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  #1  
Old 12-24-2007, 09:33 PM
CC CC is offline
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How does temperature affect an airplane's take-off distance?

In a rant in Newsweek, a writer goes on about how global warming is going to mean less dense air, and consequently, we'll need to build longer airport runways, since planes need to run a longer distance before they lift off in less dense air. I imagine we could get a little idea of how accurate that assertion is by looking at take off distances now, comparing cooler weather with warmer weather. Since yearly temperatures can vary a great deal - at O'Hare, temps can go as low as -15 almost every winter, and we get above 100 occasionally, too, we have a pretty wide range over which to examine take off distances. About 100 degrees at least. Do pilots at O'Hare, or anywhere, have to take this into consideration? What's the relationship between temperature and the distance an airliner takes to lift off? I have never heard of such a relationship and I wonder why we all don't notice that we taxi longer in winter, if this is so.
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  #2  
Old 12-24-2007, 10:06 PM
Santo Rugger Santo Rugger is offline
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Here's a chart that shows the relationship between temperature, windspeed, and altitude:

http://virtualskies.arc.nasa.gov/wea...eoffChart.html

Here's one explaining how pressure can be related to altitude:

http://www.pilotfriend.com/training/...g/aft_perf.htm
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  #3  
Old 12-24-2007, 10:07 PM
Desert Nomad Desert Nomad is offline
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You don't taxi longer in the winter because you always start the take of roll at the end of the runway... just that if it is 100F outside you'll roll a lot further down the runway than if it is 40F. And yes, they do take this into account.

In the summer in Reno, Denver etc (high altitude) there are sometimes weight restrictions that cause passengers to get bumped and they leave with empty seats to reduce weight.

Last edited by Desert Nomad; 12-24-2007 at 10:07 PM..
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  #4  
Old 12-24-2007, 10:15 PM
Napier Napier is offline
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There was a stretch not too long ago when Sky Harbor airport in Phoenix was closed for several days in a row because the temperature wsa over 120 C and the charts don't go that high. If I remember right, that is.
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  #5  
Old 12-25-2007, 08:54 AM
LSLGuy LSLGuy is offline
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Former airline pilot ...

Air temperature very definitely affects takeoff performance. All else equal, hot air is less dense. The result is that you must be moving faster through it to generate enough lift to take off. Unfortunately, the less dense the air, the less power the engines develop, so it takes even longer to to accelerate to the needed faster speed.

In the free atmosphere, it gets colder as you go higher at a pretty uniform rate (ballpark 2 degrees C/1000 feet). But when the ground is at a high altitude (e.g. an airport in the mountains), the temps can be much higher than you'd find at the same altitude over the plains or an ocean. For example, Albuquerque in the summer can be over 100 degrees F at ~5400 feet while over the plains or the ocean the expected temp is more like 40 F.

The total effect of all of this is longer runways are required at places where the weather gets hot, or where the ground is at high elevation, or worse yet, both.

The effect as seen by the passengers is just a longer than average takeoff roll, followed by a shallower climb-out.

For jet airliners, the apparent effect is somewhat reduced by the common practice of using less than full takeoff power unless needed. So when it's not too hot and the runway is long enough, and we're not carrying too much weight, we'll compute a lower thrust setting which is sufficient for the conditions and take off using that. The result is a longer takeoff roll than would be the case if we used max power all the time.

So as a passenger, you'd see most takeoffs from most runways between about 60 & 90 degrees feeling about the same to you since we're in effect adjusting the thrust to make them feel the same.


Returning to the article referenced by the OP, that person is a fool. Global warming is talking about a degree or two here or there, where even though the average impact on the climate may be huge, the actual day to day temp increment is not so much. And so the impact on air operations will likewise be minimal. If indeed daily highs became routinely 20F hotter than they are today, that would become an issue for many airports.


And Napier, you remember correctly. Both Boeing & Airbus publish takeoff data up to 50C = 122F. So if the surface temperature exceeds that, you can't legally compute your speeds, run distance, thrust required, etc. So you can't legally take off.

When that happened, Boeing offered to sell the airlines the next 10 degrees of data for an extortionate price; a lot of goodwill was lost that day in PHX.
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  #6  
Old 12-25-2007, 09:13 AM
beowulff beowulff is online now
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Quote:
Originally Posted by Napier
There was a stretch not too long ago when Sky Harbor airport in Phoenix was closed for several days in a row because the temperature wsa over 120 C and the charts don't go that high. If I remember right, that is.
Not several days.
A few hours.
I live here, and that was the famous "122.6 F" day.
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Old 12-25-2007, 09:34 AM
CC CC is offline
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Quote:
Originally Posted by LSLGuy
Former airline pilot ...
...So as a passenger, you'd see most takeoffs from most runways between about 60 & 90 degrees feeling about the same to you since we're in effect adjusting the thrust to make them feel the same...Returning to the article referenced by the OP, that person is a fool. Global warming is talking about a degree or two here or there, where even though the average impact on the climate may be huge, the actual day to day temp increment is not so much...
Yes, that was my point. Even if, as you confirm, there is a relationship, it's minor and can be accounted for (between 60 and 90 it feels about the same). The writer in that article made a number of nonsense claims, but that was one I figured I could verify fairly easily on the good ol' SDMB. Thanks. (bolding mine) xo, C.
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Old 12-25-2007, 10:35 AM
Desert Nomad Desert Nomad is offline
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Quote:
Originally Posted by LSLGuy
And Napier, you remember correctly. Both Boeing & Airbus publish takeoff data up to 50C = 122F. So if the surface temperature exceeds that, you can't legally compute your speeds, run distance, thrust required, etc. So you can't legally take off.
This is also why Dubai airport (and others around the Gulf... esp. Sana'a which is at 7000' or so) are most busy between 1am and 3am, and have little traffic in the middle of the day.

