We have two key instruments that give us the raw data we need — Page 109, Lesson 107

I want to walk you through how we actually get True Airspeed from Calibrated Airspeed using the instruments we have in the aircraft. We have two key instruments that give us the raw data we need. First, we have an instrument that gives us temperature — the air thermometer. Now, the thermometer actually measures what we call Total Air Temperature, which includes the heating effect of the aircraft moving through the air. We have to correct that down to Static Air Temperature, or SAT — that's the actual temperature of the undisturbed air outside. We know how to do that correction, so we can get SAT. Second, we have an instrument that gives us the atmospheric pressure at our level — the altimeter. Now, as you already know, the altimeter does not actually measure altitude directly. It measures pressure. The altimeter is, in fact, a form of barometer, but its readout is in feet. The key point is that the relationship between altitude in the International Standard Atmosphere and pressure in hectopascals is known. So we can use the altimeter reading to get pressure altitude. So here's the principle: we can use pressure from the altimeter and temperature from the thermometer to find the relative density of the air. Once we have that relative density, we can then find True Airspeed from Calibrated Airspeed. If you tried to do this longhand with a calculator, it would take quite a long time. However, the Navigation Computer — your CRP-5 flight computer — uses precisely this method and does it quickly and easily. Let's return to our previous example. We have a Calibrated Airspeed of 100 knots, we're at Flight Level 200, and the Static Air Temperature is minus 25 degrees Celsius. The density at those conditions is 653 grams per cubic metre. Now, look at the Navigation Computer. We have four windows marked AIRSPEED, COMP CORR, ALTITUDE, and DENSITY ALTITUDE. In the AIRSPEED window, we align Flight Level 200 against minus 25 degrees Celsius. This alters the position of the inner scale relative to the outer scale. What you've 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 0.7301. Now, you have Calibrated Airspeed on the inner scale against True Airspeed on the outer scale. As our theoretical explanation showed, 100 knots Calibrated Airspeed at Flight Level 200 and minus 25 degrees Celsius gives 137 knots True Airspeed. You do not need to know or understand the theoretical explanation behind this. All that is required is to be able to use the Navigation Computer practically. Let me give you another example, just 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, or RAS — that's Calibrated Airspeed, also called Rected Airspeed — is 170 knots. What is True Airspeed? In the AIRSPEED window, set pressure altitude — 18,000 feet — against COAT — that's Corrected Outside Air Temperature, which is minus 30 degrees Celsius. Find 170 knots CAS on the inner scale. Move the cursor if you need to. Now read off the value on the outer scale — that gives you 220 knots True Airspeed. So the process is straightforward: set pressure altitude against temperature in the AIRSPEED window, then read CAS on the inner scale against TAS on the outer scale. That's the practical method we use.