Air Temperature Measurement — Page 32, Lesson 40

Let’s start with the core definitions, because everything else in this chapter hangs on them. We have four terms you must know cold: Total Air Temperature, Ram Air Temperature, Ram Rise, and Recovery Factor. Total Air Temperature, or TAT, sometimes written as Tt, is the maximum temperature the air can reach when it is brought to rest adiabatically. Let me unpack that. “Adiabatically” means without any heat being added or removed from outside — the temperature change comes purely from compressing the air as it slows down. So if you take moving air and stop it dead, all its kinetic energy converts into heat, and the absolute hottest temperature it can possibly reach is the TAT. That’s the theoretical ceiling. Ram Air Temperature, or RAT, is simply the measured air temperature. That’s the actual reading you get from the probe, which is always a bit less than the theoretical TAT because no probe is perfect. Ram Rise is the difference between two temperatures. There are two flavours. Total Ram Rise is the difference between SAT and TAT. Measured Ram Rise is the difference between SAT and RAT. So you can see the pattern: SAT is the true outside air temperature, TAT is the theoretical maximum, RAT is what you actually measure, and the ram rise is the gap caused by the air being compressed and heated as it hits the aircraft. Recovery Factor, usual symbol Kr, is the percentage of that Ram Rise that a TAT probe actually senses and recovers. No probe recovers 100% of the theoretical rise — it recovers a fraction, and that fraction is Kr. Now let me walk you through the worked example, because it ties all four together. Assume an aircraft experiencing a SAT of -60°C at a speed where the theoretical Total Ram Rise would be 30°C. Assume a Recovery Factor of 0.9. Start with SAT = -60°C. Add the Total Ram Rise of +30°C, and you get the theoretical Total Air Temperature of -30°C. That’s the absolute maximum the air could reach if fully stopped. Now apply the Recovery Factor. Measured Ram Rise is +30°C multiplied by 0.9, which gives +27°C. So the probe only recovers 27 of the 30 degrees. Add that to the SAT of -60°C, and you get a Ram Air Temperature of -33°C. That’s what the gauge actually indicates. So the gauges would show -33°C, not -30°C. To get back to the true SAT, you apply a correction factor of -27°C — that’s the measured ram rise — and you arrive at -60°C. That correction can be applied by one of several methods, which we’ll get to shortly. Here’s a critical point for real-world flying: theoretically, uncorrected air temperature gauges give you RAT, not TAT. But in many aircraft, the gauges are incorrectly labelled “TAT.” So don’t be fooled by the label — understand what the instrument is actually doing. Now, the terminology gets a little tangled, so let me sort it out. SAT is sometimes called COAT, which stands for Corrected Outside Air Temperature. TAT is sometimes called IOAT, which stands for Indicated Outside Air Temperature. And OAT on its own is usually taken to mean SAT. So when someone says OAT, they generally mean the true outside air temperature. Let me give you the summary relationships, because they’re clean and worth memorising. TAT equals SAT plus Ram Rise. And IOAT, or OAT, equals COAT plus Ram Rise. So the indicated value is always the corrected value plus the heating effect. Now, how do you actually correct TAT or RAT back to SAT in practice? There are five methods listed: a rapid formula for use in the air, a CRP5 or similar navigation computer, an accurate formula, data tables, and the Air Data Computer. Let’s start with the rapid formula, because it’s the one you’ll use in the cockpit. The formula is: Ram Rise equals v over 100, all squared, where v is TAS in knots. So if TAS is 200 knots, ram rise is 4°C. If TAS is 300 knots, ram rise is 9°C. If TAS is 400 knots, ram rise is 16°C. And so on. You can see the pattern — it’s a square law, because kinetic energy grows with the square of speed. But here’s the caveat: this formula should not be used in JAA exams, because it’s not quite as accurate as the CRP5 solution. It’s good enough for practical use in the air when you need a quick answer and it’s not convenient to find a navigation computer or look up tables. So it’s a cockpit tool, not an exam tool. The navigation computer method uses the slide rule face of the CRP5, which has a blue scale. The outer side of that scale is TAS, and the inner side is Ram Rise. In the example, a TAS of 400 knots gives a Ram Rise of 17°C on the CRP5 — note that’s 17, not the 16 from the rapid formula. That one-degree difference is exactly why the CRP5 is considered more accurate. Let me show you the bimetallic strip and helix thermometer, because that’s the physical mechanism behind the direct-reading instruments. Here we have a bimetallic strip — two metals bonded together that expand at different rates when heated. The strip is drawn out into a helix, which gives greater pointer movement for a given temperature change. That’s the trick: coiling it amplifies the tiny expansion into a readable deflection. And here’s the bimetallic helix thermometer mounted on the windscreen or fuselage, with the tube protruding out into the airflow. This shows the placement on a PA28 Warrior. But here’s the limitation: the direct reading thermometer is not a workable solution for a fast aircraft, because you can’t read it from the cockpit — it’s outside. That’s why we need remote reading and the total temperature probe. Here’s a total temperature probe. Notice the bleed holes in the intake — those are crucial. And this shows why it’s necessary to measure air temperature in the first place. The probe is designed to bring the air to rest and recover as much of the ram rise as possible, which is where the Recovery Factor comes in. So to pull it all together: you measure RAT with the probe, you know the Recovery Factor of that probe, you apply the correction to get SAT, and you use one of the five methods to do that correction. The rapid formula for quick cockpit work, the CRP5 for better accuracy, the accurate formula and data tables for precision, and the Air Data Computer for the automated solution. That’s the complete picture of air temperature measurement.