Temperature Measurement in Aircraft (OAT, TAT, RAT)
Outside Air Temperature (OAT)
In aviation terminology, the outside air temperature (OAT) or static air temperature (SAT) refers to the temperature of the air around an aircraft but unaffected by the passage of the aircraft through it. The sensor which detects OAT must be carefully sited to ensure that airflow over it does not affect the indicated temperature.
OAT measurement is essential to
- To convert CAS to TAS
- For Air Data Computer calculations
- For determination of ac performance parameters
Total Air Temperature (TAT)
TAT = OAT + Adiabatic Heating Error (Ram Rise)
The actual outside temperature can be determined by subtracting adiabatic heating error from the indicated temperature
Corrected outside air temperature (COAT) = IOAT – Ram Rise
Majority of Ram Rise is due to adiabatic compression as a result of the airflow coming to a rest at a point, with only small amount of it due to friction between surface of ac and high speed airflow. As you measure temperature moving at high velocities, your outside thermometer will measure a higher temp than what is actually outside (what a non-moving thermometer would get). That’s because as the air rams into your thermometer it gets a little bit compressed, and that makes it heat up a little bit.
If temperature is measured by means of a sensor positioned in the airflow, kinetic heating will result, raising the temperature measured above the OAT. The temperature measured in this way is known as the Total Air Temperature (TAT) and is used in Air Data Computers (ADCs) to calculate True Airspeed (TAS). Careful design and siting of the TAT probe is necessary to ensure accurate measurement of TAT. In other words it can be said that the stagnation temperature is known as TAT and is measured by the temperature probe out side the ac.
The amount of adiabatic heating which can be sensed with the probe depends on the type of probe. Generally TAT is a term used for high speed ac that fly with reference to Mach No.
Problem is, not all instruments will compress that air in the same way, and not all of them will pick up this Ram Rise entirely. So they will publish a K, a ram-rise-coefficient (also called recovery factor) meaning how much of from the theoretical Ram Rise they actually pick up. Term TAT is used when The value of K is 100%.
Ts = Tm/1+0.2 KM²
where Ts = SAT in deg absolute
Tm = Indicated Temp in deg absolute
K = recovery factor and M = Mach No. ( If recovery factor is 100% K = 1 and if 80% the K= 0.8)
Ram Air Temperature
It is used for slow speed ac and is definition wise similar to TAT. That is the temperature measured by the probe plus temperature rise because of compressibility. RAT is used when value of K as explained above is less than 1. That means recovery factor is less than 100%.
RAT = SAT + (K x Ram Rise)