We all know that every combustion engine (except maybe in rockets), rely on an external air source for combustion. Problem for aircraft is that, the air entering the combustion chamber may not always have the ideal temperature for a healthy combustion.
Cold air usually found at high altitudes for example, is denser than air at sea level. Cold air being denser, requires more fuel otherwise an air-fuel ratio imbalance occurs and the combustion will be erratic. If the combustion in an aircraft engine is erratic, and you know what will happen. So, when the air is colder than optimum temperature, there are two things you can do – either warm the air or squirt more fuel. Obviously, the latter is not an ideal solution because the aircraft can only carry as much fuel as the tanks will hold.
But before you can do anything, you need to know what the air temp is and that is where the induction air temperature or IAT probe in aircraft comes in. The function of the modern induction air temp therefore, is to sense and digitize the temperature of the air that is to flow into the combustion chamber.
Modern day aircraft have an onboard computer that automatically balances the air-to-fuel ratio. The IAT probe (apart from sending data to the Aircraft Engine Monitor) provides air temp data to the onboard engine computer that controls the aircraft engine. The onboard computer in turn, dynamically alters the air-to-fuel ratio by changing the timing of the injector pulses.
The Intake Air Temperature (IAT) probe is mounted in the air intake manifold of the aircraft. The tip of the IAT probe is exposed to external air entering the aircraft engine. Essentially, the IAT probe is a thermistor, so its electrical resistance changes in response to changes in the temperature of the sensor. This means that the return voltage from the IAT probe changes in proportion to changes in air temperature.
If the aircraft is experiencing combustion related issues, chances are, the IAT probe might be malfunction and that can happen if it gets coated with oil, sooth, feathers, dust and so forth. Loose or corroded wiring or connectors can also have the same effect. If the IAT probe transmits inaccurate voltage, the onboard Engine Monitoring Systems will miscalculate the air-to-fuel mixture and this could result in a rich or lean fuel mixture.
The resistance and voltage test specifications for your aircraft’s IAT probe can be found in your aircraft service manual and the leaflet that came with the IAT probe (if you’ve purchased a new one).
Once every few weeks, it would be a good practice to test your aircraft’s IAT probe and also check area around the probe to ensure it is free of oil and anything else that might contribute to inaccurate temp feedback.