Public Domain Aeronautical Software (PDAS)

In addition to the standard atmosphere, it is important to be able to calculate the performance of aeronautical vehicles under non-standard conditions. In particular, it is crucial that one estimate the change in takeoff field length on a hot day as compared to a standard day or cold day. The committee that defined the standard atmosphere did not define any such non-standard atmospheres. As a result, there may be more than one such definition in practice today.

Regardless of the definition, all of the non-standard atmosphere definitions that I have examined are simply a different table of temperature versus altitude. At a given altitude, the computational algorithm is

• Compute the pressure at this altitude from the standard atmosphere. Assume this does not change.
• Compute the temperature from the revised schedule.
• Compute density from the pressure and temperature using the perfect gas law.

There are two techiniques I have examined for making an alternate table of temperature versus altitude for these non-standard conditions.

• A fixed increment in temperature that is applied at all altitudes to the standard atmosphere.
• A completely new definition, based on observational data.

The standard atmosphere routine has been augmented with an optional variable called deltaT that represents the desired increment in temperature (in kelvins) from the standard atmosphere. Now you can set deltaT = 5.0 and refer to calculations as for a +5K standard atmosphere. This would be a warm day. On the other hand, deltaT = 25.0 would represent a very hot day and deltaT = 30.0 would result in a life-threatening noontime in the desert. Similarly, setting deltaT = -30.0 would produce arctic winter conditions.

One difficulty with the above approach is that one person might choose deltaT = 15.0 for a hot day while another person might choose 25.0 and both would say they had done hot day calculations. In a US Department of Defense document, MIL-STD-210A, there are definitions of four non-standard atmospheres, referred to as hot, cold, tropical, and polar. I have created four new subroutines that implement each of these atmospheres. The definition of these atmospheres only extends to 100000 ft (30.5 km).

Public Domain Aeronautical Software (PDAS)