General Principles - Cruise — Page 263, Lesson 318

We're now into the cruise phase of the flight, and I want to walk you through the concept of optimum altitude. This is one of the most important ideas in jet performance, because it directly governs how efficiently you burn fuel on a long sector. Let's start with the definition, because it's precise and you'll need it word-for-word. The optimum altitude is defined as the pressure altitude which provides the greatest specific range or fuel mileage at a given weight and speed. So it's not just any altitude — it's the one that gives you the best fuel mileage for the weight you're carrying and the speed you're flying. Fly higher than it, or lower than it, and your range decreases. That's the key takeaway: there's a sweet spot, and deviating from it costs you range. Now, here's the critical thing to understand: the optimum altitude is not fixed. It changes as the flight progresses. Let me walk you through the chain of cause and effect, because this is where the logic lives. You'll recall that as weight decreases through fuel burn, the drag curve moves down and to the left. That means the best range speed — which for a jet is 1.32 times VMD, the speed for minimum drag — falls. And the total drag decreases as well. So as the aeroplane gets lighter, it needs to slow down to maintain that best range speed. Now, as it slows down, the Mach number also decreases. That's important because it means the aeroplane is no longer limited by the high Mach number and the corresponding high drag that comes with it. That freedom — that margin below the Mach limit — is what allows the aeroplane to climb a little. Here's the beautiful part of the logic. As the aeroplane climbs, the Mach number increases again, back up to its previous limiting value, and drag increases back to its previous value. So you're not gaining anything on drag — you're back where you started. But here's the payoff: the higher altitude has decreased the specific fuel consumption. That's the key benefit. Because fuel consumption is lower at altitude, the specific air range — the distance you get per unit of fuel — increases during that little climb. So the net effect over time is this: as weight decreases with fuel burn, the optimum altitude increases. And notice something else — as the optimum altitude increases, the specific range also increases. So the aeroplane actually becomes more efficient as it gets lighter and climbs. Let me point you to the figures so you can see this visually. In Figure 5.25, you can see the effect of altitude on specific range for a jet at high and low weight. Using the left-hand blue line, you'll notice there's an altitude at which the specific range is greatest — in the example given, that's just below 33,000 feet. That's the optimum altitude for that weight. And by comparing the specific range line for high weight versus low weight, you can see how the peak shifts. Then in Figure 5.26, you can see the optimum altitude plotted over time — it's a graph showing the optimum altitude increasing with a reduction in weight as the flight progresses, for a typical jet aeroplane. Now, here's the operational consequence. In order for the aeroplane to maximize the specific range, it must stay with the optimum altitude as that altitude slowly increases. In other words, the aeroplane must climb along the green line shown in Figure 5.26. Climbing in this way is sometimes called a — and the text cuts off there, but you can see the direction this is going: it's a step-climb or cruise-climb technique, where you follow the rising optimum altitude rather than holding a fixed level. So let me tie it all together. The optimum altitude is the pressure altitude giving the greatest specific range at a given weight and speed. It's not fixed — it rises as weight decreases through fuel burn. The mechanism is: lower weight → lower best range speed → lower Mach number → margin to climb → climb restores Mach and drag but reduces specific fuel consumption → specific air range increases. And to capture that benefit, you climb along the rising optimum altitude as the flight progresses. That's the complete picture of optimum altitude in cruise.