[klem]

Originally Posted by ACE-OF-ACES

Roger that..

But my point is it is NOT realitive to 'I_Altitude'..

As far as I can tell it is realitive to 'Z_AltitudeAGL' and/or 'Z_AltitudeMSL'.

In short, the Z_AmbeintAirTemperature value your using may be the OAT realitive to another 'altitude' and not the one the plane is currently at..

For an example of which I speak, look at the TIME TO CLIMB picture I posted

Note at the higher altitudes (around 17.5Kft) the "Z" altitudes are quite different (higher) from the indicated, pressure and density.. Thus in and around that area when you use Z_AmbeintAirTemperature (OAT) your getting the temperature 'realitive' to the "Z" altitudes and not the OAT realitive to indicated, pressure, and density. How big of an error that will introduce.. Not sure have not calcualed it yet, but knowing how a little temperature difference can have a big effect on the calcuation of density altitude one should take pause.

Again, not 100% sure yet, need to test that to prove it to myself! I just wanted to bring it up should you come across it while testing

OK, well there is the parameter I_AmbientTemp. William's example calls it Actual Temperature. It looks as though that is what he means, actual emperature as measured in the aircraft environment, "the outside air temperature (OAT) or static air temperature (SAT) refers to the temperature of the air around an aircraft".

Now, regarding:
Density_Alt = Pressure_Alt + (T_std/TLR) * (1 - Math.Pow((T_std / I_AmbientAirTemperature),0.2349690));
(now with I_AmbientAirTemperature)

Where:
T_std = 288.15 - Alt_ind × TLR
TLR = 0.0065

and

if ( height < tropopause ) then
TLR = 6.49(°C/1,000m) = 1.98(°C/1,000ft) = 3.56(°F)/1,000ft
else
TLR = -56.5(°C) = 216.65(°K)

I have two questions.

1. TLR is a rate so presumably below the Tropopause the Temperature Lapse is - Alt_Ind x TLR but above the Tropopause the Temperature Lapse is simply 56.5C ?
and similarly......
2. Within the DA calc, T_Std/TLR would appear to calculate an altitude. Does it hold good above the Tropopause where the lapse rate is zero or should I be capping T_Std at 288.15 - 56.5 = 231.65K once T_Std/TLR hits 56.5?

EDIT: Sorry, ref Q.2, I meant should I replace T_Std/TLR with some other expression above the Tropopause??

EDIT2: Think I've got it

if ( 15 - Pressure_Alt_ft*0.0019812 < -56.5)
{
TropAlt = PressureAlt - 56.5/0.0019812
TempStd = 288.15 - 56.5;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812 + TropAlt) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));
else
TempLapse= Pressure_Alt_ft*0.0019812;
TempStd = 288.15 - Templapse;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));
}
TempStd = 288.15 - TempLapse;
Density_Alt_ft = Pressure_Alt_ft + (TempStd/0.0019812) * (1 - Math.Pow((TempStd / I_AmbientAirTemperature),0.2349690));