General Principles - Climb — Page 188, Lesson 220

Let’s pick this up right where the climb performance story gets interesting. We’ve already established that an aircraft climbs because of Excess Thrust — the Thrust Available left over after Drag is balanced. Now I want to walk you through what happens to that climb when the air gets thinner, and then what happens when you try to accelerate while climbing. First, the density effect. Look at Figure 3.30. You’ll see two Thrust Available lines — one labelled “High air density” and one labelled “Low air density.” The key idea is this: any decrease in air density — which we express as an increase in Density Altitude — will reduce Thrust Available. That means the Thrust line on the graph moves downwards. Why? Because a jet engine produces thrust by accelerating a mass of air; if the air is less dense, there’s less mass to accelerate, so the thrust falls off. Now, because decreased density reduces Excess Thrust, the maximum climb angle is reduced. And here’s the important part: Excess Thrust will continually decrease with increasing Density Altitude. So the maximum angle of climb will continually decrease as the aircraft climbs. It’s not a one-off drop — it’s a steady degradation the higher you go. But here’s a subtle point that often trips people up. The speed for maximum climb angle — that’s VX — will remain constant with changes in air density. Why? Because at a constant Indicated Airspeed, or IAS, equal to VX, the Drag will not vary. Drag depends on dynamic pressure, and at a constant IAS the dynamic pressure is fixed, regardless of density. However — and this is the catch — as air density decreases, True Airspeed must be increased to maintain that required dynamic pressure. So although the IAS for VX is constant with increasing Density Altitude, the TAS for VX will of course increase. In other words, the airspeed indicator reads the same, but the actual speed through the air is getting faster as you climb. There’s also a note about humidity. High humidity will also decrease air density, and therefore it will also decrease aeroplane performance. Water vapour is less dense than dry air, so moist air is lighter. Now, this is already factored into the performance charts, so it’s not something you need to allow for in normal operations. But basic theory questions in the exam may require your knowledge of the fact — so keep it in mind. Now let’s move to the second part: the effect of accelerating on climbing. We’ve said the ability to climb depends on Excess Thrust — the Thrust Available remaining after Drag is balanced. Hence an aircraft’s maximum climb angle is limited by its maximum Excess Thrust. Now, if there is a need to accelerate the aircraft while climbing, or a need to climb while accelerating, some of that Excess Thrust must be used for the acceleration. And that means the maximum climb angle will be reduced. Think of it as a budget: the Excess Thrust is a fixed amount of energy, and if you spend part of it on speeding up, you have less left to push the aircraft upward. So the climb angle suffers. That’s the core of this section: density reduces Excess Thrust and therefore climb angle, VX stays constant in IAS but increases in TAS, humidity acts like a density decrease, and acceleration during climb steals from the Excess Thrust budget, reducing the climb angle.