Total air temperature

This article includes a list of general references, but it lacks sufficient corresponding inline citations. Please help to improve this article by introducing more precise citations. (March 2011) (Learn how and when to remove this message)

In aviation, stagnation temperature is known as total air temperature and is measured by a temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest, kinetic energy is converted to internal energy. 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: $${\displaystyle {\frac {T_{\mathrm {total} }}{T_{s}}}={1+{\frac {\gamma -1}{2}}M_{a}^{2}}}$$ where:

• ${\displaystyle T_{s}=}$ static air temperature, SAT (kelvins or degrees Rankine)
• ${\displaystyle T_{\mathrm {total} }=}$ total air temperature, TAT (kelvins or degrees Rankine)
• ${\displaystyle M_{a}=}$ Mach number
• ${\displaystyle \gamma \ =\,}$ ratio of specific heats, approx 1.400 for dry air

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:

$${\displaystyle {\frac {T_{\mathrm {total} }}{T_{s}}}={1+{\frac {\gamma -1}{2}}eM_{a}^{2}}}$$

(1)

where e is the recovery factor (also noted C_t)

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