The probe is designed to bring the air to rest relative to the aircraft.
The air is compressed and experiences an adiabatic increase in temperature.
Therefore, total air temperature is higher than the static (or ambient) air temperature.
Total air temperature is an essential input to an air data computer in order to enable the computation of static air temperature and hence true airspeed.
The relationship between static and total air temperatures is given by:
where: In practice, the total air temperature probe will not perfectly recover the energy of the airflow, and the temperature rise may not be entirely due to adiabatic process.
In this case, an empirical recovery factor (less than 1) may be introduced to compensate: where e is the recovery factor (also noted Ct) Typical recovery factors Platinum wire ratiometer thermometer ("flush bulb type"): e ≈ 0.75 − 0.9 Double platinum tube ratiometer thermometer ("TAT probe"): e ≈ 1 Other notations Total air temperature (TAT) is also called: indicated air temperature (IAT) or ram air temperature (RAT) Static air temperature (SAT) is also called: outside air temperature (OAT) or true air temperature The difference between TAT and SAT is called ram rise (RR) and is caused by compressibility and friction of the air at high velocities.
In practice the ram rise is negligible for aircraft flying at (true) airspeeds under Mach 0.2.
For airspeeds (TAS) over Mach 0.2, as airspeed increases the temperature exceeds that of still air.
This is caused by a combination of kinetic (friction) heating and adiabatic compression.
The total of kinetic heating and adiabatic temperature change (caused by adiabatic compression) is the Total Ram Rise.
If we use the Mach number equation for dry air:
and By solving (3) for the above values with TAS in knots, a simple accurate formula for ram rise is then: