Air Temperature Measurement — Page 32, Lesson 40

Right, let's get into air temperature measurement. This is a really important one because the temperature you see on the cockpit gauge is almost never the actual outside air temperature. Let me walk you through the key terms first, because everything else builds on them. We have four core definitions to nail down. First, Total Air Temperature, often written as Tt. This is the maximum temperature the air can reach when it's brought to rest adiabatically. Let's unpack that. "Adiabatically" means without any heat being added or removed from the outside—it's purely the effect of the air being compressed and slowed down. So, if you take moving air and stop it dead, all its kinetic energy turns into heat, and that maximum possible temperature is the TAT. Second, Ram Air Temperature, or RAT. This is simply the measured air temperature—what the probe actually senses. It's not the theoretical maximum; it's the real-world reading. Third, Ram Rise. This is the difference between the SAT and the TAT—that's the Total Ram Rise—or the difference between the SAT and the RAT, which is the Measured Ram Rise. So ram rise is the temperature increase caused by the air being rammed into the probe. Fourth, the Recovery Factor, with the usual symbol Kr. This is the percentage of the ram rise that a TAT probe actually senses and recovers. No probe is perfect, so it doesn't capture 100% of the theoretical ram rise—it captures a fraction, and that fraction is Kr. Now, let's put these together with a worked example, because this is where it clicks. Imagine an aircraft flying where the SAT is -60°C. At its speed, the theoretical Total Ram Rise would be +30°C. So, the theoretical Total Air Temperature is SAT plus ram rise: -60 plus +30, which gives -30°C. But our probe has a Recovery Factor of 0.9. So the measured ram rise is the total ram rise multiplied by the recovery factor: +30 × 0.9, which is +27°C. Now, the Ram Air Temperature is the SAT plus the measured ram rise: -60 plus +27, which gives -33°C. So the gauge actually indicates -33°C, not -30°C. To get back to the true SAT of -60°C, you'd need to apply a correction factor of -27°C—that's the measured ram rise. Theoretically, uncorrected air temperature gauges give you RAT, not TAT. But here's a real-world quirk: in many aircraft, the gauges are incorrectly labelled "TAT" even though they're really showing RAT. There's also some terminology overlap you need to know. SAT is sometimes called COAT, which stands for Corrected Outside Air Temperature. And TAT is sometimes called IOAT, which is Indicated Outside Air Temperature. When you just see "OAT" on its own, it usually means SAT. Let me give you the summary equations, because they're clean and worth memorising. TAT = SAT + Ram Rise. And IOAT = COAT + Ram Rise, where IOAT is the same as TAT and COAT is the same as SAT. So both equations say the same thing—the total temperature is the static temperature plus the ram rise. Now, how do we actually correct 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 look at the rapid formula first. It's a good, simple approximation: Ram Rise = (v / 100)², where v is the TAS in knots. So if your TAS is 200 knots, ram rise is (200/100)², which is 2², so 4°C. At 300 knots, it's 3², so 9°C. At 400 knots, it's 4², so 16°C. You get the pattern. But—and this is important—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 a quick practical answer in the air when you don't want to dig out a nav computer or look up tables. Finally, the Navigation Computer method. On the slide rule face of the CRP5, there's a blue scale. The outer side of that scale is TAS, and the inner side is Ram Rise. So you line up your TAS on the outer scale and read the ram rise on the inner scale. In the example given, a TAS of 400 knots gives a ram rise of 17°C on the CRP5—slightly different from the 16°C from the rapid formula, which shows you why the exam wants the more accurate CRP5 answer. That's the core of air temperature measurement. The key takeaway is that the gauge reading is a measured value, and you always have to account for ram rise and the recovery factor to get the true outside air temperature.