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Need to understand basics of runway alignment and use

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Need to understand basics of runway alignment and use

Old 20th Nov 2019, 12:01
  #1 (permalink)  
Thread Starter
Join Date: May 2010
Location: Prague, Czech Republic
Posts: 4
Need to understand basics of runway alignment and use

I understand next to nothing about flying so these are very basic questions.

I understand that the direction of takeoff and landing depends on wind direction. However I am not sure of the basic principles at work. If, e.g. I am coming into London City how does the wind direction affect whether (coming in from Prague) we come straight in down the river, - or alternatively do that epic twirl round the Shard? - which seems to happen more rarely. Does a plane ideally want the wind behind it when taking off?

Then, questions about my now home airport, which is Prague. It has two runways. The most regularly used one has the designation 6/24. It seems to be used in both directions like the single London City one. But it also has another one, with the designation 12/30. This runs at a tangent across the main runway. Presumably it is used when the wind is in a certain direction, but does that designation indicate which wind direction would require this runway to be used?

Finally Prague Airport is talking about building a runway parallel to the main one (6/24), and it says that if they build it, then they can cancel use of 12/30, which has a flightpath taking it over densely populated areas of Prague . But then my question is, what will they do if the wind direction is such that today they would use 12/30?

Looking forward to learning stuff, many thanks in advance
RHPrague is offline  
Old 21st Nov 2019, 00:11
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Join Date: Jul 2018
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Hi - very briefly, aircraft will always prefer to take-off and land into the wind. If an aircraft has an approach airspeed of 120kt, in still air its groundspeed will also be 120kt. If it's flying into a 20kt wind, it will still fly its required airspeed of 120kt (remember it's the speed of the air flowing over the wings which is important), but its groundspeed will only be 100kt. This has a number of advantages:
1. The momentum of the aircraft as it touches down is much, much less - imagine the energy of a 100 ton object moving at 100kt vs 120kt - it's much less energy which the brakes and tyres have to stop.
2. If the aircraft is flying down a 3-degree glidepath, its vertical speed reduces as the forward speed does, in the example above it is 120*(sin3) vs 100*(sin3) or 6.28kt vs 5.23kt. Again, about 20% less vertical energy to manage.

But...if it now has a 20kt tailwind, its groundspeed becomes 140kt and so there is much, much more vertical and forward energy to manage - this makes the landing roll much longer and might mean the aircraft can't stop safely on the runway length.

On a dry runway, aircraft can cope with a few knots of tailwind. On a wet or snowy runway, when braking performance is reduced, there is much less ability to accept a tailwind.

Your example of replacing 12/30 with another 06/24 is very much a business decision by the airport - there will be a few occasions where the wind is strong and across 06/24, where 12/30 would have been better for the aircraft. The airport has to decide whether it's better to accept generally higher capacity with two parallel runways, at the risk of occasionally having crosswinds which stop some aircraft using 06/24. Such decisions will usually have very detailed analysis of prevailing winds, likely aircraft types using the airport [bigger, heavier aircraft might be able to accept more crosswind than smaller types], and so on.
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Old 21st Nov 2019, 00:27
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In general an airplane takes off and lands into the wind. They can take off and land with a tail wind but that uses more runway which is undesirable. Sometimes they have to take off or land in a particular direction due to noise abatement rules. For example if the rich people live on the north of the city, you'll take off to the south and land to the north (from the south). Or there might only be a suitable instrument approach from one runway, so you have to use that regardless of wind direction. Most large jets can typically land with a 10-15 knot tail wind. Sometimes you may have to fly an instrument approach to one runway, then circle around and land on the opposite runway.
As for runway orientation, in an ideal world they would point towards the prevailing wind, but that's not always possible and airport designers have to work with what they've got if they're in a valley for example or on an island. More runways gives more options, both with wind direction and the amount of traffic they can receive. Some airports use one runway for departures and the other for landing to increase flow. Runways come in literally all shapes and sizes: I, X, L, ∆, > as well as parallels like 13L/13R or even 10L/10C/10R. Often there will be a preferred runway that is longer and a crossing runway that is shorter and not suitable for large aircraft. The wind rarely blows straight down the runway, so if you are landing to the east with a wind from the north - you just have to accept that you're landing with a crosswind.
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Old 21st Nov 2019, 11:07
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flightcatcher rudestuff Thank you so much for your answers, they are very helpful

That of course explains why a Prague flight to London (City or LHR) usually comes in straight down from the east, as previaling winds in London are more usually from the West. I sort of suspected that, but did not quite appreciate why. Got it now!

Edit..on reflection..another dumb question. I now understand well why a plane wants to land into the wind. But how does taking off into the wind assist it?

