Combustion whether in aircraft or cars or anything that has an engine, not only relies on accurate mixture of air with fuel before ignition, it also requires the air and fuel to be at optimal temperature - this ensures instant and clean combustion.
Aircraft typically fly at altitudes where the air is very cold. This cold, dense and often freezing air impedes combustion and may even need to be pre-heated before mixing with fuel. So how does the aircraft know if the air is at the correct temperature or not? This is where the Intake Air Temperature (IAT) sensor comes into play.
An automobile engineer’s solution to the cold air problem would be to increase the fuel in the fuel-air mixture. Cold air being dense, the extra fuel in the mixture corrects the fuel-to-air ratio and ensures proper combustion. The increased fuel consumption is not a problem as a land-based vehicle can stop at the nearest fuel station and fill up – this however, is not an option for aircrafts.
The aircraft therefore, is designed to pre-heat cold air so it is ‘combustion-friendly’. Whether to heat or not, is triggered by the onboard engine controller that is fed with data from the IAT probe. The IAT probe directly feeds air temp. data into the air temp indicator / onboard controller. Based on the air temp data received from the IAT probe, the engine controller dynamically alters the air-to-fuel ratio by changing the timing of the injector pulses.
When installing the Intake Air Temperature (IAT) probe, only the tip of the probe is exposed to external air entering the aircraft engine. The IAT probe itself is mounted just inside the air intake manifold of the Aircraft EGT/CHT Monitors.
How the IAT probe works:
Technically, the IAT probe is a thermistor. This means its electrical resistance is directly proportional to the changes in the air temperature i.e. the output voltage from the IAT probe varies depending on the air temperature. The output voltage for your aircraft’s IAT probe is usually mentioned in your aircraft service manual (or the leaflet that came with the IAT probe -if you’ve purchased a new one).
On ground, the pilot needs to eyeball the IAT probe to ensure it is not coated with sooth, oil or bird feathers as these could cause the IAT probe to malfunction. A malfunctioning OAT Probe will in turn the on-board engine computer miscalculating the air-to-fuel mixture.
More information on IAT probes for aircraft can be found here: https://www.jpinstruments.com/product-category/probes-sensors/
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