What is FAT in Aviation? (Free Air Temperature)

In aviation, understanding and monitoring weather conditions is crucial for safe and efficient flight operations. One important meteorological parameter that pilots and aviation professionals need to take into account is the Free Air Temperature (FAT) or ambient air temperature. The Free Air Temperature refers to the temperature of the surrounding air, unaffected by aircraft engines or other heat sources. It plays a significant role in various aspects of aviation, including aircraft performance, operational planning, and flight safety.

The Importance of Free Air Temperature in Aviation

Free Air Temperature has a direct impact on aircraft performance, especially in relation to engine power, air density, and aerodynamic efficiency. The FAT is used to calculate the density altitude, which is a crucial factor in determining an aircraft’s performance capabilities at a particular location and altitude. The density altitude accounts for variations in temperature, humidity, and air pressure in comparison to the International Standard Atmosphere (ISA) model.

When the free air temperature increases, the air density decreases. This means that in hotter conditions, the air is less dense, reducing an aircraft’s engine output and overall performance. As a result, the aircraft may take longer to achieve the required speed for takeoff, require a longer runway for takeoff and landing, or have a reduced rate of climb. Therefore, pilots and flight planners need to consider the free air temperature to ensure that the aircraft can safely take off, climb, and operate within its performance limitations.

Measuring Free Air Temperature

Free Air Temperature can be measured using various instruments, including the in-flight thermometer and the Total Air Temperature (TAT) probe. In-flight thermometers provide an approximation of the external air temperature during flight. These thermometers are typically located on the aircraft’s outside air temperature (OAT) gauge and are calibrated to compensate for heat generated by the aircraft’s structure or engine.

The Total Air Temperature (TAT) probe, on the other hand, measures the total temperature of the air entering the engine. The TAT probe is typically located outside the aircraft, ahead of the engine inlet, to capture the true temperature of the oncoming air. This measurement includes the effect of air compression due to the aircraft’s forward motion, also known as ram rise or ram effect. The TAT probe readings are essential for aircraft engine control and the accurate estimation of the air density and performance parameters.

Throughout aviation history, various methods have been employed to measure free air temperature. These include using mercury-in-glass thermometers, resistance temperature detectors (RTDs), and more advanced digital temperature sensors. Regardless of the method used, the accuracy and reliability of the temperature measurement are crucial for accurate aircraft performance calculations and flight planning.

Effect of Free Air Temperature on Aircraft Operations

The impact of free air temperature on aircraft operations extends beyond performance considerations. High temperatures can also affect aircraft systems, particularly those involved in the cooling of engines, electronics, and other critical components. Higher temperatures increase the workload on the aircraft’s cooling systems, potentially leading to reduced system efficiency and increased risk of overheating.

In addition to engine cooling, free air temperature plays a crucial role in determining the aircraft’s fuel consumption and range. Higher temperatures result in decreased air density, which leads to reduced engine efficiency. As a consequence, the aircraft may consume more fuel to maintain a given speed and altitude when compared to operations in cooler temperatures. Pilots and flight planners need to take this into account when considering fuel requirements for a specific flight and route.

Furthermore, the temperature of the surrounding air affects aircraft icing conditions. When the FAT is near or below freezing, moisture in the air can freeze on the aircraft’s surfaces, leading to ice accumulation. This can impact the aircraft’s aerodynamics, increase drag, and affect performance. Understanding the free air temperature is crucial for pilots to assess the potential for icing and take appropriate measures to mitigate the risks, such as activating anti-icing or de-icing systems.


The Free Air Temperature in aviation is a vital parameter that impacts aircraft performance, operational planning, and flight safety. By understanding the temperature of the surrounding air, pilots and aviation professionals can accurately calculate aircraft performance capabilities, determine fuel requirements, assess potential icing risks, and make informed decisions. Monitoring and accounting for the free air temperature is essential for safe and efficient flight operations, ensuring the optimal performance of both the aircraft and its systems.

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