Now, here's the interesting part — Page 263, Lesson 321

Altitude and speed control in the cruise — that's the heart of what we're looking at now. Let me walk you through how altitude affects range, first for the jet, and now for the propeller aeroplanes. For the turbo-propeller aeroplane, the key point is that most of them operate significantly lower than their jet counterparts. They seldom operate above 30 000 ft, so they never really suffer from the effects of getting close to the speed of sound. That's a crucial contrast — the jet's high-altitude cruise is limited by compressibility and Mach effects, but the turbo-prop stays well below that regime. Now, here's the interesting part. A turbo-propeller is based on the same engine design as a pure jet — it's a gas turbine at heart, just driving a propeller. So the effect of altitude on the turbo-prop is very similar to the jet. As altitude increases, the increasing true airspeed — the TAS — and the slightly decreasing specific fuel consumption both help to improve the specific range. Specific range, remember, is the distance flown per unit of fuel — the more you get per pound of fuel, the better. But this benefit is offset a little by the increasing power required at higher altitude. So whilst specific range does improve with altitude, above 10 000 ft it only improves by a small amount. That's a real practical limit — the gain flattens out. So the choice of altitude for a turbo-prop may depend more on the wind considerations, and on the time and fuel considerations involved in climbing to the selected altitude. In other words, you might choose a lower altitude to save the climb fuel, or to ride a favourable wind, rather than chasing a marginal range gain. Now the other type of propeller aeroplane — the piston engine. Here the picture is different. The piston engine has a more or less fixed specific fuel consumption, even though specific fuel consumption is lowest at high manifold pressures, low rpm, and with the mixture correctly set. So those are the conditions you'd aim for to get the best fuel economy — high manifold pressure, low revolutions per minute, correct mixture. Because the specific fuel consumption is essentially fixed, the only remaining variables in the specific air range formula for the piston engine aeroplane are the true airspeed and the power required. So for the piston engine, range is governed by those two factors — how fast you're flying and how much power you need to maintain that speed. Let me show you the graphs that illustrate these effects. shows the effect of altitude on specific range for a jet at high and low weights. shows how the optimum altitude increases as weight reduces during the flight. So to tie it together: for the turbo-prop, altitude improves specific range, but only modestly above 10 000 ft, and wind and climb considerations often dominate the choice. For the piston engine, the specific fuel consumption is fixed, so your range is purely a matter of true airspeed and power required. That's the altitude story for propeller aeroplanes.