Multi-engine Class B - En Route and Descent — Page 360, Lesson 432

Let’s pick this up right where the rule leaves off. I want to walk you through the second compliance rule and then the drift down itself, because this is where the twin-engine aeroplane starts behaving very differently from the single-engine one. First, the second compliance rule. When you calculate the descent range to work out whether the aeroplane can actually reach an airfield, you must increase the gross gradient of descent by 0.5%. That adjusted gradient is called the net gradient. All it is, is a safety margin built into the descent range calculation. The key point here is that an airfield must lie within the net descent range — not the gross descent range. So you do the calculation on the pessimistic, safety-adjusted figure, and that’s the one that has to clear the airfield. Now, the drift down. Calculating the descent range of a twin-engine aeroplane after an engine failure is not as easy as it was for the single-engine aeroplane. For the single-engine aeroplane, you simply took the height of the aeroplane, divided it by the descent gradient, and multiplied by 100. That gave you the descent range. Simple, fixed numbers. But for a twin, the gradient of descent following engine failure is constantly changing. Let me explain why, because this is the heart of it. In straight and level flight, the forward force of thrust balances the rearward force of drag. When the engine fails, you suddenly have more rearward force than forward force. Because of that excess drag, the aeroplane will slow down if you try to maintain level flight. To keep the speed — and the speed should be kept at VMD, which is the speed for minimum drag — the thrust generated by the remaining live engine must be augmented so the forces can once again be balanced. Now, how do you augment that thrust? The only way is to lower the nose. By lowering the nose, weight can act forward and provide what we call weight apparent thrust — enough of it to balance the excess drag. If you lower the nose by a sufficient amount, the forces balance again and VMD can be maintained. The only side effect is that the aeroplane is descending. That’s the drift down. But here’s the twist. As the aeroplane descends in the atmosphere, the air density increases. That means the thrust being produced by the remaining engine increases, which reduces the excess drag. So now there’s no need for so much weight apparent thrust, because the excess drag has reduced. To reduce the amount of weight apparent thrust, the nose is raised a little. So you see the cycle: you lower the nose to get weight apparent thrust, you descend, density increases, thrust increases, excess drag drops, and you raise the nose a little. That’s why the gradient is constantly changing — it’s not a fixed number like the single-engine case. That’s exactly why the drift down calculation has to be split into manageable segments, which is what we’ll look at next.