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runway analysis chart interpolation

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General Aviation

Does anybody know how to interpolate? The numbers below are from a runway analysis landing chart. In the instructions I was given, they somehow got 195.000 pounds as the approach climb limit with an outside air temperature of 8 degrees Celsius. What is the actual formula for getting this answer?

Temp (c):    -20            19
Climb WT: 209.1      190.0

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2 Answers



  1. Dcopymope on Jan 28, 2017

    Temp (c): -20 / 19
    Climb WT: 209.1 / 190.0

    I think I more or less figured it out with the six step process below:

    [quote]1. -20 – 19 = 39

    2. 19 – 8 = 11

    3. 11 / 39 = 0.28

    4. 209,100 – 190,000 = 19100 lbs

    5. 19,100 * 0.28 = 5,348

    6. 4348 + 190.000 = 195,348[/quote]

    Its not exactly 195,000 pounds but its pretty close, which means I must be doing something right.

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  2. John Scarry on Jan 30, 2017

    Your first line is wrong -20 -19 is -39. But that’s OK because it is the absolute value of the number that you want to use in the rest of your calculations. Also, if you round in each step, you get a slightly different answer than if you do all of the calculations in one step.

    Interpolation is just figuring out how far apart two things are and then figuring out where the third thing is in relation to them. Your example is a little more confusing because of the negative number, but the same principle applies.

    If you remember number lines it is easier do the calculations.

    FIrst, what is the distance between the two temperatures? In other words, how many degrees are there between the two temperatures. Well, there are 20 degrees between zero and -20 and 19 degrees between zero and 19, so the total is 39.

    If you want to do it with a formula, it is |19 – (-20) | or | -20 – 19 | where | | is the absolute value of the difference.

    Next, how far from -20 is 8. You get 28. So 8 is 28/39 of the way from -20 to 19.

    Do the same thing with the weight. Since there are no negative numbers, this one makes more intuitive sense. 209.1-190 = 19.1.

    So now you want to find out the weight that is 28/39 of the way from 209.1 to 190.

    28/39 * 19.1 = 13.7 away from 209.1 or 195.4.

    To check yourself, do it in reverse. How far from 19 is 8? (The way you did it.)

    11/39 * 19.1 – 5.4 away from 190 or 195.4

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