Let’s start the descent chapter — Page 221, Lesson 254

Let’s start the descent chapter. I want to walk you through the very beginning of it, because this sets up everything else. First, the big picture. Descent performance is all about the forces acting on the aircraft during the descent, and what factors govern that descent. In a normal flight, the descent begins at a point we call the "top of descent," and that point can be up to 200 miles before the destination aerodrome. So you can see, this is a long, planned phase, not a last-minute dive. But a descent is also required in an emergency, specifically following an engine failure or a depressurization. In that latter situation, the descent is forced early. And here's the critical part: it's important for the pilot to be aware of what determines the characteristics of the descent, so that obstacle clearance can be maintained. That's the whole reason we study this — safety in an unplanned descent. Now, there are two ways of measuring the descent performance of an aircraft. Either by angle of descent, which is sometimes called descent range, or by rate of descent, which is sometimes called descent endurance. So angle of descent is about how far you travel horizontally while descending — the range. Rate of descent is about how long the descent takes — the endurance. Keep those two pairs in mind: angle/range, and rate/endurance. Let's focus on the angle of descent first. To initiate a steady descent, thrust is normally reduced. When you pull the power back, the forward force of thrust becomes less than the rearward force of drag, and the aircraft slows down. Now, the value of drag that exceeds the thrust force is given a specific name: it's called excess drag. That's the key term here — excess drag is the amount by which drag is greater than thrust. To balance the forces and maintain speed, the nose is lowered. When you lower the nose, the weight of the aircraft now has a component acting forward along the flight path. That forward component of weight is what balances the excess drag. And this is shown in Figure 4.1. Now the aircraft will maintain this steady descent angle at a constant speed, because the forward and rearward forces are in balance once again. Let me give you the exact force balance equation, because this is the heart of it: Drag, which I'll call DA, is being balanced by the thrust, T, plus the weight apparent thrust, which is W sin γ. So the equation is: DA = T + W sin γ. Let me unpack each symbol. DA is the drag force. T is the thrust. W is the weight of the aircraft. And γ — that's the Greek letter gamma — is the descent angle. So W sin γ is the component of the aircraft's weight acting along the flight path, pulling it forward down the slope. That's what I called the weight apparent thrust. So in a steady descent, the total rearward force, which is drag, is exactly balanced by the sum of the forward forces: the engine thrust plus the forward component of weight. That's the balance that keeps the descent angle steady at a constant speed. That's the core of the angle of descent. Next we'll look at the rate of descent, which is a different way of measuring the same descent performance.