Winds and runway selection. Which one and how do you know?

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Still Learning
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Winds and runway selection. Which one and how do you know?

Post by Still Learning »

Hiya,
I have a METAR that I printed out from aviationweather.gov/metar from the 3 APR @ 2111 UTC.

For those of you that can read the language of METAR, that's great, I however need what they call the"Decoded" version. (fortunately, for me they exist.)

Here goes: KLAS 032056Z 04009G16KT 10SM SCT250 22/M15 A3003 RMK A02 SLP151TO2221150 58013

Out of all that mess, the operative part is, 04009G16KT, relative to my question, anyway...In other words, that translates out to:Winds are from 40* at 10 MPH or 9 KTS Gusting up to 18 MPH or 16 KTS.

So, KLAS, has Runways: 19/1 L&R, and 8/26 L&R. So, given that report, (if you were in an MD-80, ) vs, a 172, which RWY would you take and why? Could you provide an example?

Thanks in advance,

Still Learning
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HighBypass
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Re: Winds and runway selection. Which one and how do you kno

Post by HighBypass »

A quick rule of thumb is always pick the runway heading closest to the wind direction i.e. avoid a tailwind whenever possible, yet the selection also depends upon the runway's suitability for the aircraft you are flying. The MD-80 would not in all likelihood take off from a grass strip :)
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Piper_EEWL
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Re: Winds and runway selection. Which one and how do you kno

Post by Piper_EEWL »

Yes you usually pick the runway that gives you the most headwind component. In your example runway 01.generally there’s no difference in this concerning airplane size. But there might well be a difference between VFR and IFR since in IFR it might depend on which approaches are available for each runway.

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Re: Winds and runway selection. Which one and how do you kno

Post by Still Learning »

Explained that way, I get it.
But ever since I was knee high to a duck, I thought...here let me give you an example:
Here I am on a lazy Sunday afternoon sitting outside in my favorite lawn chair. All of a sudden, a nasty wind starts blowing out off my left shoulder, which just so happens, to be aligned with North. and I know, the wind is coming from up the block (North) it nearly flipped the chair over onto it's right side.

So, back to the airport... if those were the winds at the runway, out of the north, say 360 at 4 kts, why wouldn't take off from RWY18, as the wind is blowing from 360. (remember the guy in the armchair?) So to fly into the direction the wind is blowing, (out of the North) wouldn't you fly Rwy 18?

What am I missing?

Thanks guys, I appreciate the help.

Still Learning
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Oracle427
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Re: Winds and runway selection. Which one and how do you kno

Post by Oracle427 »

You want to take off INTO the wind, not with the wind. Correctly stated you want to take off upwind, not downwind.

The reason is that the wind velocity will reduce your velocity over the ground therefore requiring less runway to be used up to get up to flying speed.

To put it another way, if you start with a 30 knot wind and you are facing upwind and you need to reach 50 knots to take off, then you only need to accelerate by 20 knots to takeoff.

On the other hand, if you are facing downwind, then you are short by 30 knots, so you must gain 80 knots just to reach liftoff speed. You will use up a huge amount of runway trying to achieve that in a light aircraft, while in a heavier aircraft you may just run out of runway altogether.

Now all that said, keep in mind that the airplane has no idea what the air velocity over the ground is. All the airplane knows is that it needs 50 knots of velocity through the air to break ground.

I've had fun landing a 182 on days when the wind are steady steady down the runway at nearly 30 knots. It feels like you are descending vertically to the surface because the airplane is flying through the air at 65 knots, yet is only moving over the ground at around 35. It allows you to use very little runway during the landing. Does that make sense?
Last edited by Oracle427 on 06 Apr 2018, 17:06, edited 2 times in total.
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Piper_EEWL
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Re: Winds and runway selection. Which one and how do you kno

Post by Piper_EEWL »

You always takeoff and land to the wind! Never with a tailwind. The lift that makes the plane fly is created by air flowing over the wing. The amount of lift is distinguished by the speed of which the air is flowing over the wing. To lift of let’s say you need to have 50kts at the wing. If you take off on runway 18 in your example at still stand you have -4kts at the wing. So you need to accelerate to 54kts ground speed to get 50kts at the wing. If you take off on runway 36 on the other hand you have +4kts at the wing at Stillstand. So you only need to accelerate to 46kts GS. A 8kts difference! And that’ll reduce your takeoff and landing roll drastically!

If tried in real life and trust me you can tell the difference!

