Class A - En Route — Page 452, Lesson 555

We’re now into the en-route phase, Class A. The climb is done, the aeroplane has reached the top of climb, and it levels off at the cruise altitude. The key idea I want you to hold onto is that this cruise altitude should ideally coincide with the optimum altitude. You’ll recall the optimum altitude is the altitude for maximum specific range — in plain terms, maximum fuel mileage, the altitude where you get the most distance per unit of fuel. Now here’s the catch: that optimum altitude is not constant. As the flight burns fuel, weight decreases, and as weight decreases, the optimum altitude increases. So the aeroplane that wants to stay at optimum would not actually fly level — it would be slowly climbing throughout the cruise. But ATC requires level flight to maintain vertical separation from other traffic. So in congested airspace, the compromise is to fly in segments of constant altitude, each segment kept as close as possible to the optimum. Those level segments are established within 2000 ft of the optimum altitude. That procedure is the step climb — you’ve seen it in the General Principles cruise chapter. Staying within that 2000 ft band keeps the range at 99% of the maximum specific range. There may be several step climbs during the flight, and the aeroplane gains altitude through the process. But there is a limit to how high it can go. That limit is the maximum altitude. As altitude increases, the thrust required to maintain a given speed increases. Eventually you reach an altitude where thrust is at its maximum cruise value, and you cannot climb any higher without exceeding thrust limits. That altitude is the maximum altitude. Note the temperature effect: the hotter the atmosphere, the lower this maximum altitude becomes. In exceptionally hot atmospheres, the maximum altitude is almost the same as the optimum altitude. So the aeroplane cannot operate above the maximum altitude — but there are other altitude limits as well. That brings us to the aerodynamic ceiling and the manoeuvre ceiling. To understand these, we need to talk about stalling. When you reduce speed, to still produce enough lift to balance weight, the angle of attack must increase. Below a certain speed, the angle of attack is such 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 the turbulent flow. Those eddies buffet the elevators or tailplane. That phenomenon is the low speed buffet. Fly below that speed and lift drops dramatically — a full stall ensues. So we have two ceilings to consider: the aerodynamic ceiling and the manoeuvre ceiling. Let me walk you through each.