General Principles - Cruise — Page 246, Lesson 293

Right, let's get into the cruise phase. We've been looking at how the aeroplane behaves at altitude, and now we're going to tie the speeds together and then move into fuel management. First, let's look at the relationship between calibrated airspeed, true airspeed, and Mach number as altitude changes. Let's start with a constant indicated airspeed. If you hold calibrated airspeed, or CAS, constant as you climb, the air density is falling. Because the air is thinner, the aeroplane has to move faster through it to generate the same dynamic pressure, so the true airspeed, the TAS, increases. Now, here's the key part: as you go higher, the local speed of sound actually decreases. So if your TAS is increasing while the speed of sound is decreasing, the ratio between them—which is your Mach number—must increase. So, holding CAS constant means TAS goes up and Mach number goes up. Now, let's flip it. What if you hold true airspeed constant with increasing altitude? On the graph, the TAS line would be drawn vertically, straight up. In that case, your calibrated airspeed decreases, because the air is thinner and you're not getting the same pressure, and your Mach number increases, because the speed of sound is still falling while your TAS stays put. And one more scenario: what if you hold Mach number constant? Then the Mach line is drawn straight up. In that case, both TAS and CAS decrease with increasing altitude. Notice that pattern: whichever speed you hold constant, the other two move in predictable ways. Now, these graphs work for descent too. You just follow the lines down instead of up. So, looking at the constant Mach number graph, if you descend at a constant Mach, your EAS and TAS will both increase as you get into denser air. Here's a memory aid for drawing these lines. The lines for C, T, and M—calibrated, true, and Mach—always appear from left to right in that order. You can remember it with the acronym CTM, and a handy way to recall that order is "Chicken Tikka Masala." C, T, M, left to right. Now, let's shift to fuel flow. In a turbojet, fuel flow is proportional to thrust. So, as thrust increases, fuel flow increases. But for aeroplanes driven by a propeller—regardless of whether it's a piston or a turboprop engine—fuel flow is proportional to power, not thrust. That's why, when we talk about range and endurance, we treat turboprop aeroplanes as propeller aeroplanes. That brings us to the two big cruise performance parameters: range and endurance. 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? Let's deal with endurance first. Endurance is the time an aeroplane can remain airborne on a given quantity of fuel. Another way to put it: endurance can be expressed as fuel used over a given airborne time. Now, when would a pilot actually fly for maximum endurance? The only time is when the aeroplane is in a holding pattern over its destination—for instance, when there are long landing delays and running out of fuel starts to become a problem. That's the situation where you want to stretch the fuel as long as possible in the air.