General Principles - Cruise — Page 246, Lesson 293

Let’s pick this up right where the speed relationships leave off, because the next idea is the one that ties the whole cruise picture together. We’ve been looking at how calibrated airspeed, true airspeed, and Mach number behave as altitude changes. Now I want you to remember the order those three lines appear on the graph, because it’s a classic exam trap. The lines for C, T, and M — calibrated airspeed, true airspeed, and Mach number — always appear from left to right in that order. And here’s the memory aid the book gives you: think of Britain’s favourite food, Chicken Tikka Masala. C, T, M — Chicken, Tikka, Masala. That’s the order they appear on the graph, left to right, every time. Now, let’s look at what happens when you descend. These graphs work for descent too — you just follow the lines down instead of up. So look at Figure 5.12, where Mach number is held constant. If the pilot descends at a constant Mach number, then both the equivalent airspeed and the true airspeed will increase as you come down. That’s a key relationship to hold onto. Now we shift gears completely. We’re moving from speed relationships into fuel flow, and then into the two big cruise performance parameters: range and endurance. First, fuel flow. In a turbojet engine, fuel flow is proportional to thrust. So as thrust increases, fuel flow increases — they move together. But for aeroplanes driven by a propeller, regardless of whether it’s a piston engine or a turboprop, fuel flow is proportional to power instead. That distinction matters a lot, because when we talk about range and endurance, turboprop aeroplanes are treated as propeller aeroplanes. So you have to know which category your aircraft falls into: turbojet, where fuel flow tracks thrust, or propeller-driven, where fuel flow tracks power. Now, the two performance parameters. When we fly for range, we’re asking: how much fuel will the aeroplane use per unit distance? When we fly for endurance, we’re asking: how much fuel does the aeroplane use per unit time? Those are two different questions, and they lead to two different answers. Let’s deal with endurance first. The endurance of an aeroplane is the time it can remain airborne on a given quantity of fuel. Put another way, endurance can be expressed as fuel used over a given airborne time. So it’s a time-based measure — how long can you stay up on the fuel you have. And here’s the practical reality: the only time a pilot will fly for maximum endurance is when the aeroplane is in a holding pattern over its destination. Think about it — you’re circling, waiting for a landing slot, and if there are long landing delays, running out of fuel starts to become a real problem. That’s when maximum endurance matters most. So to recap where we are: we’ve got the C-T-M order on the graphs, we’ve got the descent relationship where constant Mach means EAS and TAS increase, we’ve got fuel flow proportional to thrust in turbojets and proportional to power in propeller aircraft, and we’ve defined endurance as time airborne on a given fuel quantity. That sets us up perfectly for range, which we’ll tackle next.