General Principles - Descent — Page 221, Lesson 261

We’re now into the descent part of General Principles, and I want to pick up right where the glide angle left off, because the next idea builds directly on it. We just established that VMD is the speed for the minimum angle of descent, and that the lift-drag ratio reaches its maximum at 4 degrees angle of attack. Now here’s the critical operational warning that follows from that. If, following an engine failure, you fly at VMD, your aeroplane will be flying at the smallest possible glide angle. That is the flattest glide you can achieve. The temptation, of course, is to raise the nose a little to stretch the glide. But never try to stretch the glide by raising the nose. If you do, the speed will decrease, and the glide angle will steepen. In other words, at any speed other than VMD, your glide angle will be steeper than the optimal glide angle. Let me make sure you understand why that happens, because it’s counterintuitive. Raising the nose gives you an impression from the cockpit that the glide is being extended. But if the nose were raised, the weight apparent thrust would decrease, and the aeroplane would slow down. As a result, you would no longer be flying at VMD, so you would no longer have the best lift-drag ratio. The result would be a steeper descent angle, despite the fact that the aeroplane had a slightly higher nose attitude. So the cockpit impression is misleading. The discipline here is to follow the procedures for flying at the optimum glide angle as laid down by the manufacturer or the operator in the aeroplane flight manual. Don’t improvise with the nose. Now, we shift to the second way of assessing descent performance. Remember at the beginning of the descent section we said there were two ways. We have covered the angle of descent, which relates to descent range. Now we consider the rate of descent, which relates to descent endurance. Just as the rate of climb is a function of both climb angle and velocity, the rate of descent is a function of descent angle and velocity. Here’s the formula for the rate of descent. It is: Rate of Descent equals D V minus T V, all over W. Let me unpack that. D is drag, T is thrust, V is velocity, and W is weight. So D V is drag times velocity, and T V is thrust times velocity. The difference between them, divided by weight, gives you the rate of descent. Now, you may recall that force times velocity gives us power. So we can rewrite that formula in terms of power. The correct formula for the rate of descent is: Power Required minus Power Available, divided by weight. That is the excess power required. So the rate of descent is essentially the excess power required, expressed as a rate of losing height. That ties the descent performance directly back to the power relationship you already know from climb. So to summarise where we are: we have two complementary ways to assess descent. The angle of descent, optimised at VMD with the lift-drag ratio at its maximum, governs how far you can glide. The rate of descent, governed by the excess power required over power available, governs how quickly you lose height. Both are functions of velocity, and both are critical to managing a descent properly, especially after an engine failure.