General Principles - Descent — Page 228, Lesson 268

Let’s pick this up right where the descent picture gets interesting. We’ve already seen how weight affects the descent, and now I want to walk you through the next big factor: the aeroplane’s configuration — meaning the position of the flaps and the undercarriage, or gear. To make this clean, we’re going to assume idle thrust, so the engine is essentially producing no power. That way, any change we see in the descent is purely down to the aeroplane’s shape and drag, not the throttle. Here’s the core idea. Look at Figure 4.11. If we deploy the flaps or lower the undercarriage, the excess drag increases. Excess drag is the drag that isn’t being balanced by thrust — it’s what pulls the aeroplane forward and down along its flight path. When that excess drag goes up, the aeroplane wants to slow down and steepen its path. To restore a balance of forces, the pilot lowers the nose. That action increases what we call the weight apparent thrust — effectively, the component of weight acting along the flight path that now helps pull the aeroplane forward. So the forces come back into balance, but here’s the important part: that balance is achieved at a higher angle of descent, and therefore a higher rate of descent. So the takeaway from the configuration effect is: more drag from flaps and gear means a steeper glide path and a faster sink. Now, we can see the same thing graphically. Figure 4.12 shows the drag curve for both the jet and the propeller aeroplane, with the excess drag shown as the purple area. When we deploy flaps and undercarriage, you’ll recall the curves move up and to the left. That shift increases the excess drag, which increases the angle of descent for any given speed. And notice something else: the speed for the minimum angle of descent — that’s VMD, the speed that gives the shallowest glide angle — is lower when the gear and flaps are out. The same logic applies to the rate of descent, and we can see it in Figure 4.13 using the power required graph. Here the purple area represents excess power required. With flaps and gear deployed, the power curves move up and left, which increases the excess power required and therefore increases the rate of descent. And again, the speed for the minimum rate of descent — that’s VMP — is lower. So let me summarise that clearly, because it’s a classic exam point. With gear and flaps deployed, the angle of descent increases and the rate of descent increases, but the speeds for minimum angle and minimum rate of descent both decrease. More drag, steeper and faster descent, but you achieve your best glide at a slower speed. Now let’s move on to the next factor: wind. Figure 4.14 shows the effect of headwinds and tailwinds on the angle of descent. A headwind steepens the glide angle and decreases the descent range — you don’t travel as far horizontally for the same loss of height. A tailwind does the opposite: it decreases the glide angle and increases the descent range, so you glide further. But here’s the subtle part that often catches people out. The aeroplane flying in a headwind or a tailwind reaches the same descent altitude in the same time as the aeroplane flying in zero wind conditions. That tells us something important: a headwind or tailwind has no effect on the rate of descent. The rate of descent is purely a function of the aeroplane’s performance and configuration, not the wind. The wind only changes the ground track — how far you travel over the ground — not how fast you sink through the air. So to tie it all together: configuration changes the drag, which changes both the angle and rate of descent, and lowers your best-glide speeds. Wind changes the angle and the range over the ground, but leaves the rate of descent untouched. That’s the full descent picture for configuration and wind.