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Thread: Determining ils altitude without plates

  1. #1
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    Question Determining ils altitude without plates

    OK I have a very interesting problem and maybe someone who has the answer can share with us folks who do not use plates for ils landings to determine the base altitude for a ils landing.

    Example: Airport A has a elevation of 0 . what is the base altitude for an ils landing with a 3 degree sloop and 5 miles.

    Now we have to work backwards to get the answer.

    All input is welcome on this little mind bender.

    Denis Davis
    Pilot

  2. #2

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    3 degrees are approx. 0.05236 radians, which is a slope of 5.24078% (about five meters on the y axis per 100 meters on the x axis). So over a distance 5 miles you will descend about 420 meters or 1383 feet, as google tells me.
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    My (rough) rule of thumb is 1,500 ft above airport elevation. 2,000 ft if I want to intercept farther out (flying jets). Somewhat consistent with the previous reply. Experts please correct me if my method is wrong.

    But this is just for simming. Aviation rules have a specific reason in mind to require the use of an exact intercept altitude for airports (noise reduction, obstacle, etc.).
    Last edited by jomni; 11-18-2010 at 03:08 AM.

  4. #4
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    A 5% slope over 5 miles gives 1/4 mile vertical. Now don't forget that distance in aviation is measured in nautical miles, not statute (1nm = 6075ft vs 1sm = 5280ft). So, in round numbers, 1/4 nautical mile is 1500ft. (1519ft, to be exact)

    Now with extra BLOGiness!

  5. #5
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    According to the FAA published descent table a 3 degree slope is equal to 318 ft/NM X 5NM = 1590 feet.
    WWOD---What Would Opa Do?
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  6. #6

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    Orion1969 -
    .....ils landings to determine the base altitude for a ils landing.
    Not quite sure just what you're asking, but if it refers to an INTERCEPT Altitude, then the advice given about 1500 feet above airport elevation works nice as long as you're more than 5 miles from the airport but not so far out as to have a terrain problem. The main thing to keep in mind is to intercept BELOW the Glide Slope, preferably OUTSIDE the Outer Marker. Hope this helps.
    Remember: Aviation is NOT an exact Science!

  7. #7

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    You may want to have a look into

    AIR FORCE MANUAL 11-217, VOLUM E 3
    http://www.e-publishing.af.mil/share...AN11-217V3.pdf
    Chapter 3 THE 60-TO-1 RULE AND OTHER AVIATION CALCULATIONS

    Provides a lot of mental math useful for flying.

    Relating to glideslope:
    1 down is about 1 ft down per 60 ft distance, or
    100 ft per 1 nm (6000 ft)
    i.e. 3 is 300 ft/nm
    orat 5 nm it's 1500 ft

  8. #8

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    Generally an ILS app is started at 3000ft above aerodrome level. This is usually 10nm from the runway. If your flying something like a B737 or anything with similar approach speeds, you will need to start a 500-600 fpm decent from 10nm. This should put you at around 1500ft by 5nm and within a few hundred feet by the threshold.
    Cheers,
    DHC8 FO

  9. #9

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    The very close relationshiip is Height (ft) = 316 * Distance (nm). ubder the 1 in 60 rule it's 300 * Distance (nm). At 5 nm the exact height is 1590 ft: under the 1 in 60 rule its 1500 ft.

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