There are two important effects if you measure temperature on an aircraft:
- At low airspeed, when the heat exchange between the air and the temperature sensor is low, the temperature error is dominated by radiation (e.g. sun heating up the sensor) and the heat transfer between the body of the aircraft and the OAT sensor (e.g. during a climb where the fuselage is still hot but the air temperature is already cold).
- At high airspeed the air is decelerated. As the air is brought to rest, kinetic energy is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore measured total air temperature (TAT) is higher than the static/ambient air temperature (SAT). TAT is an essential input to an air data computer. SAT is used to compute true airspeed (TAS).
In practice, the total air temperature probe will not perfectly recover the energy of the airflow. The Recovery factor is used to compensate this effect. The recovery factor is always less than 1. It is measured in the wind-tunnel.
Static air temperature is computed in the air data computer using the measured TAT, Mach-number and the Recovery factor. You can use the SAT data label on the RS-485 data interface to get the “real” static/ambient temperature. At low airspeed SAT equals TAT but as airspeed increases there will be a small delta.
We carefully designed the OAT-sensor in the wind-tunnel to minimize the error due to the heat transfer and radiation. This guaranties a fast response time.
To get the best performance from the PSS-8 OAT sensor we recommend the following:
- Install the OAT sensor below the wing, the fuselage etc. where it is protected from direct sunlight
- Install the OAT sensor away from hot parts (e.g. engine) and out of any hot air-stream (e.g. cooling air, exhaust gases)
- Install the OAT sensor in the front part of the aircraft where the boundary layer is still small (the air in the boundary layer picks up the temperature of the fuselage, wing, etc)
- Install the OAT sensor such that it is exposed to the full amount of air-stream to improve the heat exchange at low airspeed.