Last edited by Desert Nomad; 12-25-2007 at 10:36 AM..
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  #9  
Old 12-25-2007, 10:57 PM
Raguleader Raguleader is offline
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Quote:
Originally Posted by LSLGuy
Former airline pilot ...

Air temperature very definitely affects takeoff performance. All else equal, hot air is less dense. The result is that you must be moving faster through it to generate enough lift to take off. Unfortunately, the less dense the air, the less power the engines develop, so it takes even longer to to accelerate to the needed faster speed.
I had a teacher in high school who used to fly the big B-52 bombers. He said that at one point, they had retired all but the latest model of B-52 (the H series, IIRC), only to find that the turbofan engines used on them couldn't generate enough thrust to allow them to operate out of one of their main bases in North Dakota due to the altitude. The easiest solution, it turned out, was to un-retire some of the turbojet-equipped B-52s to fly out of that base until the turbofan engines could be improved and refitted.

Mind you, I don't know if high temperatures are ever a problem in Minot, North Dakota. Anybody know if it gets hot there in the summer? I hear the winters are something of a shock for the airman on his first duty station after training in Texas and Mississippi.
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  #10  
Old 12-26-2007, 08:02 AM
LSLGuy LSLGuy is offline
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Quote:
Originally Posted by CC
Yes, that was my point. Even if, as you confirm, there is a relationship, it's minor and can be accounted for (between 60 and 90 it feels about the same). The writer in that article made a number of nonsense claims, but that was one I figured I could verify fairly easily on the good ol' SDMB. Thanks. (bolding mine) xo, C.
To further clarify (or maybe muddy) ...

At any given airport, the elevation is fixed & the runway length is fixed, net of a lengthy & costly construction effort that may or may not be politically achievable.

Depending on the types of aircraft that frequent that airport, and where they are going, that airport may have runways much longer than necessary, or just barely long enough. As the world globalizes and more long-haul (read big & heavy) aircraft serve more minor cities, the runway criticality may increase.

There are airports & aircraft today where the combination of runway length, altitude, peak summer temps, and desired destination & payload almost work. Or almost don't. The airlines either leave a few seats empty, carry a little less fuel than normally desirable, or put a higher performance aircraft on the route.

Made up examples: Right now you couldn't serve London from Des Moines at all because Des Moines' runways are too short. But you could serve London from Charleston, SC, at least on days when the temp is below 85F. If over 85F occurs just 10 days a year and none exceed 90F, you can make that work economically. But if it's 60 days a year over 85F and 25 of those above 90F, probably not.


To the degree GW increases peak temps, these ciritcal situations will increase in both number of affected days and number of city combinations.

My understanding of GW is that it won't manifest as major increases in daily highs. So I don't see even the near-term-doom-scenario version of GW as posing a gut-buster for air commerce.

A countervailing factor is that newer aircraft are generally a little higher powered than older designs. Airports sized for the long takeoff rolls of 727s & 747s are generously sized for the later 737, 767, 777 & corresponding Airbus products.

This is an integrated industry. Boeing does not design machines without considering the airport & climatic environment in which they operate. Despite all the hoopla about the A380 needing airfield improvements wherever it goes (a gross exaggeration), the airplane has a lot of design compromises to fit almost completely within the existing infrastructure.

If summer temps throughout the US spiked by 15F for three months starting in 2009 we'd have a mess. But as I understand the GW predictions, that's just not the case.
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  #11  
Old 12-26-2007, 12:09 PM
Sam Stone Sam Stone is online now
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No one is predicting 15 degree temperature increases. The most we're talking about is a global average temperature increase of between 1.5 and 4.5 degrees C, over the next hundred years. Over the lifespan of the current aircraft fleet, it would be less than half that.

And most of that increase is projected to be increased night-time temperatures.

No one flies commercial aircraft in with tolerances so tight that a difference of 2 degrees would result in a runway that's too short. There's far more margin than that. Hell, there's more variance than that just in the temperature between the monitoring station and the runway itself.

And if it did make a real difference, it wouldn't mean building new airports - it would mean recertifying aircraft so that their gross takeoff weight was slightly lower. No infrastructure changes at all. And 99% of all aircraft movements are aircraft that only need a fraction of the runway they are taking off on. The runway is sized for the largest aircraft that needs it, and most aircraft are smaller.

The author was engaging in speculative scaremongering without a shred of truth in what he was saying.
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  #12  
Old 12-26-2007, 05:36 PM
Richard Pearse Richard Pearse is offline
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Quote:
Originally Posted by Sam Stone

And if it did make a real difference, it wouldn't mean building new airports - it would mean recertifying aircraft so that their gross takeoff weight was slightly lower. No infrastructure changes at all.
It wouldn't even require that. All that would happen is that the manufacturers would produce performance data for the higher temperatures. The new data would involve lower climb weight limits and may or may not put a lower runway and obstacle weight limit on the aircraft depending on the particular runway in question. But it's worth noting that this would just be an incremental change from the limits already imposed by the high temperature extremes on the current charts.
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  #13  
Old 12-26-2007, 10:13 PM
DSYoungEsq DSYoungEsq is offline
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Quote:
Originally Posted by Sam Stone
The author was engaging in speculative scaremongering without a shred of truth in what he was saying.
His punishment should be to be put in a plane on a treadmill. In hot air.
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