General Principles - Climb — Page 207, Lesson 242

Let's pick this up right where the graphs leave off. We've been looking at how decreasing density—whether from high temperature, high altitude, or high humidity—shifts the Power Available curve down and right, and the Power Required curve up and right. The net effect is less Excess Power, which means a reduced rate of climb for both jet and propeller aeroplanes. Now, here's the subtle part that catches many pilots. Notice from the graphs that the true airspeed for VY increases a little with decreasing density or increasing altitude. But we fly using indicated airspeeds, not true airspeeds. So we need to understand what happens to the indicated airspeed of VY. Here's the key relationship: if the true airspeed increases only slightly with altitude, the indicated airspeed will still fall. Why? Because at higher altitude, the air is less dense, so for a given indicated airspeed, the true airspeed is higher. Since VY as TAS only creeps up a little, the IAS must drop to compensate. So although VY as a true airspeed increases with decreasing density or increasing altitude, VY as an indicated airspeed decreases. In fact, VY will eventually fall to become the same value as VX. Let me summarize that clearly: reduced density decreases the indicated airspeed of VY, and it decreases the rate of climb. Two separate effects, both working against you as you climb. Now let's think about altitude itself. As the aeroplane flies higher, the Excess Power available diminishes, and therefore the maximum achievable rate of climb will decrease. There will come an altitude where the Excess Power available decreases to zero. At that point, the rate of climb also decreases to zero. That altitude is known as the absolute ceiling. Let me show you what that looks like on a graph. Here's how to read that excess power graph. VY is the speed that gives the maximum Excess Power available and therefore the maximum achievable rate of climb—that's the top of each curve. VX, on the other hand, can be found where the tangent out of the origin touches each curve. As altitude increases, the Excess Power available, the achievable rate of climb, and the indicated airspeed for VY all decrease. Eventually, there will be an altitude where VX and VY are the same speed, there is no more Excess Power, and the rate of climb is zero. That's the absolute ceiling again. Now, here's an important practical point. At its absolute ceiling, the performance of an aeroplane is so reduced that it is unable to manoeuvre. So the absolute ceiling is a rather abstract concept for a pilot. It's more useful to know the aeroplane's service ceiling. And that's what we're about to define next.