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Thread: Why 68 Deg. for inspection

  1. #21
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    Quote Originally Posted by Willy View Post
    . . .What is the "standard" for density?
    It is 'Sea Level' density at 29.92 and 59 deg.


    Quote Originally Posted by BCRider View Post
    That's because pilots don't care about air density.... . . .They are after a setting for their barometer so they can set "zero" for their altimeters. . .
    Density is of ultimate importance to a pilot. It is the main factor affecting aircraft and engine performance. And pilots don't 'zero' their altimeters unless they are exactly at sea level. (American Airlines used to set their altimeters so that at touchdown wherever they were the altimeter would read zero, but I think they gave that up decades ago.) The Kollsman window (where the altimeter adjustments are made) has been marked in both hPa and in.Hg for at least 75 years. Those adjustments are used to obtain the most accurate possible actual altitude above sea level as the aircraft moves through areas of varying barometric pressure and temperature.

  2. #22
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    You're right, "zero" was the wrong word to use. They dial the local barometric pressure into the altimeter so it provides the proper local altitude rather than "zero".

  3. #23
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    As to why inches of mercury, rather than other expressions of atmospheric pressure, it almost certainly stems from the fact that the mercurial barometer has long been the standard for the actual pressure measurement, and remains so today.
    Yes, on a day-to-day basis readings will be taken and used from an aneroid barometer or a barograph, but those devices are required to be regularly calibrated against a mecurial barometer.
    It's also convenient that .01 of that 29.92 in. of mercury very closely corresponds to 10 feet of altitude.

    For helicopter pilots, particularly in hot climates, density (in the form of density altitude) is of PRIMARY concern. Density altitude being simply pressure altitude adjusted for temperature.

  4. #24
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    Why not just use "bar" for air pressure?? and then specify a temperature with that, to get the density??

  5. #25
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    Quote Originally Posted by nickel-city-fab View Post
    Why not just use "bar" for air pressure?? and then specify a temperature with that, to get the density??
    Well, in day-to-day meteorological measurements and transmitted weather reports, the "bar" is what's used ...except it's expressed in thousandths, i.e. millibar. The weather man only converts it to inches of mercury for consumption by the aviation community.

    But when you see a weather map, with Highs and Lows, and isobars, that was generated from the reported sea level pressure from a network of stations such as North America, all expressed in millibars, not in" of Hg.

    (added) I guess I didn't really address your main point. As to rotary wing operations, I'm not really sure of all the details, as to why they need density altitude expressed as such, rather than just taking pressure and applying a temperature adjustment themselves. Probably purely as a matter of convenience to the chopper pilots. "It's easier to have the weather man do it and pass it along to us."
    Tho that's a bit of an unkind statement. The weather guy is already in possession of all the data need to make the calculation; it's only natural to have him do it one time and feed to all who need that DA value.
    Last edited by lynnl; 07-09-2019 at 01:10 PM.

  6. #26
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    Quote Originally Posted by BCRider View Post
    At least for some materials measured in millionths of a meter the term is microns. And I agree that it's rather meaningless at first glance. Micrometers would be a much better tie in to the system.
    The SI unit of length is the micrometre, which can't be confused with anything else, unlike micrometer and micron.

    George

  7. #27
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    Quote Originally Posted by chipmaker4130 View Post

    It is 'Sea Level' density at 29.92 and 59 deg.

    Duh!
    Home, down in the valley behind the Red Angus
    Bad Decisions Make Good Stories

  8. #28
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    I'm not aware of any standard for air density. But it would have to account for water vapor, e.g. dry air is denser than moist air ...other factors being equal.

  9. #29
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    Quote Originally Posted by lynnl View Post
    I'm not aware of any standard for air density. But it would have to account for water vapor, e.g. dry air is denser than moist air ...other factors being equal.
    Thanks Lynnl for the clarification, this is probably along the lines of my thinking when I first raised the question earlier.
    Home, down in the valley behind the Red Angus
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  10. #30
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    Quote Originally Posted by Doozer View Post
    Sounds to me that you are too lazy to remember things like one miles is 5280 feet.
    Are you afraid your brain will leak out your ear????

    -Doozer
    Nope, just worried that if I fill my head with fluff that I don't use in daily life, I'll turn into an obnoxious jerk who entertains myself by insulting strangers on the internet

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