General Principles - Climb — Page 207, Lesson 242

Let's pick this up right where the density discussion left off. We've established that when density decreases—from high temperature, high altitude, or high humidity—the Power Available curves move down and right, and the Power Required curves move up and right. The net effect is less Excess Power available, which directly reduces the rate of climb for both jet and propeller aeroplanes. Now, here's the critical part for you as a pilot. Look at Figure 3.51. Notice that the true airspeed for VY—that's the speed for best rate of climb—increases a little with decreasing density or increasing altitude. But here's the catch: we fly using indicated airspeeds, not true airspeeds. So we need to understand what happens to the indicated airspeed of VY. Let me explain this carefully. Using Figure 3.52, you'll see that even though the true airspeed increases only slightly with altitude, the indicated airspeed still falls. Why? Because at higher altitude, the air is less dense, so the same indicated airspeed corresponds to a higher true airspeed. So if VY as a TAS increases only a little, the IAS must actually decrease to compensate for the lower density. Here's the key summary: as density decreases, VY as a true airspeed increases, but VY as an indicated airspeed decreases. And eventually, VY will fall to become the same value as VX—that's the speed for best angle of climb. So in summary, reduced density decreases the indicated airspeed of VY and decreases the rate of climb. Now let's think about altitude specifically. As the aeroplane flies higher, the Excess Power available diminishes, and therefore the maximum achievable rate of climb decreases. There will come an altitude where the Excess Power available decreases to zero, as shown in Figure 3.53. At that point, the rate of climb also decreases to zero. This altitude is known as the absolute ceiling. Let me make sure you understand what's happening at the absolute ceiling. At that altitude, there is no more Excess Power available—the Power Available curve just touches the Power Required curve, so there's no surplus power to climb with. The rate of climb is zero. Now look at Figure 3.54. This shows the excess power for a typical aeroplane at various altitudes. Notice that VY is the speed that gives the maximum Excess Power available and therefore the maximum achievable rate of climb—that's shown by the top of each curve. Also note that on this graph, VX can be found where the tangent out of the origin touches each curve. That's a geometric way of finding the best angle of climb speed. As altitude increases, notice that 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, and there is no more Excess Power, so the rate of climb is zero. That altitude is the absolute ceiling. 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 not a practical operating altitude. What's more useful for a pilot is to know the aeroplane's service ceiling. And that's what we're about to define next.