that doesn't describe your airport, all bets are off!
The only way to judge definitively, is to do the landing approach and see if you can handle the actual winds (even if it is 5 Kts :rolleyes:), and be prepared for a go-around.
 
DGlaeser said:
The POH shows 'demonstrated' x-wind from the manufacturer. That is not a 'limit'.
I look for higher wind days to do x-wind work with students (I'm one of those masochistic instructors :eek:)
Actual winds at the runway can vary a LOT from the 'reported' winds, especially if there are buildings surrounding the runway. A windsock on the ground near the runway is very helpful - but not always/often available.
Note that the formula works for 'flat grassy areas'. If that doesn't describe your airport, all bets are off!
The only way to judge definitively, is to do the landing approach and see if you can handle the actual winds (even if it is 5 Kts :rolleyes:), and be prepared for a go-around.
Click to expand...

^^^^^ What he said. Aside from buildings, trees affect the wind too.
 
Not exact, but easier than an exact calculation on final....

30 degrees or less use 50% of the wind speed
45 degrees use 75%
60 degrees or more use 100%
 
Another issue from a "can I handle it" perspective is that important aspects of wind behavior aren't always reflected in the ASOS report. All you are given is the steady wind speed, wind gusts, and range of variability (120V240). Turbulence is not reflected in those values, though if you know the terrain you can sometimes infer it. I'm not sure over what time range the variability report is taken from, but I've seen conditions where the wind, though not particularly strong, was continually changing direction by over 90* more than once per second, usually due to trees and other obstacles on the ground. That kind of behavior, even if it is reported as only 07G14KT, is A LOT harder to handle than 15G25KT from a constant direction, even if it's a direct crosswind.
 
It's not a limitation and in the hands of a skilled pilot, current with the plane, can be well exceeded.

Also keep in mind, especially in something like a 7ECA, the paved runway isn't your only option for landing, taxiways or flat soft surfaces are good options, depending on the folks on the grounds feelings about it and your assessment of the conditions.
 
Mike5250 said:
Not exact, but easier than an exact calculation on final....

30 degrees or less use 50% of the wind speed
45 degrees use 75%
60 degrees or more use 100%
Click to expand...

I was taught the same. Think of a clock. 30 minutes is 1/2 an hour, so 50%. 45 minutes is 3/4 an hour, so 75%. And of course 60min is 100% of an hour
 
Thanks to the comments I edited the OP to remove references to "limits" and "legal".

My main interest in this thread is to discuss what information the pilot has available and how to apply it.

Anyone care to share thoughts on how best to interpret ASOS wind information for your own type of plane?
 
jnmeade said:
Thanks to the comments I edited the OP to remove references to "limits" and "legal".

My main interest in this thread is to discuss what information the pilot has available and how to apply it.

Anyone care to share thoughts on how best to interpret ASOS wind information for your own type of plane?
Click to expand...


It's a ASOS, so it's not always 100% in the first place

I'll keep a eye on any smoke, water, tree tops, my GPS track and GS, vs DG and TAS.

For ground ops I'll usually give it a shot no matter what, shy of it getting super sporty as I get closer, water operations I don't mess with much about 15kts and even then it often is ruff water so I don't NEED to subject my plane to that, so I don't.

I've have my plane for just shy of a year now, but I'm still getting used to her. My last plane I was VERY comfortable with, landed in some very major crosswinds, the ground handling was harder than the landing.

For major cross winds I also like to pull my flaps when I touch down, works well with manual flaps, which the only kind of flap I'll have in a personal plane, I also have found tailwheel planes are easier for me to handle major crosswinds in, especially in a 2 point configuration.
 
I don't try to micro analyze the winds from ASOS reports other than to know which runway to set up for. When I get there, I deal with the winds as they are.
 
Who really cares what the wind is, measured 20' or 10' or 5' above the ground making a 15 kt wind 13.3 or 7 kts on the surface, think of it only as a general guidance.
What really matters is what it's doing to you RIGHT NOW depending on WHERE you are.
 
The reported wind might vary significantly from the actual wind at surface level. But how consistent is that? If you know the ASOS reports 20 kts, does it really matter that much that the actual surface wind at the Hotel intersection in a Warrior is really 16kts? I don't know, and I don't know too many people that really bother that much. If it's consistent, and the numbers say 20, and you always feel the same amount of wind when the number says 20, haven't you essentially recalibrated yourself to that value?

