Air Temperature Measurement — Page 32, Lesson 36

Right, let's get into the Total Air Temperature probe. This is the heart of how we measure the air temperature that actually matters to the aircraft in flight. First, the construction. The probe is a small strut with an air intake, and it's made of nickel-plated beryllium copper. That material choice is deliberate — it gives good thermal conductivity and strength. It's fixed to the fuselage at a point that keeps it away from the aircraft's boundary layer. The boundary layer is the slow-moving, disturbed air right next to the skin, so we want to sample clean, undisturbed air. Now, the clever part — how it separates water. As the air flows through the tube, the airflow is made to turn through a right angle before it passes round the sensing element. That sharp turn throws water particles out of the flow, so they don't hit the sensor and give a false reading. Then, the bleed holes in the intake casing permit boundary air to be drawn off because of the higher pressure inside the intake. The sensor itself is a resistance wire made of pure platinum. It has very high thermal conductivity and a rapid response to change — so it reacts quickly to temperature variations. There's also an inbuilt heating element to prevent ice formation. And here's a nice detail: it's self-compensating. As the temperature rises, so does the heater resistance, which reduces the heater current. So the heater naturally backs off when it's warm. The heater can have a small effect on the temperature readings, but it introduces an error of less than 1°C, which is not significant. Now, we also need to measure air temperature on the ground. Modern aircraft don't use full power for take-off, to avoid unnecessary thermal stress on the engines. The take-off is carried out using the minimum power necessary to ensure safety, but no more. Runway length, weight, altitude, and temperature are all taken into consideration in the calculation of the required power — and it's perfectly normal to carry out a take-off at, say, 93% power. To measure air temperature on the ground, we use an air-to-air ejector, also called an aspirator. Bleed air from either an APU or a running engine creates a negative differential pressure within the casing, so outside air is drawn through it even when the aircraft is stationary. This prevents the temperature of stagnant, heat-soaked air from within the casing being measured. In the diagram, the engine bleed air is positive pressure from one of the engine compressor stages. Although it's blowing, not sucking, the flow is arranged such that the rearward movement of the engine bleed air creates a suction effect past the sensing element. Finally, let's talk about errors. Air temperature gauges are subject to three types. Instrument error — imperfections in manufacture. Environmental errors — solar heating of the probe, overcome by shielding in the strut, and ice accretion on the probe, overcome by the heater. And heating error — adiabatic, that's compression heating, and kinetic, that's friction heating. Of these, by far the greatest is heating error — the effects of compression and kinetic heat. That's the one that really dominates.