The Total Air Temperature (TAT) is measured on A320FAM by two TAT probes installed on the aircraft nose, one on the Captain side, one on the First Officer side.
The TAT is mainly used to compute the Static Air Temperature (SAT) (also to calculate true air speed).
But have your observed air coming out of TAT probes when aircraft is on ground with packs on?
Wonder why?
Here is the answer.
Those are called aspirated TAT probes or ventilated TAT probes.
When A/C is on ground during a sunny day, non-aspirated TAT probes may give you inaccurate temperature measurement (much hotter than the actual temperature) because of the solar radiations.
The aim of the aspirating system is to reduce the solar radiation error by creating an airflow inside the probe whereas the A/C is stopped.
Pressurized air is blown in the pneumatic tube. Then, the air escapes through the holes located at the end of the pneumatic tube. In the area where the air is ejected, a depression is generated thanks to the air velocity.
Consequently, cool air is aspirated from the outside of the probe (due to the pressure difference between the outside and the low pressure area), and pass through the sensing element.
You must have also observed black/dirt traces on A/C skin just where air exits out of aspirated TAT probes.
Well this occurred due to ingested icing fluid being burnt by the heating of the TAT sensor, while being drawn through the probe itself due to the aspiration system of the of TAT sensor. The burnt icing fluid residue is then blown onto the aircraft skin which causes these traces.
Author – Sushanth Singh (B1.1 Aircraft Maintenance Engineer)