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

Let’s pick this up right where the drift down begins to flatten out. I want you to picture the aeroplane with one engine failed, still in the cruise. It has lost half its thrust, so drag now exceeds thrust, and the aeroplane must descend. But here is the key idea: as it descends, the air gets denser, and the remaining engine produces more thrust. So the descent angle can slowly reduce. This process can continue until the remaining engine generates sufficient thrust to balance the drag without any need for weight apparent thrust. At the altitude where this balance occurs, the aeroplane is able to level off. Let me unpack that phrase, "weight apparent thrust." When you are descending, part of the weight acts along the flight path and helps pull the aeroplane forward, effectively adding to the thrust. That component is the weight apparent thrust. So early in the drift down, you need that extra help because the one engine cannot yet match the drag. As you descend and the engine produces more thrust, you need less and less of that weight component. Eventually, the engine alone balances the drag, the weight apparent thrust drops to zero, and you can fly level again. 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. This procedure is known as the drift down procedure, and it produces a drift down flight profile similar to the one shown in Figure 12.2. Now, the construction of the drift down profile. Earlier I told you that calculating the descent range for a twin-engine aeroplane after engine failure was complicated. The reason, which is now hopefully apparent, is that the descent gradient, or descent angle, is constantly changing. It is not a single straight line down. In the absence of a drift down graph, the only feasible way of calculating the descent range is to break the profile down into manageable segments and carry out several calculations, as shown in Figure 12.2. Each of these calculations will need two things: the net descent gradient at that altitude, and the vertical interval of that segment. The net descent gradient is the slope of the descent path at that particular altitude, expressed as a ratio or percentage. The vertical interval is simply the height lost over that segment. Multiply or divide those two together, depending on how the gradient is expressed, and you get the horizontal distance covered for that segment. To find the descent range, simply add all the horizontal distances in all the segments. After 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. Once that is known, it can be added to the descent range of the drift down profile to give the total range of the aeroplane following engine failure. Now here is where the regulation comes in. Therefore, 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, whose radius is the total single-engine range, is drawn around each airfield between the departure and destination points. To comply with the regulations, the aeroplane 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 this: engine fails, you drift down along a curved path, you calculate that curved descent range by splitting it into straight segments, you add the level single-engine cruise range, and that total becomes the radius of a circle around each airfield. Your planned track must stay inside those circles so that at any moment, if the other engine fails too, you can still reach an aerodrome. That is the EU-OPS requirement in practical terms.