General Principles - Descent — Page 221, Lesson 261

I want to walk you through the descent phase now, and we're going to start with the single most important speed in a glide — VMD. That stands for the speed for minimum drag, and it's the speed at which your lift-to-drag ratio, L/D, is at its maximum. Let me make that crystal clear, because it drives everything else. At VMD, your aeroplane is producing the most lift for the least drag. And here's the key number to remember: that maximum lift-to-drag ratio occurs at a fixed angle of attack of 4 degrees. So VMD is always flown at 4 degrees angle of attack. Because L/D is at its maximum there, VMD is also the speed for the minimum angle of descent — the shallowest glide angle you can achieve. That's why VMD is also called the speed for L/D Max. Now, what happens if you deviate from those 4 degrees? If your angle of attack is greater than 4 degrees, or less than 4 degrees, the speed will change, the lift-to-drag ratio will decrease, and consequently the angle of the glide will increase — meaning your descent gets steeper. So there's exactly one sweet spot, and it's locked to that 4-degree angle of attack. Let me put this into a real operational scenario, because this is where pilots make a classic mistake. Suppose you have an engine failure. If you fly at VMD, your aeroplane will be flying at the smallest possible glide angle — that's your best chance to cover the most distance before you touch down. Now, here's the temptation: you want to stretch the glide, so you instinctively raise the nose. Do not do that. Never try to stretch the glide by raising the nose. If you raise the nose, the speed will decrease, and the glide angle will steepen. At any speed other than VMD, your glide angle will be steeper than the optimal glide angle. Let me explain why that happens, because it's counterintuitive. When you raise the nose, the weight apparent thrust decreases — that's the component of weight acting along the flight path that's helping pull you forward. With less of that thrust, the aeroplane slows down. Once you slow down, you're no longer at VMD, so you no longer have the best lift-to-drag ratio. The result is a steeper descent angle, even though your nose attitude looks higher from the cockpit. It gives the impression that the glide is being extended, but it's actually being shortened. So be disciplined. Follow the procedures for flying at the optimum glide angle as laid down by the manufacturer or the operator in the aeroplane flight manual. Now let's move on to the second way of assessing descent performance. Earlier in the descent section, we said there are two ways to look at it. We've just covered the angle of descent, which relates to descent range — how far you can go. Now we consider the rate of descent, which relates to descent endurance — how long you can stay in the air. You've already learned that rate of climb is a function of both climb angle and velocity. The rate of descent works the same way — it's a function of descent angle and velocity. Here's the formula for rate of descent: Rate of Descent equals D times V minus T times V, all divided by W. Let me unpack that. D is drag, V is velocity, so D times V is the power required to overcome drag. T is thrust, so T times V is the power available from the thrust. The difference between them — power required minus power available — is the excess power required. And W is weight. So the rate of descent is the excess power required divided by the weight. Now, you may recall that force times velocity gives us power. So the correct way to write that formula is: Rate of Descent equals Power Required minus Power Available, divided by Weight. That's the excess power required divided by weight. In a descent, you're using more power than you have available — that deficit is what drives your rate of sink. So the bigger the power deficit, or the lighter the weight, the faster you descend. That's the complete picture of descent performance: VMD gives you the shallowest angle for maximum range, and the rate of descent formula ties your sink rate to the power deficit and your weight.