Thank you so much for clarifying this and sending me the link. Great service! Highly appreciated!

I will need some days to go through the details. I'll let you know how it works out.

The ISA calculations are more advanced than a simple percentage as that is a simplification that only holds at lower altitudes. The function which performs the conversion takes the airspeed and density altitude as parameters. There are some formulas on the page below, and taking a quick look at the page I can confirm that ours matches the one given that works for low-speed aircraft and density altitudes below 36,000 feet.

http://williams.best.vwh.net/avform.htm#Mach

Ok. But SD take the altitude into account, right? Since I notice the higher the altitude, the higher the calculated GS (wind changes taken into account), which means higher calculated TAS. Higher altitude normally means lower temperature, reducing TAS. However, higher altitude also means lower pressure, increasing TAS.

So that may explain why SD calculates a higher GS/TAS than what I get when taking also temperature into account. I'll remove the temperature dependencies from my equations and see if I now get the same GS as SD. I have seen both 2% and 3% being used for increased TAS per 1000 ft. I'll post my results later.

SkyDemon uses the International Standard Atmosphere model to convert IAS to TAS. It therefore does not take account of pressure or temperature changes as these are specifically not use in the standard atmosphere model.

You are misunderstanding. I am talking about the GS calculated by SD in the static PLOG in Planning mode. SD gathers wind info from the web, uses my TT (True Track) specified for the leg I want to fly, and calculates the HDG and GS that would be seen during flight. The calculated GS is directly related to TAS corrected for wind data and is the basis for calculating ETE. The calculated TAS must be adjusted for the altitude specified for the leg and the temperature collected from weather info. I have implemented such altitude and temperature corrections in my spread sheet, but the models I am using, are giving me a lower TAS (and GS) than SD reports in the PLOG. So SD is using slightly different models.

So I am asking SD to tell me what their model/mathematics is so we get similar results. It should be in SD's interest too to get consistent results.

I have read dozens of articles on the web on how to adjust TAS taking temperature and altitude into account. The equations I am using give me a very good match with those calculations, but pretty far off with SD. So I hope SD can tell me how they calculate TAS (and by that GS) so I can adjust my equations. That shouldn't be any secret. I need the mathematics.

My airspeed ground and true matches with my garmin 496 exactly

Cheers Mike
Anything i say is my opinion you can take it or leave it

SD constantly seems to calculate a far too high GS. That is my conclusion after caøculating dozens of scenarios.

Here is what i have done:
Android version only supports Static Pilot Log, so I had to create something to have an Operating Flight Plan (=Live Pilot Log). I have asked for support of Live Pilot Log in another thread, but SD answered they can't tell when to support that.

So I created a spreadsheet where I have implemented Live Pilot Log (+ more features than Live Pilot Log). In the spread sheet I enter IAS, temperature, altitude, wind etc. I calculate TAS taking the following into account:
- adjust TAS as a function of temperature and altitude using this formula: http://en.wikipedia.org/wiki/True_airspeed
- increase TAS by 2% per 1000'
- decrease temperature by 2 degC per 1000'
This gives me TAS spot on with the manual E6B (checked with examples on the web). I also compared with the Android app "FlightTools E6B", which is within 1%. So seems my equations are very accurate.

However, SD is constantly giving TAS too high. I compared SD with E6B and my equations by creating routes on SD at different places with different altitudes, temperature and wind conditions. I adjusted the track to get straight tail or head wind (no xwind) such that GS=TAS+/-head wind. Then I looked at the PLOG to get the wind, altitude, TRK and GS data.

Could SD explain why the results differ and how SD calculates TAS taking altitude and temperature into account?