General Principles - Cruise — Page 263, Lesson 320

We’ve just been looking at the cruise climb, where the aeroplane gradually climbs to stay at the optimum altitude as weight falls. But as I said, that’s not always possible, because air traffic control and airspace congestion may predetermine the flight cruising levels. So when that happens, we use step climbs instead, and those are shown by the dashed yellow line in Figure 5.26. Let me walk you through what a step climb actually is. Essentially, the aeroplane climbs to about 2000 feet above the optimum altitude and levels off. Now, as fuel is used and weight falls, the optimum altitude will increase. It will rise until it is again 2000 feet above the aeroplane’s current level — but here’s the catch, it can take up to 3 hours for it to do so. So at its current level, the aeroplane can then climb 4000 feet and level off, so that it will once again be 2000 feet above the optimum altitude. That’s the step — you climb 4000 feet, and you end up sitting 2000 feet above the optimum. But there’s an important limitation. If the last step climb is within 200 nautical miles of the top of descent, then the fuel saving is negated, and the aeroplane should remain level. In other words, don’t bother climbing if you’re that close to starting your descent — you won’t save any fuel. The step climb process can be repeated throughout the cruise, and this helps explain why the cruise altitudes at the end of the flight are higher than at the start. As you burn fuel and get lighter, you step up to higher levels. Now, let’s talk about the cost. 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 may not sound like much, but over a year, a typical 747 would have used an extra 34,000 tonnes of fuel. And if an aeroplane didn’t even step climb — if it 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. That really demonstrates just how important altitude and speed control are in the cruise for a typical commercial flight. So the key numbers to hold onto: 2000 feet above optimum, a 4000-foot climb step, up to 3 hours between steps, 200 nautical miles from top of descent as the cutoff, 1% fuel penalty for stepping, and 10% penalty for staying level the whole way.