Air Temperature Measurement: SAT, TAT, and Ram Rise

022-02-02Pablo Asensio Martínez2026-04-011 min

Accurate air temperature measurement is crucial for calculating True Airspeed (TAS) and monitoring engine performance.

Temperature Definitions

  • Static Air Temperature (SAT) / Outside Air Temperature (OAT): The temperature of the undisturbed air through which the aircraft is flying.
  • Total Air Temperature (TAT): The temperature measured by a probe sensing the airflow. It is always higher than SAT due to heating effects.
  • Ram Rise: The difference between TAT and SAT ($TAT = SAT + \text{Ram Rise}$).

Heating Effects

As the aircraft moves through the air, the temperature measured by the probe increases due to two factors:

  1. Kinetic Heating: Caused by friction between air molecules and the probe surface.
  2. Adiabatic Heating: Caused by the compression of air as it is brought to rest in the probe.

Measurement and Calculation

  • On modern aircraft with an Air Data Computer (ADC), the TAT is the measured value (using a Rosemount probe, for example).
  • There is no direct OAT probe. Instead, the ADC calculates SAT (and OAT) internally by subtracting the calculated Ram Rise and accounting for the probe's recovery factor from the measured TAT.
  • Formula: $TAT = SAT (1 + 0.2 \times K \times M^2)$, where K is the recovery factor and M is the Mach number.