General Principles - Cruise — Page 263, Lesson 320

We’ve just been talking about the optimum altitude and how it rises as the aeroplane gets lighter. Now I want to walk you through what happens when you can’t simply follow that optimum altitude all the way, and that brings us to the idea of the “cruise climb”. A cruise climb is exactly what it sounds like — the aeroplane climbs continuously, very gently, so that it stays right at the optimum altitude as weight falls. But here’s the practical problem: air traffic control and airspace congestion may predetermine your flight cruising levels. In other words, you often can’t just keep climbing whenever you want, because ATC has assigned you a specific level and the airspace is busy. So when that’s the case, to stay close to the optimum altitude, you perform what are called step climbs. Those are shown by the dashed yellow line in Figure 5.26. Let me explain what a step climb actually is. Essentially, the aeroplane climbs to about 2000 feet above the optimum altitude and then levels off. So you overshoot the optimum by 2000 feet, settle there, and cruise. Now, as fuel is used and weight falls, the optimum altitude will increase — it keeps creeping upward — until it reaches a point where it is again 2000 feet above the aeroplane’s current level. But here’s the key timing detail: it can take up to 3 hours for that to happen. So you sit at that level for up to three hours while the optimum altitude climbs up to meet you, 2000 feet above you. Once that condition is met — the optimum is now 2000 feet above your current level — the aeroplane can then climb 4000 feet and level off. Why 4000? Because if you climb 4000 feet from where you are, you’ll once again be 2000 feet above the optimum altitude. You’ve re-established that same 2000-foot overshoot, and the whole cycle can repeat. That’s the step climb process, and it can be repeated throughout the cruise. And this is exactly why cruise altitudes at the end of a flight are higher than at the start — you’ve been stepping up the whole way. Now, there’s an important limitation I want you to note. If the last step climb is within 200 nautical miles of the top of descent, then the fuel saving is negated. In other words, if you’re that close to starting your descent, the benefit of climbing is wiped out — so the aeroplane should remain level. Don’t bother stepping up if you’re within 200 NM of the top of descent. Let me also give you the cost picture, because this is where the numbers matter. Carrying out step climbs this way, rather than always staying with the optimum altitude, will increase fuel consumption by about 1%, and therefore decrease the maximum range by 1%. That 1% may not sound like much, but over a year a typical 747 would have used an extra 34,000 tonnes of fuel. Let that sink in — 34,000 tonnes, just from that 1% penalty. And to really show you how important this is, consider the alternative: if an aeroplane did not even step climb and simply remained at a constant altitude during the whole cruise, then it would increase its fuel consumption by 10% compared to flying constantly at the optimum altitude. Ten percent — that’s a massive penalty. So this whole discussion demonstrates just how important altitude and speed control are in the cruise for a typical commercial flight. The difference between stepping up properly and just sitting level is the difference between a 1% penalty and a 10% penalty. So to pull it together: cruise climb is the ideal, ATC often forces step climbs instead, you climb 2000 feet above optimum, wait up to 3 hours for optimum to rise 2000 feet above you, then climb 4000 feet to re-establish that overshoot, repeat all cruise, skip the step if you’re within 200 NM of top of descent, and accept about a 1% fuel penalty for the privilege — versus 10% if you never step at all.