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Why do takeoffs at high altitudes take longer ???

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  • ATLcrew
    replied
    This should be good...

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  • Quebec Golf
    replied
    Originally posted by shinavn View Post
    Hi,
    I'm a Denver native, and this is something that I'm very curious about. Does it have to do with the lower oxygen density? And, does that have something to do with the way the engines run, or the amount of lift an airplane can get???

    Thanks
    It's both. Lift and thrust both suffer.

    1. Lower air density (not oxygen) means a higher speed is required to achieve the same lift. This higher speed requires a longer time spent accelerating, which translates to a longer distance traveled over the runway

    2. The effect of altitude on turbofan engine thrust is a bit more involved (thermodynamics, flat ratings, operational practices, turbomachinery speed margins etc etc) but as a rule higher altitude will mean less thrust. Remember that jet engines produce thrust by accelerating a mass of air, and there is less air mass available at higher altitudes. Less thrust results in a lower rate of acceleration, again adding to the time spent accelerating and therefore the distance traveled over the runway

    Alex

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  • Michael Rodeback
    replied
    Yes, it does have to do with the air density. I'm not an expert on the subject so I don't recommend quoting me on this, but it's sort of like trying to big, heavy ball down the sidewalk with one hand tied behind your back. It takes more time and effort to get the ball rolling than it does with two hands. The idea behind rotation is to get more air hitting the bottom of the wing to push it up into the air. So if you take away some of that air it's like taking away that second hand. I remember watching an Embry Riddle training video on YouTube where they did some takeoff performance calculations. With the environmental factors they used, a Cessna 172 taking off from Daytona Beach took about 500 feet, the same factors at in Prescott, Arizona took about 800 feet. I also went planespotting in Seattle recently, which is 4,000 feet lower than my hometown of Salt Lake, and I noticed that the pitch used on short final into SeaTac was about 4-5 degrees lower than it usually is when they come into Salt Lake.
    I don't have time to watch the whole video because I have to get up early in the morning, but this one looks pretty good. I apologize if it isn't. https://www.youtube.com/watch?v=LINk0lBJwBo

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  • Why do takeoffs at high altitudes take longer ???

    Hi,
    I'm a Denver native, and this is something that I'm very curious about. Does it have to do with the lower oxygen density? And, does that have something to do with the way the engines run, or the amount of lift an airplane can get???

    Thanks
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