Air Temperature Measurement — Page 32, Lesson 38

Let's pick up right where the heating error story gets interesting. We've established that as the aircraft speeds up, the measured temperature climbs above the true still-air temperature. Now I want to nail down exactly why that happens, because it's the heart of this whole chapter. There are two distinct physical processes at work, and they're easy to confuse, so let's separate them cleanly. First, kinetic heating. This is the friction effect. Think of the Direct Reading thermometer — the one with the bimetallic strip mounted on the windscreen. As the aircraft accelerates, more and more air molecules per second slam into the flat plate at the surface of the probe. That constant bombardment is friction, and friction generates heat. So kinetic heating is a direct application of heat — molecules physically striking the probe and warming it up. Second, adiabatic heating. This is the main contributor in the Remote Reading, or Total Head, thermometer. And here's the key distinction: adiabatic heating is not a direct application of heat at all. It's a conversion of energy. Let me give you the classic example. Boiling a kettle — that's direct heat input. But pumping a bicycle pump? The barrel gets warm even though you haven't applied any flame to it. Why? Because you're compressing the air, and pressure energy is being converted into temperature energy. No heat is added from outside — the temperature rise comes from the compression itself. Now, how does that apply to the probe? In the Remote Reading thermometer, the outside airflow — which could be several hundred knots — is brought virtually to rest inside the platinum measurement chamber, and it's brought to rest very rapidly. When that moving air is decelerated, its kinetic energy has to go somewhere. It's released in the form of a temperature rise. That's adiabatic heating. Now here's the crucial relationship. The laws of thermodynamics tell us that kinetic heating and adiabatic heating always combine to give a specific figure. We call that the Total Ram Rise. It's the theoretical maximum temperature increase you'd expect from the aircraft's speed. But — and this is the practical reality — no measurement process is perfect. There are always leaks and inefficiencies in the probe. So we never actually sense the full ram rise. The amount we do sense is called the Measured Ram Rise. And this leads us to three distinct temperatures, and you must keep them straight because they're used for different purposes. If we could measure the Total Ram Rise perfectly, we would be measuring the Total Air Temperature. That's the theoretical ideal — the true total temperature including all the ram rise. But in practice, because of those inefficiencies, what we actually measure is the Ram Air Temperature, and it's lower than Total Air Temperature. That's the real, sensed value from the probe. Then there's the Static Air Temperature, sometimes written as TS. This is the temperature of the undisturbed air through which the aircraft is about to fly. Notice the wording — "about to fly." It's the temperature of the air before the aircraft disturbs it, before any compression or friction has had a chance to act. That's the true ambient temperature of the atmosphere at your altitude. So let me lay out the hierarchy for you, because this is the part that trips people up. Static Air Temperature is the lowest — it's the undisturbed air. Ram Air Temperature is what we actually measure — it's higher than SAT because it includes the measured ram rise. And Total Air Temperature is the highest — it's what we'd measure if we captured the full, perfect Total Ram Rise with zero losses. The gap between SAT and TAT is the full ram rise. The gap between SAT and RAT is only the measured ram rise — smaller, because of the probe's inefficiencies. And that inefficiency is exactly why we can't just read the probe and call it the true air temperature. Now, one thing I want to make sure you understand before we move on: the platinum measurement chamber I mentioned — that's the sensing element inside the Remote Reading probe where the air is brought to rest. It's called "platinum" because that's the material used for the sensing element, and it's chosen for its predictable resistance-temperature characteristics. But I'm getting ahead of myself — we'll cover the construction of that probe in detail shortly. For now, the takeaway is this: speed heats the air through two mechanisms — friction and compression — and the total of those two is the Total Ram Rise. We can't measure all of it, so we get Ram Air Temperature instead of Total Air Temperature, and the true ambient value we're ultimately after is Static Air Temperature, TS.