Why are the altitudes and distances in the profile view different when calculated with the fpm values in the conversion table on Jeppesen charts? To put it more simply, I find the distance between points according to the speed-fpm table, in how many minutes I will fly. I multiply the minutes I find by the fpm value in the conversion table. But I get values lower than the altitude difference between points and these values may vary in different calculations. To give an example from the image, an airplane flying at 90 knots will fly 3 DME in 2 minutes. If it descends at 478 fpm, equivalent to 90 knots on the conversion chart, it descends 956 feet in 2 minutes (2x478=956). The descent in the 3 DME should be 987 feet (3000-2013=987) in the profile view, while the descent calculations given in the conversion table should be 956 feet. Where did the 31 feet in between go? Or is there an intentional reason for this? I would be glad if you help. The image is taken from page 11-1 of "LTBU" code Corlu Ataturk airport Jeppesen chart.
So, I decided to just start from scratch and build a model that started at the TDZ and worked back out. I wanted to determine what would happen if the DME antenna was not precisely collocated with the Glide Slope antenna. I wanted to take into account the slant-range error endemic to DME as well.
As I tinkered with the model, what I stumbled upon is that when I input 3.00° GS, I got the kinds of errors you were describing. But when I solved for minimum error, guess where I ended up? At 3.09° GS, the slope that was actually published on the plate as the non-precision LOC GS.
Here's the data. Note that I used the DME slant range and not the horizontal ground distance, in my altitude computations. Also, I used the plate's 508' elevation for the baseline, whereas the airport charted elevation is 570'.
Table 1 - 3.00° GS
|Degrees||DME||DME Dist||Horiz Dist||Alt AGL||Alt MSL||Expected||Height Error|
Table 2 - 3.09° GS
|Degrees||DME||DME||Horiz||Alt AGL||Alt MSL||Expected||Height Error|
I post this as an "answer" because I needed the space and table capability that the answer block provides... But I acknowledge that this doesn't answer the question of "why?"