Added calculated values based on Koch chart in METAR derived data #463

[Munzijoy]

In the Derived Section of METAR data, I added calculated values for

• Takeoff distance increase
• Rate of climb decrease based on density altitude. The values are similar to those on a Koch chart. Formulas based on https://groups.google.com/g/rec.aviation.piloting/c/SDlMioBVqaA/m/8_x9GYsnGwAJ have been used. The calculated values have been checked using https://www.takeofflanding.com/

As the results are only a very rough approximation anyway, I rounded round up the results to next group of ten.

[kebekus]

@Munzijoy Thanks for your help! I would like to merge your PR, but I still have a few questions. Enroute is used by an incredibly diverse audience, ranging from military pilots and airline pilots to general aviation aircraft, helicopters, ultralights, paragliders with and without motor, ... basically anything that flies.

Do we have any idea what kind of aircraft your formula applies to? I presume it makes a big difference if you have a jet engine, a piston engine with(out) turbocharger, etc etc

[kebekus]

@Munzijoy Additional question: When the t/o distance increases by x%, is this relative to "normal" conditions at the given altitude or "normal" conditions at sea level?

[Munzijoy]

The values are relative to sea level conditions.

According to https://www.faasafety.gov/files/events/nm/nm09/2013/nm0951144/density_altitude.pdf "The chart may be conservative for airplanes with supercharged engines". I'm not sure whether this formula applies to propeller aircrafts only and so far I couldn't find out. As the aircraft type is a huge restriction and right now I don't have any further information on how it affects the formula, maybe we should postpone this feature and the PR shouldn't be merged.

[kebekus]

@Munzijoy Thanks for the info. I am not that negative. Give me a few days to days to discuss the matter with a couple of people here in Freiburg.