General Principles - Descent — Page 228, Lesson 266

Right, let's pick this up with the descent itself. We've covered the climb, now we're looking at how we get back down, and I want to start with the two fundamental ways we can think about descending. First, the emergency descent. For a Class A aeroplane — that's your large transport category jet — an emergency descent is flown at maximum operating speeds, with the speed brakes deployed and the thrust at idle. That's the absolute priority: get down fast. But what if the aim isn't speed, but endurance? What if we want to descend at the lowest rate of descent? To do that, the aeroplane needs to fly at a speed that gives the minimum excess power required. Let's look at Figure 4.9 — you can see that point clearly. It's found at the very bottom of the power required curve. You may recall this speed is called VMP — the minimum power speed. So, to lose height at the slowest possible rate, the aeroplane would fly at VMP. This lowest rate of descent is also known as maximum descent endurance, which essentially means the aeroplane will take the greatest time to descend. EASA sometimes refers to this as the speed for maximum glide endurance. Now, let's look at the factors affecting descent, starting with weight. For this, we'll only consider the effect in a glide — in other words, with idle power. Let's concentrate first on the minimum angle of descent, or the glide angle. Look at Figure 4.10. You can see that an aeroplane with a higher weight will have a larger amount of weight apparent thrust — that's the component of weight acting forward along the flight path. But if the aeroplane is still flying at VMD — which will be faster with a higher weight — it will also have a greater amount of drag. You'll recall from earlier that a higher weight moves the drag curve up and to the right. In Figure 4.10, notice that the forward and rearward forces along the flight path are still balanced, albeit a bit longer. But crucially, notice that the angle of descent is unchanged. This is the key point I want you to understand: weight has no effect on the minimum angle of descent, or glide angle, but it will increase the speed of the descent. In summary, weight has no effect on the minimum angle of descent, but it will increase the speed along that descent gradient, and therefore it will increase the rate of descent.