Single-engine Class B - En Route and Descent — Page 317, Lesson 395

I want to walk you through the final piece of the forced-landing rule for a single-engine Class B aeroplane, and then I'll show you exactly where this lives in CAP 698. We've been working with a 7% gradient of descent in our example. Here's the catch: the regulations will not let you use that 7% figure in your en-route calculation. EU-OPS 1.542 (b) (2) is the rule that governs this. It states that, to comply with the safe forced landing rule, the assumed en-route gradient must be the gross gradient of descent, increased by a gradient of 0.5%. Let me unpack that. The gross gradient is the actual, unadjusted descent performance the aeroplane can genuinely achieve — in our example, that's 7%. But the regulation forces you to assume a worse performance. You take that gross gradient and you add 0.5% to it. So 7% becomes 7.5%. That 7.5% is what we call the net gradient. Why add 0.5% to a descent gradient? Think about it — a steeper descent gradient means the aeroplane loses height more quickly over a given distance, so it covers less horizontal ground while descending. By forcing you to assume a steeper gradient than the aeroplane actually achieves, the regulation builds in a safety margin. It deliberately degrades your assumed descent performance. That's the whole point of the net gradient — it's the deteriorated figure the regulations insist you use. Now, what does that do to your glide distance? With the gross gradient of 7%, the aeroplane might genuinely glide 23.5 NM after an engine failure. But with the net gradient of 7.5%, the assumed glide distance drops to 22 NM. So even though the aeroplane could physically cover 23.5 NM, you must plan on only 22 NM. That's a conservative assumption, and it's exactly what the regulation demands. This has a direct consequence for your forced-landing circles. In our earlier work, we drew circles around each safe landing area representing the glide range. Now, because the net gradient reduces the glide distance, those circles must shrink to a radius of 22 NM. Look at Figure 9.5 — you'll see the net track that is required to remain within net glide range of each safe forced landing area. The planned track has to change slightly so that at every point along the route, the aeroplane is no further than 22 NM from a safe forced landing area. The track shown in that figure meets the entire set of requirements as stated in EU-OPS. Now let me show you where all of this lives in CAP 698, because the en-route regulations are scattered through that manual rather than collected in one convenient place. I mentioned at the start of the chapter that CAP 698 contains these rules, but you have to hunt for them. On page 1 of section 2, in the general requirement paragraph, point c) is the regulation about ensuring the aeroplane is not operated unless surfaces are available which permit a safe forced landing to be carried out in the event of engine failure. That's the overarching rule — you can't operate unless there's somewhere safe to put it down if the engine quits. The remaining en-route requirements are a few pages further on. You'll find them at the bottom of page 8 of section 2. The first part of the regulations there states that the aeroplane may not be assumed to be flying above the altitude at which a rate of climb of 300 feet per minute can be achieved. So there's a ceiling on your assumed altitude — you can't plan to be higher than the point where the aeroplane can still climb at 300 feet per minute. Underneath that rule, you'll see the requirement we've just been discussing: the net gradient of descent, in the event of engine failure, is the gross gradient plus 0.5%. That's the same 0.5% increase we applied to turn our 7% into 7.5%. One more thing before we move on. Although the concepts of range and endurance have been covered in a previous lesson, it's important for the pilot to be able to use the information in the aeroplane flight manual so that he may calculate the range and endurance of the aeroplane. In the aircraft manual there are the performance charts and data you'll need for those calculations — that's where you go to get the actual figures for your specific aeroplane.