PS: Oracle and I seem to have the same approach on explaining that here :wink: :lol:
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Oracle427
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Re: Winds and runway selection. Which one and how do you kno

Post by Oracle427 »

Hey Piper, great minds think alike? :p

The only other way I try to explain this is with boating. I try to get the person to envision themselves paddling across a river, upstream or downstream to help visualize the effect of the wind.
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Piper_EEWL
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Re: Winds and runway selection. Which one and how do you kno

Post by Piper_EEWL »

Oracle427 wrote:Hey Piper, great minds think alike? :p
I guess so 8)

And that boat analogy is a good one. I’ll have to remember that one!
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AKar
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Re: Winds and runway selection. Which one and how do you kno

Post by AKar »

Then also some airfields have preferred runways or even restricted runways that may require you to accept tailwind conditions. For example, sometimes given runway is used for departures for even winds up to certain magnitude from unfavorable direction due to reasons of noise abatement or traffic flow. In some other cases the reasons are more obvious, such as one end of the runway pointing directly into a hill.

In FSX/P3D, these may or may not work as expected. I understand the airport files in the sim can include runway preferences and limitations, but how these are utilized appears to vary quite a bit. Therefore it is not uncommon to receive landing instructions that are rather questionable.

By default it appears that FSX/P3D prefers landing directions that have ILS associated with. This seems to allow for couple of knots of tailwind.

-Esa

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Piper_EEWL
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Re: Winds and runway selection. Which one and how do you kno

Post by Piper_EEWL »

I have to say that thing with default ATC in FSX that annoys me the most with regard to wind and runway selection is that it changes the runway to the opposite forth and back if the wind is shifting. This wouldn’t happen in real life obviously due to traffic reasons.
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Re: Winds and runway selection. Which one and how do you kno

Post by AKar »

Yeah, that's annoying in particular with airports that should have fairly obvious preferred runway in reality. Not that much of an issue with GA planes, but if doing something like, say, FSL A320, it is fairly annoying to play the guessing game of the active runway in situations where no doubt should exist. This is even more so because the simulator lets you know of the actual runway in use only so close to the destination where you often are well into STAR and descend.

-Esa

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Re: Winds and runway selection. Which one and how do you kno

Post by Piper_EEWL »

Yes I agree. Innsbruck is a very good example for that! And I think it’s even annoying with GA when you get a clearance for rwy 08 and have a 737 coming at you from rwy 26 :shock:
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Re: Winds and runway selection. Which one and how do you kno

Post by Still Learning »

Ok, this is where I get confused.

Let's go back to my example. Winds are 040 at 9 KTS.

Lets say we're at KJFK (because they have a 4/22)

So, if the wind is blowing out at 040* wouldn't that be the same as you standing at one end of a compass rose, (that's on the floor,) with a fan. You are standing at 040, on the compass rose.

I'm standing at reciprocal, that being 220*. When you turn on that fan on HI, I should be feeling the "breeze" coming from your direction, 040 - which is directly across from me.

So, even though the winds are 040*, the optimal runway that you should use under these conditions, would be RWY 22. Not RWY 04 as you would be either taking off or landing with a tailwind.

Did I miss anything?

Thanks for helping me.

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Re: Winds and runway selection. Which one and how do you kno

Post by DHenriques_ »

Still Learning wrote:Ok, this is where I get confused.

Let's go back to my example. Winds are 040 at 9 KTS.

Lets say we're at KJFK (because they have a 4/22)

So, if the wind is blowing out at 040* wouldn't that be the same as you standing at one end of a compass rose, (that's on the floor,) with a fan. You are standing at 040, on the compass rose.

I'm standing at reciprocal, that being 220*. When you turn on that fan on HI, I should be feeling the "breeze" coming from your direction, 040 - which is directly across from me.

So, even though the winds are 040*, the optimal runway that you should use under these conditions, would be RWY 22. Not RWY 04 as you would be either taking off or landing with a tailwind.

Did I miss anything?



Thanks for helping me.

Still Learning

Yes. You've missed one word and that word is key to understanding your question.
The word is FROM. The wind is FROM 040 degrees. So if you are taking off INTO the wind what direction (heading) would you be facing?
Dudley Henriques

Still Learning
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Re: Winds and runway selection. Which one and how do you kno

Post by Still Learning »

Yes. You've missed one word and that word is key to understanding your question.
The word is FROM. The wind is FROM 040 degrees. So if you are taking off INTO the wind what direction (heading) would you be facing?
Dudley Henriques
I've always thought it was the inverse of the report, so the reciprocal 040 would be 22. You'd be facing a heading of 220*
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