Last edited by RHPrague; 21st Nov 2019 at 16:30.
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Old 21st Nov 2019, 17:12
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Think about how much runway you would need when taking off - The important factor for lift is air speed over the wing... if you take off with a tail wind you will need to acellarate to the wind speed just to be at zero - that is all wasted runway... To achieve your take off speed into wind will take much shorter runway distance than zero wind.....and consequently even less than when downwind....
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Old 21st Nov 2019, 18:11
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Ask any Navy pilot who flies off carriers ...
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Old 22nd Nov 2019, 02:31
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When an aircraft is on the ground, with a headwind, that's airspeed that it already has. It doesn't need to be accelerated to the same degree to reach lift off speed, so a headwind will make the takeoff roll shorter. A tail wind has the opposite effect.

When landing, a headwind will reduce the ground speed, and means that the aircraft has less energy relative to the ground to get rid of, so less heat in brakes, shorter landing roll etc. A tailwind will add to the groundspeed, and means we'll need more runway. (There is normally a limit on acceptable tail winds placed by the manufacturers which is generally 15 knots).

A crosswind during take off will try to push you off the runway, so you'll sometimes need quite large control inputs to keep the aircraft straight. Those inputs will generate drag, and will reduce the takeoff performance, though not dramatically. You need to be careful rotating the aircraft as you will lose the control that having some wheels on the ground gives. Generally, though, not a big deal. Landing is a bit different. You can see aircraft appearing to 'crab' sideways towards a runway, but that's not exactly what they are doing. We turn into the wind, so that the resulting drift gives us a track down the runway. Every time there is a change in the wind, we have to turn to both fix the effect that the change had (it's moved us in one direction or the other before we can correct it), plus we have to then reestablish that correct track. At the crosswind limit, the drift will be around 15 degrees.

Just before touchdown we have to flare the aircraft. Once that's done, you need to use rudder to align the aircraft with the runway. That rudder input has to be done smoothly, as a secondary effect of any rudder input is roll, and you don't want any roll near the ground, and in particular, you don't want any towards the downwind side (and that's the effect that the straightening rudder will have). Of course, as soon as you even start to straighten the aircraft up, you're cancelling out your wind correction, and the wind will start to take you towards the downwind side of the runway. In smaller aircraft, that can corrected by a little bank in the opposite direction, but it's not really an option on the big quads because of engine ground clearance (pod scrapes). On the Boeings, you can actually land with all of the drift still intact, and whilst a bit uncomfortable at the back of the aircraft, it is a very viable technique. An Airbus driver might be able to confirm, but I think they are limited to 5 degress of drift at touchdown (???).

Even once down, the wind is still trying to push you to the side of the runway, so you may have to use quite large and abrupt control inputs to ensure the aircraft stays where you want it.

And all of this becomes much more interesting in the wet....

So, after all that, headwind is generally best. Light tailwinds are fine, but can become limiting very quickly, and crosswinds can be hard work.... but fun.
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Old 22nd Nov 2019, 04:08
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Originally Posted by RHPrague View Post
flightcatcher rudestuff Thank you so much for your answers, they are very helpful

That of course explains why a Prague flight to London (City or LHR) usually comes in straight down from the east, as previaling winds in London are more usually from the West. I sort of suspected that, but did not quite appreciate why. Got it now!

Edit..on reflection..another dumb question. I now understand well why a plane wants to land into the wind. But how does taking off into the wind assist it?
The aircraft wings function by wind rushing past them, someone mentioned a take-off speed of 120kt and a 20kt wind speed, if that wind is directly down the runway the in to it the aircraft would only need to achieve 100kt for 120kt, if taking it as a tailwind the aircraft would need to achieve 140kt for 120kt, 40kt is quite significant particularly on such a length of runway as your mentioned LCY.

I recall one DC10 that took off out of PIK with a tailwind, they took out the approach lights at the other end as they blasted past, on the other hand I've knon DHC6's get airborne when they've only been attempting to taxy in high winds.
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Old 22nd Nov 2019, 06:10
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wetbehindear is offline  
Old 3rd Dec 2019, 11:19
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Location: Prague, Czech Republic
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Fascinating explanations, thank you so much!

Further one coming, concerns Prague again, and based on what I have learnt here. The airport has put out a blast of PR about its plans for the second -parallel- runway, and to quell criticism due to the new runway being closer to the built up area than the current main one, they have asserted that the new one will be used mainly for landings.

But I then wonder what that implies about the overall operation if they have the two parallel runways? I think its true that the prevailing winds are westerly. So I suppose, that they see both runways being used most of the time, and both in the same direction, correct?

But if the winds shift, and we get a Beast from the East - and that does happen - they will have to either use the new runway for take-offs, or, in order to deliver their "promise" not use the new runway until the winds allow it to be used for landing again.

Is that a fair summary of the likely plan?
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