So the pilot plans it well in advance — Page 221, Lesson 254

Let’s start with the big picture. In a normal flight, the descent doesn’t just happen anywhere — it begins at a specific point we call the top of descent, and that point can be up to 200 miles before the destination aerodrome. So the pilot plans it well in advance. But a descent can also be forced early, like after an engine failure or a depressurization. In those emergency cases, the descent is early and unplanned, and it becomes critical that you understand what determines the descent characteristics, because you need to maintain obstacle clearance — you have to stay above terrain and obstacles while you come down. Now, there are two ways we measure descent performance. One is by angle of descent, which is sometimes called descent range. The other is by rate of descent, sometimes called descent endurance. We’ll focus on the angle of descent first. To start a steady descent, the pilot normally reduces thrust. When thrust drops, the forward force of thrust becomes less than the rearward force of drag, so the aircraft slows down. The amount by which drag exceeds thrust is called excess drag. That’s the key term here — excess drag is the drag that is greater than the thrust force. Now, to balance the forces and keep the speed from decaying, the pilot lowers the nose. When the nose is lowered, the weight of the aircraft tilts forward, and a component of that weight now acts along the flight path, providing a forward force. That component is called the weight apparent thrust, and it’s expressed as W sin γ, where W is the weight and γ (gamma) is the descent angle. So the weight apparent thrust is the part of the aircraft’s weight that pulls it forward down the descent path. Once the nose is lowered enough, the weight apparent thrust provides just enough forward force to balance the excess drag. At that point, the aircraft maintains a steady descent angle at a constant speed. The forward and rearward forces are in balance again. Let me give you the exact balance equation. The drag, which we call DA, is balanced by the thrust T plus the weight apparent thrust W sin γ. So the equation is: DA = T + W sin γ Read that as: the total rearward force, drag, equals the forward thrust plus the forward component of weight. When that holds, you have a steady, constant-speed descent at a fixed angle. I want you to look at Figure 4.1, which shows this balance of forces in a normal powered descent. You’ll see the drag arrow pointing rearward, the thrust arrow pointing forward, and the weight apparent thrust component acting along the flight path. That’s the visual picture of the equation we just wrote. So the takeaway for this part: descent performance is measured by angle or rate, the descent starts at the top of descent, and in a steady powered descent the drag is balanced by thrust plus the forward component of weight, W sin γ. That balance is what holds your descent angle and your speed constant.