General Principles - Cruise — Page 267, Lesson 324

Let’s pick this up right where the wind–altitude trade-off graph left off, and move into the heart of cruise performance: the difference between Maximum Range Cruise and Long Range Cruise. First, I want you to recall the speed we already met for a jet aeroplane, the speed for maximum range. That speed is 1.32 times VMD. Now, VMD is the speed for minimum drag — the speed at which the aeroplane’s total drag is at its lowest. For a jet, maximum range happens at 1.32 times that minimum-drag speed. In commercial operations, we don’t usually call it “1.32 VMD” — we call it the Maximum Range Cruise, or MRC for short. Now, look at Figure 5.29, which shows the relationship between aeroplane speed and range. The top of the blue curve represents the point of maximum range, and the speed at which that maximum range is found. That’s your MRC point — the very peak of the curve. Here’s the key operational reality: maximum range cruise speed is seldom flown. Why? Because in commercial operations, there are costs other than fuel that matter. So we usually fly a higher speed. And here’s the beautiful part — look at the top of that graph. The line is fairly flat. That flatness means a significant speed increase can be achieved with only a small compromise in range. That higher speed is called the Long Range Cruise, or LRC. Let me give you the numbers, because they matter. The long range cruise speed is about 4% higher than the maximum range cruise speed. And as a result, the specific range reduces by about 1%. Specific range — that’s the distance flown per unit of fuel, typically nautical miles per kilogram or per pound of fuel. So you give up only about 1% of your specific range, but you gain about 4% more speed. That’s a very good trade. The reason for using this higher speed is simply that there are costs other than fuel which need to be considered in commercial operations — things like crew time, maintenance, and the value of getting the aeroplane and its payload to the destination sooner. A more detailed explanation of how those costs relate and affect operations will be dealt with later on in the book. So, to summarise the core idea: MRC gives you the absolute maximum range at 1.32 VMD, but LRC gives you nearly the same range — only about 1% less specific range — for about 4% more speed. And that’s why, in practice, commercial jets fly Long Range Cruise rather than Maximum Range Cruise.