Class A - En Route — Page 457, Lesson 566

We're now into the obstacle clearance requirements for the drift down procedure. This is where the theory turns into the hard, regulatory numbers you'll actually plan against. The crucial point here is that when an engine fails and the aeroplane is forced to descend, terrain like mountains becomes a genuine flight hazard. So when we assess that hazard, we must introduce a safety margin. And here's the key distinction: when planning routes and the flight profile, we do not use the gross flight profile. We use the net flight profile. Let me make that crystal clear. The gross flight profile is what the aeroplane can actually achieve — the real, physical performance. The net flight profile is that same profile made worse by a safety factor. In other words, we deliberately assume a gradient of descent that is worse than the aeroplane can actually achieve. We plan against the pessimistic case, not the optimistic one. Now, that safety factor is a specific number, and it depends on how many engines the aeroplane has. For a two-engine aeroplane with one engine inoperative, the gross gradient of descent is increased by 1.1%. For a three-engine aeroplane, that increases to 1.4%. And for a four-engine aeroplane, it's 1.6%. So the more engines you have, the bigger the safety margin you must apply to the gradient. Now let's bring in the regulation. This is EU-OPS, and it states that the one-engine-inoperative en route net flight path must comply with either sub-paragraph (a) or (b) at all points along the route. Sub-paragraph (a) says the gradient of the net flight path must be positive at at least 1000 feet above all terrain and obstructions along the route, within 5 nautical miles on either side of the intended track. So you need a positive gradient — meaning you're still climbing, not descending — and you need at least 1000 feet of clearance above everything, within that 5-nautical-mile corridor on each side of your track. Now, if the aeroplane is unable to satisfy that restriction, or if satisfying it would be too limiting in terms of weight, then you work out a drift down procedure. That's sub-paragraph (b). Under (b), the net flight path must permit the aeroplane to continue flight from the cruising altitude to an aerodrome where a landing can be made. And along that path, the net flight path must clear vertically, by at least 2000 feet, all terrain and obstructions along the route within the prescribed corridor. So note the difference: under (a) it's 1000 feet, under (b) it's 2000 feet. There are two additional requirements on top of that. First, the net flight path must have a positive gradient at 1500 feet above the aerodrome where the landing is assumed to be made after engine failure. So as you approach that diversion aerodrome, you still need a positive gradient at 1500 feet above it. Second, fuel jettisoning is permitted — but only to an extent consistent with reaching the aerodrome with the required fuel reserves. So you can dump fuel to lighten the aeroplane, but you must still arrive with your reserves intact. Now, how do you actually find out if the aeroplane can level off at 1000 feet above an obstacle? You use a graph in CAP 698, on page 40 of section 4. That graph has been reproduced here as Figure 17.14, and it's used to calculate the maximum mass for a given net level-off altitude. So you enter with the altitude you need to clear, and it gives you the maximum mass the aeroplane can be at to achieve that level-off. And I want you to see the two profiles side by side — Figure 17.13 shows the net and gross descent profiles for a typical twin-engine medium range jet. That's the visual of what we've been talking about: the gross profile is the real one, the net profile is the one with the safety factor built in, and it's the net one you plan against.