We have two instruments that feed this calculation — Page 109, Lesson 107

Let me walk you through how the Navigation Computer turns temperature and pressure into true airspeed. This is the practical heart of TAS and altitude conversions. We have two instruments that feed this calculation. First, the air thermometer gives us temperature. But there's a catch — the thermometer reads Total Air Temperature, or TAT, because the air is rammed and compressed as it hits the probe. We have to correct that down to Static Air Temperature, SAT, which is the actual temperature of the undisturbed air around us. You already know how to do that correction, so we just take SAT as our starting value. Second, the altimeter gives us atmospheric pressure at our level. And here's the key point — the altimeter does not actually measure altitude. It measures pressure. It is, in fact, a form of barometer, but its readout is in feet rather than hectopascals. The reason we can read feet is that the law relating altitude in the ISA atmosphere to hectopascals of pressure is known, and we can use that law. So we have pressure from the altimeter and temperature from the thermometer. From those two, we can find the relative density of the air. And once we have relative density, we can find TAS from CAS. If you tried to do this longhand with a calculator, it would take quite a long time. The Navigation Computer uses precisely this method, and it does it quickly and easily. Let me return to our earlier example. CAS is 100 knots, we're at FL200, SAT is minus 25 degrees Celsius, and the density is 653 grams per cubic metre. On the Navigation Computer, we have four windows marked AIRSPEED, COMP CORR, ALTITUDE, and DENSITY ALTITUDE. Here's the procedure. In the AIRSPEED window, you align FL200 against minus 25 degrees Celsius. That single action has altered the position of the inner scale relative to the outer scale. What you've actually done is align them in the ratio of the square root of the relative density. If you want to check this, look at the '1' — shown as '10' — on the outer scale. Against it, on the inner scale, you will see the ratio .7301. That number is the square root of the relative density, and it's the physical meaning of the alignment you just made. Now you have CAS on the inner scale against TAS on the outer scale. As our theoretical explanation showed, 100 knots CAS at FL200 and minus 25 degrees Celsius is 137 knots TAS. You read the CAS value on the inner scale, and directly opposite it on the outer scale sits the TAS. Now, I want to be clear about one thing. You do not require to know, or understand, the theoretical explanation behind this. All that is required is to be able to use the Navigation Computer practically. So don't worry about deriving the physics — just master the mechanical steps. Let me give you another example, purely using the Navigation Computer method. You are at 18,000 feet pressure altitude, and the SAT — the corrected outside air temperature — is minus 30 degrees Celsius. Your CAS, which is also called RAS, is 170 knots. What is TAS? Here's the sequence. In the AIRSPEED window, set pressure altitude, 18,000 feet, against COAT, which is minus 30 degrees Celsius. Then find 170 knots CAS on the inner scale. Move the cursor if you need to. Now read off the value on the outer scale — and that gives you 220 knots TAS. So the whole method boils down to this: align pressure altitude against SAT in the AIRSPEED window, then read CAS on the inner scale against TAS on the outer scale. Two inputs, one alignment, and the computer does the density work for you.