There was a discussion on Baidu Tieba regarding this topic, so I looked up some information. The original question was: “Why does the engine information panel display the converted TAT instead of OAT? What exactly are they used for respectively?”

1. Definitions TAT (Total Temperature): Total Air Temperature SAT (Static Air Temperature): Static Air Temperature OAT (Outside Air Temperature): Outside Air Temperature

2. The relationship between TAT and Pressure According to the introduction in Wikipedia, Total Temperature (also known as stagnation temperature or total temperature) is a term related to aerodynamics. A fluid in motion possesses pressure, temperature, density, velocity, and Mach number. If a fluid can be brought to a complete standstill via an adiabatic process, its kinetic energy is converted into internal energy, manifesting as changes in pressure, temperature, and density. The temperature at this moment is the total temperature. A practical example is the aircraft’s pitot tube measuring total temperature and total pressure at its front end.

Adiabatic compression occurs when air pressure rises, causing the gas temperature to rise as well. For example, when inflating a bicycle tire, you can feel the temperature of the pump rise. This is precisely because the air pressure increases rapidly enough to be considered an adiabatic process; heat does not escape, thus the temperature rises. Diesel engines rely on the principle of adiabatic compression during the compression stroke to ignite the mixed gas in the combustion chamber.

Adiabatic expansion occurs when air pressure drops, causing the gas temperature to drop as well. For example, when deflating a tire, you can clearly feel that the released air is quite cold. This is precisely because the air pressure drops rapidly enough to be considered an adiabatic process; the internal energy of the gas is converted into mechanical energy, and the temperature drops.

3. Aircraft TAT during high-speed flight

According to the explanation in this Japanese pilot’s blog, Regarding the TAT measured by the pitot tube, due to the adiabatic compression effect of the air pressure acting on the airframe, The TAT is significantly higher than the SAT or OAT (the actual outside air temperature). For instance, when the cruise speed is Mach 0.8, the TAT is about 30 degrees Celsius higher than the SAT.

The freezing temperature of fuel generally used by airlines is approximately minus 46 degrees. Even if the outside temperature (SAT) is minus 71 degrees, the TAT will be minus 41 degrees. Therefore, under these temperature conditions, the fuel will not freeze, and the flight is safe.

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