General Principles - Descent — Page 228, Lesson 266

Right, let's pick this up with the descent phase. We've covered the general climb and cruise performance, and now we're looking at how an aeroplane behaves when it's coming down. First, let's establish the two extreme ways we might want to descend. For a Class A aeroplane — that's your large transport category jet — an emergency descent is flown at maximum operating speeds, with the speed brakes deployed and the thrust at idle. That's the fast, dramatic way down. But what if the aim is the opposite? What if we want to descend at the lowest rate of descent? That's a different target entirely. To achieve the lowest rate of descent, the aeroplane needs to fly at a speed that gives the minimum excess power required. Now, think back to the power required curve — the U-shaped curve we plotted for level flight. The very bottom of that curve is the point of minimum power required. And you'll recall that the speed at the bottom of that curve has a name: VMP — the speed for minimum power. So, to lose height at the slowest possible rate of descent, the aeroplane must fly at VMP. This lowest rate of descent has a special name: it's called maximum descent endurance. Essentially, it means the aeroplane will take the greatest time to descend — it's stretching the descent out for as long as possible. EASA sometimes refers to this as the speed for maximum glide endurance. So remember the pairing: VMP gives you the slowest rate of descent, which is the maximum endurance in a descent. Now, let's move on to the factors affecting descent, and we'll start with weight. For this, we're only going to consider the effect of weight in a glide — that is, with idle power. And we're going to concentrate on the minimum angle of descent, which is the glide angle. Here's the key insight, and it's a subtle one. Look at Figure 4.10. An aeroplane with a higher weight will have a larger amount of what we call weight apparent thrust — that's the component of weight acting forward along the flight path, pulling the aeroplane down the glide slope. But — and this is crucial — if the aeroplane is still flying at VMD, the speed for minimum drag, that speed will be faster with a higher weight. And because it's flying faster at VMD, it will also have a greater amount of drag. You'll recall from earlier that a higher weight moves the drag curve up and to the right. So in the glide, the forward and rearward forces along the flight path are still balanced — they're just larger in magnitude, a bit longer on the diagram. But crucially, the angle of descent is unchanged. The balance point shifts, but the slope stays the same. This is the important takeaway: weight has no effect on the minimum angle of descent, or the glide angle. But it will increase the speed of the descent. So to summarise: weight has no effect on the minimum angle of descent, but it increases the speed along that descent gradient, and therefore it increases the rate of descent. A heavier aeroplane glides down the same slope, but it gets down faster because it's moving along that slope more quickly.