We're starting a brand-new chapter now: Air Temperature Measurement — Page 24, Lesson 32

We're starting a brand-new chapter now: Air Temperature Measurement. This is a short but very important chapter, because it deals with a measurement that affects everything from engine performance to true airspeed and altitude calculations. Let me give you the roadmap first. The chapter opens with an introduction, then moves to the Air Temperature Thermometers themselves. From there we get into the heart of it: the Total Air Temperature Probe, which is the main sensor we use. Then we cover Errors — and this is the meat of the chapter, because temperature measurement in flight is full of errors. Specifically, we'll look at the Heating Error caused by compression and kinetic heat, and then how we correct for that using something called Ram Rise and the Recovery Factor. After that, we'll see how to correct TAT and RAT to SAT — that's Total Air Temperature, Ram Air Temperature, and Static Air Temperature. We'll have a rapid formula for use in the air, a method using the Navigation Computer, and then a more accurate formula. We'll also touch on the Absolute, or Kelvin, Temperature Scale, because that's the scale we use in all these calculations. Finally, we'll cover Calibration of the system. So the big picture is this: the temperature we actually measure in flight is not the true outside air temperature. It's heated up by the air being compressed and slowed down as it hits the probe. Our job in this chapter is to understand that error and correct for it precisely. Let's start with the basics. The instrument we're talking about is the Air Temperature Thermometer. In modern aircraft, this is almost always a Total Air Temperature probe, often abbreviated TAT. The probe is designed to measure the temperature of the air after it has been brought to rest — that's the "total" or "stagnation" temperature. We'll get into why that matters in a moment. The key concept to grasp right now is the difference between TAT and SAT. SAT is Static Air Temperature — that's the true temperature of the undisturbed air outside the aircraft. TAT is what the probe actually reads, and it's always higher than SAT because of the heating effects we'll discuss. So as we go through this chapter, keep that distinction in mind: the probe gives us TAT, and we need to work backwards to find SAT. That's the whole game of air temperature measurement. Let's move into the details of the thermometers and the probe itself.