Total Air Temperature and TAT Probes
Aviation requires regular updates of critical metrics, such as altitude, speed, air pressure, and more. These values are used by the pilot and various computers to perform needed calculations. However, not every metric is easy to measure accurately and in real-time, particularly those values dependent on the state of gasses, since these variables are all related and can change quickly. Temperature is one such value that is critical to several aviation components with several methods of measurement. In this blog, we will discuss everything you need to know about the measurement and calculation of total air temperature, which most accurately represents the air temperature in flight.
In order to fully comprehend the idea of total air temperature, it is first necessary to review the other types of temperature measured in flight. Static air temperature (SAT) is the temperature of undisturbed air next to the aircraft. The most accurate value of this reading would take place on the ground, although measurements are considered to have sufficient accuracy until the airspeed reaches 200 knots. Like other forms of measured and calculated temperature, SAT is critical to predicting the lift of the aircraft. This is because lift is partially dependent on the air density, which shares an inverse relationship with temperature. As a result, a much hotter day or climate would lead to a lower density and, in turn, lift.
As the plane increases its speed, a phenomenon called "ram air temperature rise" takes place. This phenomenon involves a rise in temperature caused by adiabatic compression, which occurs as air is compressed at the aircraft's leading edges. The consequence of this is that the temperature "felt" by the vessel's skin is higher than that which is encompassed by the static air pressure. Up to 200 knots, this difference is generally negligible, with only 1-3°C needing to be added to the SAT. However, the temperature vs. knots trend stops being linear after 200 knots, quickly rising to values over 20°C over 400 knots.
Icing is caused by the deposition and freezing of hydrometeors on any part of the aircraft. This ice affects the aircraft differently depending on the surface it is attached to. Still, in general, the plane experiences a decrease in lift, reduction in thrust, increase in weight, and significantly more drag. Additionally, if the buildup of ice reaches the pitot tube and static pressure ports, all flight instruments dependent on accurate data from those sources will display false information. Clearly, a regularly updated accurate TAT can help the pilot gauge if and how they should increase their airspeed to avoid icing.
TAT sensors should be accurate, lightweight, and resistant to corrosion. Although they are commonly mounted directly onto the airframe, sensors may also be placed directly onto the engine to provide real-time readings at a crucial site. Additionally, boom-mounted sensors are becoming available for military in-flight refueling operations. Although their primary use is in the identification of temperatures around 0°C, sensors should also be able to withstand high-temperature extremes.
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