In the case of herbicide applications, then yeah, you probably do want a micro (local spot) measurement, rather than an ASOS that's 15 miles away and 33' in the air. At boom level there are probably so many terrain variables that the ASOS is more of a general guide to conditions than a useful measurement.
 
Our AWOS may be 180deg off from what the runway is seeing. We have a flag near the AWOS equipment and it's typically pointing in a completely different direction from the windsock. Our AWOS has also been reporting -RA almost all of the time for years.
 
What matters is do you have enogh rudder to align the airplane with the runway. The ASOS is a helpful indicator. If in doubt, go around!

I bet the wind isn't very intense below the tops of the wheat either...
 
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One of the remarks in the A/F D for KOSC:
"DURG DALGT HRS TAILWINDS MAY EXIST OVER APCH ENDS RY 06 & 24 SIMULTANEOUSLY."
 
But OSC is long enough at 11,800 the tailwind issue is non-existent.
 
For the pilot that says he landed above the demonstrated airspeed, remember the original demonstrator was probably using the local ASOS as well.

Also, landings start above the ASOS height and land through it so the ASOS wind value was part of the landing. The wind speed at the surface is zero and probably higher than the ASOS above the ASOS so a pilot can pick any value in between as his/her demonstrated value.
 
Sac Arrow said:
I don't try to micro analyze the winds from ASOS reports other than to know which runway to set up for. When I get there, I deal with the winds as they are.
Click to expand...

Exactly. Last weekend, my local airport doesn't have a weather station. But KPSM does, and its only a few miles away. They were reporting 12knts crosswind, no gusts. But when I got to my airport and overflew it for a look, the windsocks were flat. In the pattern I could tell the wind was blowing us pretty good, and we had a decent crab to stay on course. Felt like it was gusting, with some up/down drafts. I took that into account on downwind, base, and final. But as we got down below the trees, it blocked the wind, and the landing was almost like a windless day.

It doesn't always do that. There's a gap in the trees midfield where the hangers and FBO are. Sometimes you have no wind as you round out (trees), and then right as you touch down you get a crosswind (no trees), and on rollout the wind goes away (hangers).

All landings are different. Even when you're just doing laps in the pattern, they can change.
 
Honestly, gusts are FAR more of an issue than steady wind, unless that wind gets near the stall speed.

I've landed in a 25 knot steady crosswind, easy peasy. I've also landed in 10G20, and it's a lot more challenging, even if it's strictly a headwind. The real sphincter clencher is when a long gust suddenly gives up at 20 AGL. I once had that happen when I was stupid enough to try a short field approach. Suddenly losing 15 knots of airspeed over the fence is not good.

Gusts are not always reported, and when they are reported, may not be accurate. You have to feel them.
 
MAKG1 said:
Honestly, gusts are FAR more of an issue than steady wind, unless that wind gets near the stall speed.

I've landed in a 25 knot steady crosswind, easy peasy. I've also landed in 10G20, and it's a lot more challenging, even if it's strictly a headwind. The real sphincter clencher is when a long gust suddenly gives up at 20 AGL. I once had that happen when I was stupid enough to try a short field approach. Suddenly losing 15 knots of airspeed over the fence is not good.

Gusts are not always reported, and when they are reported, may not be accurate. You have to feel them.
Click to expand...

SOP around these parts. If crossing gusts and mechanical turbulence aren't enough? I've overflown my cabin strip and watched the windsock rotate 360*. I've seen it many times while on the ground. A couple of weeks ago the winds for departure were blowing about 60* off heading (narrow runway) at estimated 25G40. Trees had been falling all weekend. Judging the wind from the ground while in the lee of trees is harder than setting up to land. If you don't like the set-up you can go around several times hoping to time the gusts right to allow a landing. You only get one chance at take-off.

There's no substitute for horsepower.
 
Someday someone will invent a portable radar based wind gun. Just point it, click it and get a wind reading between you and an object you are pointing at.
 
JoeSelch said:
At Oscoda KOSC, I wouldn't be able to tell if the socks were pointing at another. :)
Click to expand...

At BJC it's relatively easy to see the windsocks at both ends. And it is all too common to see them pointing at each other. Lots of runway so no big deal for those of us in spam cans. The jets get a little picky about it sometimes...
 
cgrab said:
For the pilot that says he landed above the demonstrated airspeed, remember the original demonstrator was probably using the local ASOS as well.

