General Principles - Cruise — Page 263, Lesson 322

Let’s pick this up right at the point where we’re looking at how a piston-propeller aeroplane behaves at altitude, because the logic here is a beautiful trade-off between two opposing effects. I want you to picture the aeroplane climbing. As it operates at higher and higher altitudes, the power required to maintain the range speed will increase. That’s the first effect, and it’s detrimental to range — more power needed means more fuel burned for the same distance. But here’s the second effect: as altitude increases, true airspeed increases for any given indicated airspeed. That’s good for range, because you’re covering more ground per unit of time. Now, the key point is that this true airspeed benefit slightly more than offsets the increase of power required. So the net result is that the specific range slowly increases with altitude. Specific range, remember, is the distance flown per unit of fuel — so it’s going up, slowly, as you climb. But this can’t go on forever. The aeroplane will eventually reach an altitude where the throttle needs to be fully advanced just to maintain the selected speed. That altitude has a precise name: the full throttle height. It’s shown in Figure 5.27. Beyond this altitude, the selected power and the selected airspeed cannot be maintained — the throttle is already wide open, so you can’t add more — and the aeroplane will slow down. Very soon after this altitude, the true airspeed will also start to fall, despite the decreasing density. You’d think thinner air would let you go faster, but the power deficit wins. This fact, combined with the constantly increasing amount of power required, means that the specific range will decrease. So the maximum specific range is attained just after full throttle height — not at it, but just past it, because of that slight lag before true airspeed starts to fall. Now let’s move to the wind altitude trade-off. The effect of headwinds and tailwinds on range can play a significant role in your choice of cruising altitude. If there’s a considerable headwind at your selected cruising altitude, that’s detrimental to range — you’re fighting it the whole way. In that case, it may be beneficial to operate at a different altitude where the winds might be more favourable. In large commercial operations, most of these considerations are dealt with prior to the flight by the flight planning personnel — it’s done on the ground, in advance. But in smaller operations, and if conditions change in flight, a pilot may have to carry out a wind altitude trade-off calculation themselves. The information enabling you to do this is usually given in the aeroplane flight manual, an example of which is shown in Figure 5.28. So the takeaway for you as a pilot: you’re balancing two competing factors — the power required and the true airspeed benefit — and the wind at altitude can tip that balance, which is why you might trade one altitude for another.