General Principles - Cruise — Page 267, Lesson 324

Right, let's pick this up with the Long Range Cruise concept. We've just looked at the wind-altitude trade-off graph for an Airbus, and now I want to walk you through what happens when we actually choose a cruise speed in commercial operations. Let's start with the relationship between aeroplane speed and range. Figure 5.29 shows this relationship, and the key point is the top of the blue curve. That top represents the point of maximum range, and the speed at which this is found. For a jet aeroplane, you'll recall this speed is 1.32 VMD. Now, VMD stands for the minimum drag speed — the speed at which total drag is at its lowest. So 1.32 VMD is the speed that gives you the greatest range for a given amount of fuel. In commercial operations, this speed is more commonly referred to as the Maximum Range Cruise, or MRC. So MRC is just the operational name for that 1.32 VMD speed. Now here's the important part. Maximum range cruise speed is seldom flown. Usually, a higher speed is used. Why? Look at the top of the graph — the line is fairly flat. That flatness means a significant speed increase can be achieved with only a small compromise in range. In other words, you can go noticeably faster without losing much range. This higher speed is called the Long Range Cruise, or LRC. The long range cruise speed is about 4% higher than the maximum range cruise speed. And as such, the specific range reduces by about 1%. Let me define specific range — it's the distance travelled per unit of fuel consumed. So you're giving up only about 1% of your range per fuel unit to gain that 4% speed increase. That's a very good trade. Now, why do we bother? The reason for using this higher speed is simply that there are costs other than fuel which need to be considered in commercial operations. Fuel isn't the only expense — there's crew time, aircraft utilisation, maintenance, and so on. Flying faster means you complete the sector sooner, which can reduce those other costs. A more detailed explanation of the relationships of these costs and how they affect operations will be dealt with later on in the course. So to summarise: MRC is the maximum range speed at 1.32 VMD, but we rarely fly it. We fly LRC instead — about 4% faster, sacrificing only about 1% of specific range, because time and other costs matter in commercial flying.