First, let's define what "en route" actually means in this context — Page 317, Lesson 390

We're starting a new chapter now — Chapter 9, "Single-engine Class B – En Route and Descent." This is where we look at the performance requirements for a single-engine aeroplane during the en route and descent phases of flight. First, let's define what "en route" actually means in this context. Essentially, the en route part of the flight is 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 en route segment is bracketed by those two altitudes — 1500 feet above departure, 1000 feet above destination. Everything between those is the en route portion. Now, the governing regulation here is EU-OPS 1.542. 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 core requirement is: if the engine fails, you must be able to glide to somewhere you can land safely. To comply with that rule, the operator has to know two things about the route. First, for any given route, the operator must know the safe forced landing areas — the places 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. Whether the aeroplane can reach them depends on two parameters. The first is the altitude chosen for the flight. The second is the descent gradient of the aeroplane following engine failure. If you know both of those, you can calculate how far the aeroplane will travel after the engine fails. Let me walk you through an actual example. Assume a cruise altitude of 10,000 feet and a gradient of descent of 7% following an engine failure. What is the descent range? The descent range is the horizontal distance travelled. You work it out 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 = Vertical divided by Gradient, times 100. Let's plug in the numbers. Vertical is 10,000 feet. Gradient is 7%. So we take 10,000 divided by 7, which gives us roughly 1,428.6, and then multiply by 100. That gives us approximately 142,857 feet of horizontal distance. That's the descent range — how far the aeroplane will glide horizontally from 10,000 feet at a 7% gradient. That figure shows the descent range calculation visually. The key idea is that the gradient tells you how much height you lose per unit of horizontal distance — 7% means you lose 7 feet of altitude for every 100 feet you travel forward. So the formula inverts that relationship to find the horizontal distance from a given height. One thing to note — 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 we're pulling the relevant pieces together here in this chapter. That's the foundation of the en route and descent requirements for single-engine Class B operations. The critical takeaway is the relationship between altitude, descent gradient, and descent range — and how that determines whether you can reach a safe forced landing area if the engine fails.