Also, landings start above the ASOS height and land through it so the ASOS wind value was part of the landing. The wind speed at the surface is zero and probably higher than the ASOS above the ASOS so a pilot can pick any value in between as his/her demonstrated value.
Click to expand...

I'd imagine any pilot with say 100hrs has landed above the max demo at least a few times. Really is much to do about nothing.
 
MAKG1 said:
Radar can't see wind unless it contains something to reflect the radar. Like precipitation.
Click to expand...

Or like atmospheric "bubbles" (probably seen it when there is "mirage"). Or maybe turbulence creates enough of a boundry layer.

Or (from wiki)
The new generation of sensors use sound waves to measure wind speed and direction. The measurement is based on the time it takes for an ultrasonic pulse to travel from one transducer to another, which varies depending on - among other factors - the wind speed. The transit time is measured in both directions for several (usually two or three) pairs of the transducer heads. Based on those results, the sensor computes wind speed and direction. Compared to mechanical sensors, the ultrasonic sensors offer several advantages such as no moving parts, advanced self-diagnostic capabilities and reduced maintenance requirements.
 
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Stewartb said:
You need to ask your instructor to explain crosswind component and then teach you that an airplane's demonstrated crosswind numbers are in no way a limitation but are a statement of what the manufacturer demonstrated during flight trials.
Click to expand...

Nice typical POA turbodick response, but I don't see where he claimed the POH's DCC was a limitation. :dunno:
 
For me, I take the wind information as a general sense of the wind vector. I do calculate the xwind component when winds are high enough for it to be significant, but that is more for me to compare to my own experiences "have I ever...". I do consider the book's demonstrated xwind component and vs my own personal limitations.

I don't obsess over what the book might say one way or another because the only thing that matters is what I and getting on final.

The wind info sets up my mental picture. For example, "OK, Landing 17R, winds 230 at X gusting Y. Strong xwind from the right, expect shear and turbulence due to obstructions over there, carry x knots for gust factor, be ready for a GA if it gets unstable."

What I get on final is what I get on final regardless of what the ASOS reports.
 
petrolero said:
What I get on final is what I get on final regardless of what the ASOS reports.
Click to expand...

This is important.

Once, when heading into Half Moon Bay, ASOS reported a 10 knot left crosswind (on 30).

On downwind at TPA, I needed a strong LEFT crab. At the same time, I could see the windsock indicated wind from the RIGHT (like ASOS said).

The conclusion? Wind is going to shift at some point, possibly with some turbulence, below TPA. Sure enough, it came about at 500 AGL, over the harbor. Not that much turbulence, but the climb out after that was "fun." Strong rolling moment just off the DER. I could have sworn it was a wake turbulence encounter, except the pattern was empty but for me.

This was all interplay of the sea breeze with terrain. It rolled over the top at TPA.
 
Just because a weather station says something and a POH has a figure doesn't really mean squat. There are far too many variables in terrain, weather, and even weather station currency and accuracy to say for sure based on just two data points

The only way you'll really know what it safe for you is by starting out low and slowly pushing further and further with experience.
 
champ driver said:
Who really cares what the wind is, measured 20' or 10' or 5' above the ground making a 15 kt wind 13.3 or 7 kts on the surface, think of it only as a general guidance.
What really matters is what it's doing to you RIGHT NOW depending on WHERE you are.
Click to expand...

My observation is that people who live 45 minutes from an airport frequently use the ASOS winds to determine whether to drive over for a flight. The difference between 25 knots and 15 knots might change their mind about investing in the trip.
 
48dodge said:
Our AWOS may be 180deg off from what the runway is seeing. We have a flag near the AWOS equipment and it's typically pointing in a completely different direction from the windsock. Our AWOS has also been reporting -RA almost all of the time for years.
Click to expand...

Wind socks are typically 10' above the surface. Flags can be any height. We all know the wind usually changes direction and intensity with altitude and this discussion is just about that.
 
cgrab said:
For the pilot that says he landed above the demonstrated airspeed, remember the original demonstrator was probably using the local ASOS as well.
.
Click to expand...

Why do you say that? I would be surprised because it would mean the manufacturer artificially lowered the real demonstrated x-wind component. Why say the limit was 15 when it was really 20?

Any major manufacturer test pilot here who can talk to that?
 
Sac Arrow said:
I don't try to micro analyze the winds from ASOS reports other than to know which runway to set up for. When I get there, I deal with the winds as they are.
Click to expand...

Agree with the post above.
 
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