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

Let's pick this up with the second compliance rule, because it's the key to everything that follows. When we calculate the descent range to check whether a multi-engine Class B aeroplane can actually reach an airfield after an engine failure, we don't use the raw, measured descent gradient. We take that gross gradient of descent and we increase it by 0.5%. That adjusted value is called the net gradient. Think of it as a built-in safety margin — we're deliberately making the descent look worse than it really is, so that our calculated range is conservative. The rule is strict: the airfield must lie within the net descent range, not the gross descent range. We plan against the pessimistic number, not the optimistic one. Now, the drift down. This is where the twin-engine case gets genuinely harder than the single-engine case. For a single-engine aeroplane, calculating the descent range was simple arithmetic: you took the height of the aeroplane, divided it by the descent gradient, and multiplied by 100. Clean, fixed numbers. But for a twin after an engine failure, the gradient of descent is constantly changing. I want you to understand exactly why, because this is the heart of the drift down. Think about straight and level flight first. The forward force of thrust balances the rearward force of drag. Now the engine fails. Suddenly there is more rearward force than forward force — an excess of drag. If you tried to hold level flight, the aeroplane would slow down. To keep the speed — and the speed we want to hold is VMD, the minimum drag speed — the remaining live engine must produce more thrust so the forces can balance again. But here's the problem: a failed engine can't just magically give you more thrust. The only way to augment the thrust is to lower the nose. When you lower the nose, weight acts forward, and that forward component of weight provides what we call weight apparent thrust. That's the extra forward force that balances the excess drag. If you lower the nose by just the right amount, the forces balance once again and VMD is maintained. The only side effect is that the aeroplane is descending — that's the drift down itself. Now watch what happens as the descent continues. As the aeroplane descends in the atmosphere, air density increases. Denser air means the remaining engine produces more thrust. That increased thrust reduces the excess drag. So now we don't need as much weight apparent thrust, because the excess drag has shrunk. To reduce the weight apparent thrust, we raise the nose a little. And that's the cycle — the gradient is constantly changing because the density is constantly changing, and the pilot is constantly re-trimming the nose to hold VMD. Let me show you the forces at work in that early part of the drift down. That's the picture of the early drift down. And because the gradient keeps changing, we can't use that simple height-divided-by-gradient formula. Instead, we split the drift down profile into manageable segments, so we can calculate the descent range segment by segment. So the two big ideas to hold onto: first, the net gradient is the gross gradient plus 0.5%, and the airfield must be within the net descent range. Second, the drift down gradient is never constant — it changes because density changes with altitude, which changes the live engine's thrust, which changes the excess drag, which forces the nose to be re-adjusted to keep VMD. That's why the calculation has to be broken into segments.