Let's pick this up from the start — Page 221, Lesson 258

We're moving into a new part of the descent chapter now — the minimum angle of descent and the glide. Let's pick this up from the start. First, a quick recap of where we are. You've just learned that to maximize the descent angle, you maximize excess drag. Excess drag is the amount by which drag exceeds thrust. Now we flip that idea completely on its head. After an engine failure, your aim is no longer to get down steeply. Your aim is to cover the greatest horizontal distance, so that you have the largest possible area in which to select a suitable landing field. That means you want the smallest possible angle of descent — sometimes called the minimum glide angle. So to descend at the smallest possible angle, excess drag must be minimum. Think about that contrast: maximum angle of descent needs maximum excess drag; minimum angle of descent needs minimum excess drag. That's the whole logic of this section. Now, where does that minimum excess drag occur? Look at the drag curve. VMD is the speed found at the bottom of the drag curve — VMD stands for velocity for minimum drag. At VMD, drag itself is at a minimum. So, with no power — that's the key condition, zero thrust — VMD is the speed that gives you minimum excess drag. Therefore, to descend at the minimum possible angle, the aeroplane must be flown at VMD. That's the rule: engine failed, you want maximum range in the glide, you fly at VMD. Now there's another way to examine glide performance, and that's through the aeroplane's lift over drag ratio — commonly called the lift drag ratio. But before we get into that, there's a little more detail to discuss first. In the glide descent, the resultant of drag and lift balances the force of weight. From your Principles of Flight lessons, the term for the resultant of lift and drag is the total reaction. Here's the geometry: if you take the drag force line and move it upwards, you form a triangle of forces. The angle gamma — that's the Greek letter gamma — is the angle between the lift and the total reaction. And here's the crucial point: that angle gamma is the same as the angle of descent. So if there were any change in the lift and/or drag values, both the angle gamma and the glide angle would change together. They're the same angle. Now, a typical modern jet has a maximum lift drag ratio of about 19. Where this ratio reaches its maximum, the value of the angle gamma will be at its minimum — and therefore the glide angle is at its minimum too. That ties it all together: maximum lift drag ratio gives you the shallowest glide, which is exactly what you want after an engine failure. So the whole picture is this: engine failure, you want maximum horizontal distance, you fly at VMD, which is the bottom of the drag curve, which gives minimum excess drag, which gives the minimum glide angle — and that corresponds to the maximum lift drag ratio, about 19 for a typical modern jet.