Originally Posted by
Alpine Flyer
I have devised the following Excel Formula:
=(-"ISADEV"/-0,0019812)*LN((1+-0,0019812*"MDH")/(288,15+-0,0019812*"APAlt"))
where "ISADEV" is the temperature deviation from ISA (usually a negative number in cold weather)
"MDH" is the minimum descent height in ft
"APAlt" is the airport altitude in ft
-0,0019812 is the lapse rate in K/ft and 288,15 is ISA/sea level in K.
This results in meaningless results.
I am challenged by not really understanding why the basic formula should work and would be grateful for any hints on where I might have erred in applying it.
A quick examination of this formula reveals that for MDH over about 500 ft, the term (1+-0,0019812*"MDH") in your spreadsheet formula turns negative. The natural log (LN) of a negative number is undefined (unless you are an electrical engineer
).