PPL Ex18 (v)[Page3]: NAVIGATION: (True Air Speed -TAS Page 3)

The Standard Atmosphere is defined as follows:

1. The air is a dry perfect gas
2. The temperature at sea level is 15˚C
3. The pressure at sea level is 1013.25 millibars
4. The temperature gradient from 5,000m below sea level to an altitude at which the air temperature becomes -56.5˚C is -0.0065˚C per metre (approx. -1.98˚C/1000ft)
5. The density ρ0 at sea level under the above conditions is 0.002378 slug/ft3 (or 1.225kg/m3)

Now observe the DENSITY ALTITUDE window. The DENSITY ALTITUDE is 0. So, at zero feet density altitude RAS = TAS. At 5,000ft DENSITY ALTITUDE you will notice that the temperature is +5˚C in the AIR SPEED window at 5,000 Pressure Altitude, and the 90 on the R.A.S. scale is now opposite 97 on the T.A.S. scale. So, in a standard atmosphere, 90kts R.A.S. at 5000ft AMSL = 97Kts T.A.S. At 2000ft Density Altitude 90kts R.A.S. = 92.7Kts T.A.S.

Note: If the temperature is not standard for the altitude then use the AIR SPEED window to align the C.O.A.T. against the PRESSURE ALTITUDE. E.g. if the temperature at 5000ft is not the standard +5˚C but (say) +20˚C (ISA+15 see iv. above - Standard Atmosphere) the T.A.S. will be 99.5kts. The higher the temperature the higher will be the T.A.S because the Density is lower at that altitude. +20˚C at 5000ft altitude equates to a ‘Density Altitude’ of 6800ft. Conversely -20˚C at 5000ft altitude equates to a ‘Density Altitude’ of 1950ft.

Here is a definition of DENSITY ALTITUDE: The altitude in the Standard Atmosphere corresponding to the flight level density. Basically it is the measure of the number of molecules of air per cubic metre which can act upon the aircraft surfaces with the resulting forces of lift, drag etc. Density of a gas is determined by pressure and temperature. Density Altitude is the theoretical density of a standard atmosphere at that altitude. Aircraft performance is directly related to air density; therefore, performance is determined by DENSITY ALTITUDE regardless of the actual altitude.OK – that is TAS out of the way!