Static Air Temperature (SAT) and Outside Air Temperature (OAT)

Static Air Temperature (SAT), also known as outside air temperature (OAT), represents the temperature of undisturbed air surrounding an aircraft. It is the temperature you would observe if you could suspend a thermometer in the air without any influence from airflow passing over it.

It’s crucial to carefully position the sensor that detects OAT to ensure that the airflow over it doesn’t influence the indicated temperature. This positioning helps maintain accuracy in temperature readings, which are essential for various aspects of flight planning, performance calculations, and safety considerations.

In jets, SAT (OAT) is derived by applying correction factors to data obtained from Total Air Temperature (TAT) or Ram Air Temperature (RAT) probes. Typically, modern air-data computers in large jet aircraft handle this calculation.

Total Air Temperature (TAT)

Total Air Temperature (TAT) represents the temperature of the air as measured by a probe exposed to the airflow. It’s corrected for the effects of compressibility and friction, providing a more accurate indication of the actual temperature experienced by the aircraft during flight. TAT is crucial for calculating true airspeed, air density, and other aerodynamic parameters necessary for flight performance. It’s particularly important in high-speed and high-altitude flight, where the effects of air compression become significant.

TAT vs RAT

TAT and RAT represent different approaches to measuring air temperature in aircraft. TAT, or Total Air Temperature, accounts for compressibility and friction effects in its measurement. It provides a corrected temperature reading influenced by the aircraft’s motion through the air. This correction ensures accurate data for aerodynamic calculations. RAT, or Ram Air Temperature, is measured directly from a probe exposed to the incoming airflow, without corrections for compressibility.

Use of Anti-Ice – TAT (or RAT) / SAT (or OAT)

TAT (or RAT) is crucial in determining the activation of engine anti-ice systems. Even if SAT (or OAT) indicates temperatures below freezing, TAT  reflects the actual temperature experienced by the aircraft due to its motion through the air. Manufacturers typically recommend activating engine anti-ice when TAT (or RAT) drops below 10 degrees Celsius in visible moisture conditions to prevent ice formation.