General Principles - Descent — Page 228, Lesson 264

Let’s start with the core idea of a descent, because everything here hangs on one equation. The rate of descent is equal to the power required minus the power available, divided by the weight. So I’ll write that as: rate of descent equals (power required minus power available) divided by weight. Now, power required minus power available is what we call the excess power required. So for any given weight, the rate of descent is determined entirely by that excess power required. The greater the excess power required, the larger the achievable rate of descent. Conversely, the lesser the excess power required, the smaller the rate of descent. That’s a direct, proportional relationship. Let me make sure the terms are clear. Power required is the power the aeroplane needs to maintain its current flight condition — it’s a function of speed and drag. Power available is the power the engine is actually delivering at that moment. When you subtract available from required, you get the excess — and in a descent, that excess is what drives you downward. Now let’s apply this to a real scenario: an emergency descent. Say the pilot initiates a descent following depressurization. The aim is to reach FL100 — that’s flight level 100, which is 10,000 feet — as soon as possible. To do that, the aeroplane needs to lose height with the maximum possible rate of descent. And that can only happen if the aeroplane has the maximum excess power required. Look at Figure 4.8. It shows the power required and power available curves for both a jet and a propeller aeroplane. The areas beneath the power required curves but above the power available curves represent the excess power required — those are highlighted in purple. Notice that in normal powered flight, there is not very much excess power required. But it is possible to generate more. How do we generate more? We reduce the power available to zero. In Figure 4.9, the throttle is closed — so power available drops to zero, and now all of the area beneath the power required curve becomes excess power required. The purple areas are now as large as possible. But here’s the key point: to achieve the greatest rate of descent, the aeroplane needs to fly at a speed that achieves the greatest excess power required. And it’s obvious from the graph that this can only be achieved at very high speeds. Remember, power required is a function of speed and drag. So to maximize power required, the aeroplane needs to be configured for high drag and high speed. In fact, for a lot of aeroplanes, that means deploying speed brakes, lowering the landing gear, and extending flaps — all of which increase drag, which increases power required, which increases the excess power required, which gives you the maximum rate of descent. So the whole logic chain is: close the throttle to zero power available, configure for high drag and high speed to maximize power required, and the excess power required becomes as large as possible — giving you the maximum rate of descent for the emergency descent to FL100. Take a look at Figure 4.8 and Figure 4.9 to see those purple excess power areas grow as the throttle closes.