Air Temperature Measurement — Page 32, Lesson 36

Let’s pick up with the Total Air Temperature Probe — the TAT probe. This is the heart of air temperature measurement in flight, and I want you to see exactly how it’s built and why every part is there. The probe itself is a small strut with an air intake, and it’s made of nickel-plated beryllium copper. That material choice matters: beryllium copper gives you good thermal conductivity — so heat moves through it quickly — and it also gives you strength, so the strut can survive the airflow and vibration. The nickel plating protects it. The probe is fixed to the fuselage at a point that keeps it away from the aircraft’s boundary layer. The boundary layer is the thin layer of slow-moving, disturbed air that clings to the skin of the aircraft. If you mounted the probe inside that layer, you’d be measuring stagnant, turbulent air, not the true free-stream temperature. So placement is deliberate. Now, as the air flows through the tube, something clever happens: the airflow is made to turn through a right angle before it passes round the sensing element. That right-angle turn is what separates out water particles. Water droplets are heavier than air, so when the flow has to bend sharply, the droplets can’t follow the turn — they carry on and get separated out, so they don’t hit the sensing element and corrupt the reading. There are also bleed holes in the intake casing. Because the pressure inside the intake is higher than the surrounding boundary air, those bleed holes let boundary air be drawn off. That keeps the slow, contaminated boundary air from building up inside the probe and affecting the measurement. The actual sensor is a resistance wire made of pure platinum. Platinum is chosen because it has very high thermal conductivity and a rapid response to change — so the wire heats up and cools down almost instantly with the air temperature, and its electrical resistance changes in a known, repeatable way with temperature. That resistance change is what the instrument reads out as temperature. Now, icing. The probe has an inbuilt heating element to prevent ice from forming. And here’s the neat part: it’s self-compensating. As the temperature rises, the heater resistance also rises, which reduces the heater current. So the heater automatically backs off when it’s warm, and pushes harder when it’s cold. That’s a feedback loop built into the resistance of the heater itself. But the heater does have a small effect on the temperature readings — it introduces an error of less than 1°C. That’s not significant, so we accept it. Now, there’s a second problem: measuring air temperature on the ground. Modern aircraft don’t use full power for take-off. Why? To avoid unnecessary thermal stress to the engines. The take-off is carried out using the minimum power necessary to ensure safety, but no more. So the crew calculates the required power taking into account runway length, weight, altitude, and temperature — and it’s perfectly normal to take off at, say, 93% power. That calculation needs an accurate ground temperature. But here’s the catch: when the aircraft is stationary on the ground, the air inside the probe casing can become stagnant and heat-soaked — it absorbs heat from the hot aircraft structure and from the sun, so it’s no longer representative of the outside air. To fix that, we use an air-to-air ejector, also called an aspirator. Bleed air from either an APU — the auxiliary power unit — or from a running engine creates a negative differential pressure within the casing. That negative pressure draws outside air through the casing even when the aircraft is stationary. So instead of measuring stagnant, heat-soaked air, the probe continuously samples fresh outside air. Let me be precise about how that ejector works, because it’s easy to misunderstand. The engine bleed air is positive pressure from one of the engine compressor stages. So it’s blowing, not sucking. But the flow is arranged such that the rearward movement of that engine bleed air creates a suction effect past the sensing element. Think of it like a jet pump: the fast-moving bleed air rushing rearward lowers the pressure around it, and that low pressure draws the outside air through the probe. So even though the driving air is blowing, the effect on the sensing element is suction. Finally, let’s talk about errors. Air temperature gauges are subject to several errors, and I want you to know each one and how it’s overcome. First, instrument error — that’s from imperfections in manufacture. No instrument is perfect; small manufacturing tolerances cause small reading errors. Second, environmental errors. There are two of these. One is solar heating of the probe — the sun heats the probe itself, which would make it read too high. That’s overcome by shielding in the strut. The other is ice accretion on the probe — ice building up on the probe, which would insulate it and corrupt the reading. That’s overcome by the heater we talked about. Third, heating error — and this is the big one. It comes from adiabatic heating, which is compression heating, and kinetic heating, which is friction heating. As the air is compressed and slowed down against the probe, its temperature rises. By far the greatest of all these errors is the heating error — the combined effects of compression and kinetic heat. That’s why the probe is called a total temperature probe: it measures the total temperature, which includes that heating effect, not just the static air temperature. So to tie it together: the TAT probe is a nickel-plated beryllium copper strut, mounted away from the boundary layer, with a right-angle turn to separate water, bleed holes to draw off boundary air, a pure platinum resistance wire as the sensor, and a self-compensating heater. On the ground, an aspirator uses engine or APU bleed air to draw fresh outside air through it. And the dominant error you must always remember is the heating error from compression and friction.