General Principles - Cruise — Page 251, Lesson 304

Let’s pick this up right where we left off. We’ve already covered endurance, so now we’re moving into the second of the two cruise performance parameters: range. First, let me give you the summary of where we stand on endurance, because it sets the stage. For a jet aeroplane, maximum endurance is achieved at or above the tropopause — that’s the boundary layer in the atmosphere where the temperature stops decreasing with altitude. For a turbo-propeller aeroplane, maximum endurance comes at about 10,000 feet. And for a piston engine aeroplane, maximum endurance is at sea level. Keep those three in mind — they’re a classic comparison. Now, range. Range is actually the more useful performance parameter of the two, and it’s the one aircraft designers are constantly trying to improve. Here’s the key distinction: endurance is about airborne time — how long you can stay up. Range is about distance covered — how far you can go. Because of that, range is sometimes called “fuel mileage.” Think of it like your car: you want to know how many miles you get per gallon, not just how long you can keep the engine running. For range, the concern isn’t just to minimize fuel flow — that’s the endurance mindset. For range, the more important thing is to maximize speed. That way, for a given amount of fuel, you cover a greater distance. Now, the formal definition. Maximum range is the maximum distance an aeroplane can fly for a given fuel quantity consumed. Another way to say it — and this is the expression more commonly used in commercial operations — is the minimum fuel used by an aeroplane over a given distance. Both definitions describe the same thing, just from opposite directions: one fixes the fuel and asks how far, the other fixes the distance and asks how little fuel. As a basic formula, range is simply distance in nautical miles divided by fuel quantity in kilograms: RANGE = DISTANCE (NM) ÷ FUEL (kg) But — and this is exactly the same adjustment we made for endurance — that raw formula doesn’t give us useful information on its own. 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, which is your TAS — true airspeed. The bottom line becomes kilograms of fuel per hour — your fuel flow. So the formula now reads: SPECIFIC RANGE (SR) = TAS ÷ FUEL FLOW That’s the formula for specific range, and it’s the one that actually matters for cruise planning. It tells you how many nautical miles you get per kilogram of fuel at a given speed and fuel flow. That’s the heart of fuel mileage in aviation terms. Let me pause there — that’s the core of what we’ve got