We're starting a brand-new chapter now — Chapter 3, Air Temperature… — Page 24, Lesson 32

We're starting a brand-new chapter now — Chapter 3, Air Temperature Measurement. This is a fresh topic, so let's set the stage properly. The chapter opens with an introduction, then moves through the air temperature thermometers themselves, the total air temperature probe, and then a deep dive into errors — specifically heating error from compression and kinetic heat, and something called ram rise, which involves a recovery factor. After that we get into the correction of TAT and RAT to SAT, a rapid formula for use in the air, the navigation computer method, an accurate formula, the absolute or Kelvin temperature scale, and finally calibration. Now, before we get into the meat of it, I want to flag the key acronyms you'll see throughout this chapter, because they're going to come up again and again. TAT stands for Total Air Temperature. RAT is Ram Air Temperature. And SAT is Static Air Temperature. These three are the backbone of everything we're about to cover. The core idea here is that when an aircraft moves through the air, the air doesn't just sit there — it gets compressed and heated as it hits the aircraft. So the temperature you actually measure on the aircraft is not the true temperature of the outside air. It's elevated. The whole chapter is about understanding that difference, measuring it accurately, and correcting for it. Let me walk you through the structure. First, the introduction sets up why we care about air temperature at all — it's fundamental to so many other systems. Then we look at the actual thermometers used, and specifically the total air temperature probe, which is the sensor mounted on the aircraft that measures the ram air temperature. That probe is designed to capture the air and measure its temperature, but because of the compression and kinetic heating effects, what it reads is not the static air temperature. That's where the errors section comes in. We have heating error, which is caused by compression and kinetic heat. And then ram rise — that's the temperature increase caused by the air being brought to rest at the probe. The recovery factor is the key here — it's a measure of how much of that ram rise the probe actually captures. A perfect probe would recover all of it, but in practice, probes recover a fraction, and that fraction is the recovery factor. Then we correct TAT and RAT back to SAT. TAT is the temperature you'd get if the probe recovered 100% of the ram rise — the theoretical maximum. RAT is what the probe actually measures, accounting for the recovery factor being less than one. SAT is the true static air temperature of the undisturbed air. The correction takes you from what you measured, through the recovery factor, back to the true outside temperature. For doing this in the air, there's a rapid formula — a quick approximation you can use on the fly. And there's also the navigation computer method, which is the standard way pilots actually do it in practice. Then there's an accurate formula for when you need precision. Finally, we get to the absolute or Kelvin temperature scale. This is important because all the thermodynamic relationships in aviation — and in the corrections we're doing — work in Kelvin, not Celsius. Zero Kelvin is absolute zero, where all molecular motion stops. And then we finish with calibration, which is how we ensure the probe and the instruments read accurately in the first place. So that's the roadmap. We're going to start with the introduction and the basic thermometers, then build up to the probe, the errors, and the corrections. Let's begin with the introduction and the air temperature thermometers.