General Principles - Descent — Page 228, Lesson 264

Let's start with the descent itself. The rate of descent is equal to the power required minus the power available, divided by the weight. So, power required minus power available gives you what we call the excess power required. For any given weight, the rate of descent is determined 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. Now, think about an emergency descent. Say the pilot initiates one following depressurization. The aim is to reach FL100, that's flight level 100, 10,000 feet, as soon as possible. To achieve that, the aeroplane needs to lose height with the maximum possible rate of descent. That can only happen if the aeroplane has 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, and they're highlighted in purple. Notice there isn't very much excess power required there, but it is possible to generate more. To create the conditions of the greatest excess of power required, the power available, represented by the green lines in the previous diagram, must be reduced to zero. That's shown in Figure 4.9. With the throttle closed, no power is available, so all of the area beneath the power required curve becomes excess power required. The purple areas are now as large as possible. To achieve the greatest rate of descent, the aeroplane needs to fly at a speed that achieves the greatest excess power required. 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. Therefore, the aeroplane needs to be configured for high drag and high speed in order to achieve maximum power required. In fact, for a lot of aeroplanes, that means deploying the speed brakes, the airbrakes, and possibly the landing gear to increase drag, while flying at the maximum permissible speed. That combination gives you the maximum excess power required, and therefore the maximum rate of descent for the emergency descent.