General Principles - Cruise — Page 251, Lesson 304

Right, let's get into the cruise phase properly. We've already covered endurance, which is all about how long you can stay airborne. Now we're shifting to the other big performance parameter: range. First, let me quickly recap where we left off with endurance, because the altitude picture matters. In summary: jet aeroplanes achieve maximum endurance at or above the tropopause — that's the boundary between the troposphere and the stratosphere, roughly 36,000 feet. Turbo-propeller aeroplanes reach maximum endurance at about 10,000 feet, and piston engine aeroplanes have their maximum endurance at sea level. So the engine type dictates the best altitude for staying airborne longest. Now, range. I want you to think of range as the more operationally useful parameter. Endurance is about airborne time; range is about distance covered. In fact, it's sometimes referred to as "fuel mileage" — that's a good way to picture it. For range, the concern isn't just to minimize fuel flow; more importantly, we want to maximize the speed. That combination — low fuel burn and high speed — is what lets the aeroplane travel a greater distance. Let me give you the formal definition. Maximum range is the maximum distance an aeroplane can fly for a given fuel quantity consumed. Or, put the other way, it's the minimum fuel used by an aeroplane over a given distance. That second expression is the one more commonly used for commercial operations, because in airline flying you're usually given a distance and you want to burn the least fuel covering it. As a basic formula, range is simply distance in nautical miles divided by fuel quantity in kilograms: RANGE = DISTANCE (NM) ÷ FUEL (kg) But just like we did with endurance, we have to adjust this formula so it gives us useful, practical information. The range an aeroplane can achieve is determined by two things: the speed of the aeroplane and the fuel flow. So we rebuild the formula. The top line becomes nautical air miles per hour — that's your TAS, true airspeed. The bottom line becomes kilograms of fuel per hour — that's your fuel flow. So the formula now reads: SPECIFIC RANGE (SR) = TAS ÷ FUEL FLOW That's the formula for specific range. And that's the key performance measure we'll build on — it tells you how many nautical miles you get per kilogram of fuel at a given speed and fuel flow. We'll take it from there.