General Principles - Climb — Page 188, Lesson 220

I want to walk you through the climb performance section now, and we're going to start with how air density affects the climb. This is a turbojet graph, Figure 3.30, and it plots Thrust Available and Thrust Required against Indicated Airspeed, or IAS. On that graph you'll see two Thrust Available lines. One is labelled "Thrust Available (High air density)" and sits higher up, and the other is "Thrust Available (Low air density)" and sits lower down. The key point is this: any decrease in air density — which is the same as an increase in Density Altitude — will reduce Thrust Available. So the Thrust line moves downwards on the graph, exactly as shown. Now, why does that matter for climbing? Because the climb depends on Excess Thrust. Excess Thrust is the amount of Thrust Available remaining after Drag is balanced. In other words, it's the leftover thrust once you've overcome drag. Since decreased density reduces Thrust Available, it reduces Excess Thrust, and therefore the maximum climb angle is reduced. And here's the continuous 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 keeps getting worse the higher you go. Now, there's a very important subtlety here, and it's about the speed VX. VX is the speed for maximum climb angle. Notice that VX will remain constant with changes in air density. Why? Because at a constant IAS — that is, at VX — Drag will not vary. Drag depends on the dynamic pressure, and if you hold IAS constant, you hold dynamic pressure constant, so drag stays the same. But here's the catch you need to remember from earlier lessons: as air density decreases, True Airspeed must be increased to maintain the required dynamic pressure. So although the IAS for VX is constant with increasing Density Altitude, the TAS for VX will of course increase. The indicated speed stays the same, but the true airspeed behind it grows as the air gets thinner. One more factor on density: high humidity will also decrease air density, and therefore it will also decrease aeroplane performance. Now, this is already factored into performance charts, so it's not something you need to allow for in practice. But basic theory questions in the exam may require your knowledge of that fact, so keep it in mind. Now let's move to the effect of accelerating on climbing. I already said the ability to climb depends on Excess Thrust — the Thrust Available left after Drag is balanced. So 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. The thrust that could have gone into the climb is being diverted into speeding the aircraft up instead. So to tie it together: climb angle is governed by Excess Thrust, density altitude erodes that excess thrust and lowers the maximum climb angle while VX stays constant in IAS but rises in TAS, and any acceleration during the climb steals from the excess thrust and reduces the climb angle further.