Let me set the scene — Page 317, Lesson 390

We're starting a brand-new chapter now — Chapter 9, "Single-engine Class B – En Route and Descent." This is where we leave the take-off and landing phases behind and look at what happens in the middle of the flight, and then as we come down to the destination. Let me set the scene. The en route part of the flight is defined very precisely here. It's considered to be from 1500 feet above the airfield from which the aeroplane has taken off, down to 1000 feet above the destination airfield. So the moment you climb through 1500 feet above your departure field, you're in the en route phase, and you stay in it until you descend to 1000 feet above the destination. That's the envelope we're working with in this chapter. Now, the governing regulation here is EU-OPS 1.542. I want you to hear this rule exactly, because it drives everything in this chapter. It states that an operator must ensure that the aeroplane, in the meteorological conditions expected for the flight, and in the event of engine failure, is capable of reaching a place at which a safe forced landing can be made. So the whole point of the en route performance work for a single-engine Class B aeroplane is: if that one engine quits, you must be able to glide to somewhere you can put the aircraft down safely. To comply with that rule, the operator has to know two categories of information. First, for any given route, the operator must know the safe forced landing areas — the places along the route where a forced landing is actually possible. Second, the operator needs to know whether the aeroplane will be able to reach those areas if the engine fails while en route. And whether the aeroplane can reach them depends on two things. The first is the altitude chosen for the flight. The second is the descent gradient of the aeroplane following engine failure. If you know those two parameters, you can calculate how far the aeroplane will travel following engine failure. That distance is what we call the descent range. Let me walk you through a concrete example, because this is the heart of the calculation. Assume a cruise altitude of 10,000 feet, and assume a gradient of descent of 7% following an engine failure. The question is: what is the descent range? Here's the relationship. The horizontal distance travelled — the descent range — is found by taking the height of the aeroplane above the ground, dividing that by the gradient, and then multiplying by 100. So the formula is: Horizontal Distance equals Vertical divided by Gradient, times 100. Let's plug the numbers in. Vertical is 10,000 feet. Gradient is 7%. So we take 10,000 divided by 7, which gives us about 1,428.6, and then multiply by 100. That gives us 142,857 feet of horizontal distance. If you want that in nautical miles, you'd divide by 6,076 feet per nautical mile, which gives you roughly 23.5 nautical miles. So with a 7% descent gradient from 10,000 feet, this aeroplane can cover about 23 and a half nautical miles before it reaches the ground. That's the descent range calculation in its simplest form. The figure on screen shows exactly this — the height above ground, the gradient of descent, and the resulting horizontal distance. Now, one thing I want to flag before we go further. The CAP 698 manual does show the en route performance requirements, but they're scattered through the document, which doesn't make for ease of reference. So this chapter is essentially pulling those requirements together for you in one place. We'll be looking at the en route and descent requirements in detail, the information contained in CAP 698, and then the typical range and endurance graphs. That's the structure of what's coming. So the key takeaway from this opening: the en route phase is 1500 feet above departure to 1000 feet above destination. The rule is EU-OPS 1.542 — you must be able to reach a safe forced landing area if the engine fails. And the descent range is simply your height above ground divided by the descent gradient, multiplied by 100. That's the tool you'll use to check whether the forced landing areas along your route are actually within reach.