Why don't the Altitude-DME values in the approach profile match the Groundspeed-FPM values in the conversion table?

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When the calculation is made with the values given in the conversion table, the altitude values in the profile view are significantly different. To give an example from the picture, we see a profile descending from 2000 feet to 900 feet at an angle of 3 degrees. Assuming we descend at 70 knots, the vertical velocity we need to maintain has to be 372 fpm. We fly the distance of 3.4 DME between waypoints at a speed of 70 knots in 2.9142857 minutes. In 2.9142857 minutes, at 372 fpm, we would descend to 2.9142857*372=1084.1142804 feet. Although the altitude difference is 2000'-900'=1100', we calculated the altitude value to be descended as 1084'. Where does the difference of 1100'-1084'=16 feet come from?

Also, you can reach another question I asked very similar to this question from this link: Why are the altitudes and distances in the profile view different when calculated with the fpm values in the conversion table on Jeppesen charts?


1 Answer 1


Have you ever seen a DME display show hundredths of a NM?

To make the math work correctly, you will need to descend at 3.4498297885 NM on the DME.

$$ \frac{2000-900}{372} X \frac{70}{60} = 3.4498207885 $$

In both of your questions, you are assuming that we are supposed to descend exactly at 3.4NM but in reality there is an assumed error due to capability of the receiver only showing tenths of a NM.

Is it better to reach 900' MSL before the MAP or after when you start to descend at 3.5NM.

  • $\begingroup$ Wow, I never thought of it that way. What you say seems logical, but shouldn't the fpm value be given according to the DME value? So I mean why isn't fpm information given according to 3.4 DME? Why should the 3.4498207885 DME point on the approach path be chosen deliberately and given fpm over that point according to the altitude-DME information? Shouldn't the selected point be 3.4 DME and the appropriate fpm value should be given in the table?... $\endgroup$
    – pilot162
    Feb 19 at 14:27
  • $\begingroup$ ...The answer to this question would be "maintaining the 3-degree approach path". But I don't understand why the logic of what I wrote doesn't fit the situation in the other question. $\endgroup$
    – pilot162
    Feb 19 at 14:31
  • 1
    $\begingroup$ I find the original question and your follow-up questions quite fascinating. There is theory and then there is the art of executing that theory. You are missing the art portion of the equation. Very few pilots will start their descent exactly at 3.4 NM on the DME. It might be 3.35 or 3.45 or earlier or later or somewhere in between. That is the art. The idea of these descent rate table is to get you close enough to where you should be to execute the approach and landing successfully. No one can also maintain the exact FPM listed on the table. It is impossible. Pilot executes the art $\endgroup$
    – wbeard52
    Feb 19 at 18:03

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