In this lesson, we will learn about the Static Air, Ram Air and Total Air Temperatures which are shortened to SAT, RAT and TAT. In our earlier post we had understood about the Errors in Temperature Measurement.
STATIC AIR TEMPERATURE
Static air temperature which is also called SAT is the actual temperature of undisturbed air outside the aircraft. Our aim is to measure this temperature correctly. Unfortunately, SAT cannot be obtained directly but only by correcting for errors in TAT probe. That is the reason that SAT is also called as Corrected Outside Air Temperature or COAT.
TOTAL AIR TEMPERATURE – TOTAL RAM RISE
Total Air Temperature or TAT is the temperature attained by air when brought to rest by the probe. Total Ram Rise is the actual increase in temperature due to adiabatic heating. We can easily guess that TAT will always be higher than the SAT due to Total Ram Rise. However, Measured Ram Rise will be lesser than the Total Ram Rise due to leaks and other inefficiencies in measurement.
RAM AIR TEMPERATURE – MEASURED RAM RISE
Ram Air Temperature or RAT is the temperature as detected and indicated by the TAT probe. The measured temperature will be higher than the Static Air Temperature by a amount equal to the Measured Ram Rise. We had seen that Measured Ram Rise is the increase in temperature as measured by the TAT Probe. In other words, RAT is the SAT affected by Measured Ram Rise. RAT is also called Indicated Outside Air Temperature or IOAT.
RECOVERY FACTOR
Total Ram Rise is the difference between Total Air Temperature (TAT) and Static Air Temperature (SAT). Measured Ram Rise is the difference between Ram Air Temperature (RAT) and Static Air Temperature (SAT). We are aware that the Measured Ram Rise is the Ram Rise as measured by TAT probe which will be lesser than Total Ram Rise due to loss in measurement. Recovery factor is the ratio between Measured and the Total Ram Rise.
In this post, we had learnt about the various terms used in measurement of Outside Air Temperature. In the next lesson let us see how to calculate the Static Air Temperature from the Measured Temperature