The Navigation Computer - Distance, Speed, Time and Conversions — Page 94, Lesson 92

Let's start with the yellow scale, because it's the simplest thing on the computer and it sets up how we read everything else. The yellow scale exists for one purpose only: to convert an answer from minutes into hours and minutes. When you use the black '60' triangle as your hourly rate, the computer gives you answers in minutes — even when the answer is more than 60 minutes. But in aviation, we usually want the answer expressed as hours and minutes, not just a big number of minutes. So suppose a calculation gives you 142.6 minutes. You look at the yellow ring and you can see that 142.6 minutes sits between 2 hours and 2 hours 30 minutes. Since 2 hours is 120 minutes, the extra 22.6 minutes takes you to 2 hours 22.6 minutes. That's the entire job of the yellow scale — nothing more. Now the blue scale, which is more interesting. The blue scale gives you the value of ram rise. Ram rise is defined as the difference between Static Air Temperature and Total Air Temperature. In symbols, Ram Rise equals TAT minus SAT. Let me define those two temperatures carefully, because they're easy to confuse. Static Air Temperature, SAT, is the temperature of the ambient atmosphere surrounding the aircraft, with no inaccuracies caused by the measurement process. It's the true temperature of the air out there. This is the temperature we need to calculate True Airspeed from Calibrated Airspeed and Pressure Altitude, or to calculate TAS from Mach number. But here's the problem: what we actually measure with our temperature probe is the Total Air Temperature, TAT. TAT is affected by compressibility and by kinetic or adiabatic heating. Because the air is compressed and heated as it hits the probe, TAT is always higher than SAT. So the ram rise — the difference between them — is the heating caused by the aircraft's motion through the air. There's a good approximation for ram rise, and it's this: Ram Rise equals TAS divided by 100, all squared, where TAS is in knots. So if TAS is 400 knots, 400 over 100 is 4, and 4 squared is 16 — so ram rise is approximately 16°C. If TAS is 500 knots, 500 over 100 is 5, squared is 25 — approximately 25°C. And so on. Now, that approximation is usually good enough for practical purposes, but it tends to be slightly low. It's not as accurate as the answer produced by the Navigation Computer — and for EASA ATPL examinations, the computer's answer is the one expected. So you need to know how to read it off the blue scale. Here's how. Suppose your TAS is 400 knots. You align the cursor with 400 knots on the upper portion of the blue scale. Then you read off the ram rise on the lower portion of the blue scale. For 400 knots, it reads about 17°C — slightly higher than the 16°C approximation, as I said. Then you can use that to convert between TAT and SAT. So if the TAT at 400 knots TAS were, say, minus 22°C, then the SAT would be minus 39°C. Because SAT is TAT minus ram rise: minus 22 minus 17 gives minus 39. Now, fuel consumption problems work exactly like distance, speed and time calculations. We normally get a fuel consumption per hour, we use base 60, and we read the answer in minutes. If it comes to more than 60 minutes, we use the yellow scale to read the answer in hours and minutes. Here's the worked example. Your fuel consumption is 2300 kilograms per hour. Your time to destination is 1 hour 37 minutes. How much fuel will you use? Remember the key rule: minutes are always on the inner scale. So first, set the black triangle against 2300. Then align the cursor with 1 hour 37 minutes — which is 97 minutes — on the inner scale. Read off the answer: 37.2. The correct answer is obviously 3720 kilograms. And to reassure you of the computer's accuracy: using a calculator gives you 3718.33 kilograms. The navigation computer is very accurate — in fact, probably considerably more accurate than your fuel gauges. So to tie it together: the yellow scale converts minutes to hours and minutes, the blue scale gives ram rise from TAS, and fuel consumption uses the same base-60 setup as distance and speed, with minutes always on the inner scale.