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

I want to walk you through the next part of the navigation computer — the yellow scale, the blue scale, and fuel consumption. Let's start with the yellow scale. Its purpose is simple: it lets you convert minutes into hours and minutes. When you use the black '60' triangle for an hourly rate, the answer comes out in minutes — even if it's more than 60 minutes. But in aviation, we often want the answer in hours and minutes, not just minutes. So the yellow scale is there to make that conversion. For example, suppose an answer comes to 142.6 minutes. If you look at the yellow ring, you can see that 142.6 minutes falls between 2 hours and 2 hours 30 minutes. So it must be 2 hours and 22.6 minutes. That's the sole purpose of the yellow scale — nothing else. Now let's move to the blue scale. The blue scale gives you the value of ram rise. Ram rise is the difference between Static Air Temperature, or SAT, and Total Air Temperature, or TAT. So the formula is: Ram Rise equals TAT minus SAT. Let me define those terms carefully. Static Air Temperature is the temperature of the ambient atmosphere surrounding the aircraft, with no inaccuracies caused by the measurement process. This is the temperature we need to calculate True Airspeed, or TAS, from Calibrated Airspeed, or CAS, and Pressure Altitude — or to calculate TAS from Mach number. However, what we actually measure with our temperature probe is Total Air Temperature, TAT. TAT is affected by compressibility and kinetic or adiabatic heating, and it is always a higher temperature than SAT. There's a good approximation for ram rise: Ram Rise equals (TAS divided by 100) squared, where TAS is in knots. So if TAS is 400 knots, ram rise is approximately 16 degrees Celsius. If TAS is 500 knots, ram rise is approximately 25 degrees Celsius, and so on. But this approximation tends to be slightly low — it's usually good enough for practical purposes, but it is not as accurate as the answer produced by the navigation computer. And for EASA ATPL examinations, the navigation computer's answer is the one that will be expected. Let me show you how to use the blue scale. 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. It comes out to about 17 degrees Celsius. So if the TAT at 400 knots TAS were, say, minus 22 degrees Celsius, then the SAT would be minus 39 degrees Celsius — because you subtract the 17-degree ram rise from the TAT. Now let's talk about fuel consumption. 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 the answer comes to more than 60 minutes, we use the yellow scale to read the answer in hours and minutes. Here's an example. Your fuel consumption is 2300 kilograms per hour. Your time to destination is 1 hour and 37 minutes. How much fuel will you use? Remember — minutes are always on the inner scale. So you set the black triangle against 2300. Then align the cursor with 1 hour 37 minutes — which is 97 minutes. Read off the answer: 37.2. The correct answer is obviously 3720 kilograms. If you use a calculator, you get 3718.33 kilograms. That should reassure you that the navigation computer is very accurate — in fact, probably considerably more accurate than your fuel gauges. So to summarise: the yellow scale converts minutes to hours and minutes. The blue scale gives ram rise — the difference between TAT and SAT. And fuel consumption works just like distance, speed, and time, using the same scales.