Multi-engine Class B - En Route and Descent — Page 360, Lesson 435

We've just seen how, after an engine failure in the cruise, the aeroplane is forced to descend. Now I want to walk you through what happens next, because this is the heart of the drift down procedure. As the aeroplane descends, the air gets denser. That means the remaining engine can produce more thrust. So, the descent angle can slowly be reduced. This process continues until the remaining engine generates sufficient thrust to balance the drag without any need for what we call "weight apparent thrust." Let me unpack that term, because it's crucial. When you're descending, part of the weight acts along the flight path, helping to pull the aeroplane forward. That component of weight is doing some of the work of overcoming drag. That's the "weight apparent thrust." At the altitude where the engine's thrust alone balances the drag, the aeroplane no longer needs that help from weight, and it is able to level off. So, in summary: after engine failure in the cruise, the aeroplane is forced to descend, but as it descends, it can slowly reduce the descent angle until it can once more fly level. That whole procedure is the drift down procedure, and it produces a drift down flight profile, which is shown in Figure 12.2. Now, the tricky part: calculating the descent range for a twin-engine aeroplane after engine failure is complicated. And the reason, which is now hopefully apparent, is that the descent gradient—or descent angle—is constantly changing. It's not a single fixed number. In the absence of a drift down graph, the only feasible way to calculate the descent range is to break the profile down into manageable segments and carry out several calculations, exactly as shown in Figure 12.2. For each of these segments, you will need two things: the net descent gradient at that altitude, and the vertical interval of that segment. The net descent gradient is the actual slope of the descent at that particular altitude, accounting for the engine failure. The vertical interval is simply the height lost over that segment. Multiplying those together—or rather, using them together—gives you the horizontal distance covered for that segment. To find the total descent range, you simply add all the horizontal distances from all the segments together. Once the descent range has been calculated, and the aeroplane is able to fly straight and level, the last thing to do is find out the one engine inoperative cruise range. That's the distance the aeroplane can fly level on the single remaining engine. Once you know that, you add it to the descent range of the drift down profile. That gives you the total range of the aeroplane following engine failure. And here's where the operational regulation comes in. At any point along the flight, there must be an airfield at which a landing can be made within the range of the aeroplane after engine failure. To ensure this, a circle is drawn around each airfield between the departure and destination points. The radius of that circle is the total single-engine range. To comply with the regulations, the aeroplane's track must fall inside these circles. In doing so, the aeroplane will comply with EU-OPS, which states that in the event of engine failure, the aeroplane is capable of continuing flight to an aerodrome where a landing can be made. So the whole picture is: engine fails, you drift down, you calculate the descent range segment by segment, add the single-engine cruise range, and that total range defines a circle around each airfield. Your track must stay inside those circles to stay legal under EU-OPS. That's the complete drift down range story.