Class A - En Route — Page 452, Lesson 555

We've just finished the climb and we're level at the top of climb. Now, the big question: where do we cruise? The ideal is to be at the optimum altitude. You might remember that's the altitude for maximum specific range — in other words, maximum fuel mileage. But here's the catch: that altitude is not a fixed number. As we burn fuel and the aeroplane gets lighter, the optimum altitude actually increases. So if we wanted to fly at the absolute optimum all the time, we wouldn't be flying level at all — we'd be in a very slow, continuous climb throughout the cruise. Now, that's a problem for Air Traffic Control. ATC needs us in level flight to maintain vertical separation from other traffic. So in congested airspace, we can't just drift upward. The compromise is the step climb. We fly in segments of constant altitude, and each segment is established within 2000 ft of the optimum altitude. That's the key tolerance. By staying within 2000 ft of optimum, we keep our range at 99% of the maximum specific range. So we lose almost nothing in fuel efficiency, but we give ATC the level segments they need. There may be several step climbs during the flight, and overall the aeroplane is gaining altitude throughout the process. But there's a hard limit on how high we can go. As altitude increases, the thrust required to maintain a given speed increases. Eventually we reach an altitude where thrust is at its maximum cruise value — we can't climb any higher without exceeding thrust limits. That's the maximum altitude. And it's not a fixed number either. The hotter the atmosphere, the lower this maximum altitude becomes. In exceptionally hot conditions, the maximum altitude can be almost the same as the optimum altitude. So the envelope can really squeeze shut. Now, before we talk about the other altitude limits, we need to understand stalling. When we reduce speed, to keep producing enough lift to balance weight, the angle of attack must increase. But below a certain speed, the angle of attack gets so high that the airflow over the wing starts to separate from the boundary layer, producing turbulent airflow. The separation point fluctuates back and forth along the wing, creating strong eddies in that turbulent flow. Those eddies buffet the elevators or tailplane. That's the low speed buffet. If we fly below that speed, lift drops dramatically and a full stall ensues. So we have two ceilings to think about. The aerodynamic ceiling and the manoeuvre ceiling — we'll dig into those next. But for now, hold onto this: the optimum altitude rises as we get lighter, we step-climb within 2000 ft of it to keep 99% range, and the maximum altitude is set by thrust limits, which drop in hot air.