General Principles - Cruise — Page 263, Lesson 322

I want to walk you through the cruise performance of a piston-propeller aeroplane now, because it behaves quite differently from the jet we just looked at. Let me start with the key idea: as the aeroplane climbs higher, two opposing things happen. First, the power required to maintain the range speed increases. That's detrimental to range — more power means more fuel burned. But second, as altitude increases, true airspeed increases for any given indicated airspeed. That's good for range, because you're covering more ground for the same indicated reading. The key point here is that this true airspeed benefit slightly more than offsets the increase in power required. So the net effect is that specific range slowly increases with altitude. Now, specific range — just to be clear — is the distance you can fly per unit of fuel. So as you climb, you're getting slightly more distance per fuel unit, up to a point. That point comes when the throttle needs to be fully advanced just to maintain the selected speed. We call this altitude the full throttle height. You can see it marked on Figure 5.27, which shows specific range against altitude for a typical piston-propeller aeroplane. Beyond full throttle height, the selected power and selected airspeed can no longer be maintained — the aeroplane will slow down. And here's the subtle part: very soon after this altitude, the true airspeed will also start to fall, despite the decreasing density. Normally, lower density would let true airspeed rise, but the engine just can't deliver the power anymore. That falling true airspeed, combined with the constantly increasing power required, means specific range now decreases. So the maximum specific range is attained just after full throttle height — not at it, but just beyond it. Now let me move to the wind altitude trade-off. Headwinds and tailwinds can play a significant role in choosing your cruising altitude. If there's a considerable headwind at your selected cruising altitude, that's detrimental to range — you're fighting the wind. In that case, it may be beneficial to operate at a different altitude where the winds are more favourable. In large commercial operations, most of these considerations are dealt with before the flight by flight planning personnel. But in smaller operations — and if conditions change in flight — a pilot may have to carry out a wind altitude trade-off calculation yourself. The information you need for this is usually given in the aeroplane flight manual, and there's an example of that in Figure 5.28. So the takeaway for you as a pilot: for a piston-propeller aeroplane, maximum range sits just past full throttle height, and wind at altitude can shift where you want to cruise. That's the core of the wind altitude trade-off.