Class A - En Route — Page 457, Lesson 566

We’re now at the heart of the en-route phase for a Class A aeroplane — the drift down and, critically, the obstacle clearance requirements that govern it. Let me walk you through this carefully, because this is where the safety margins are defined and where the regulations become very specific. First, the core problem. When an engine fails, the aeroplane is forced to descend — we call that the drift down. The danger is obvious: terrain, such as mountains, may present a flight hazard. So when we assess the terrain hazard, we must introduce a safety margin. And here’s the key principle: when planning routes and planning the flight profile, we do not use the gross flight profile — the actual performance the aeroplane can achieve. Instead, we use the net flight profile. In other words, the flight profile must be made worse by a safety factor. Let me be precise about that safety factor. It is based on assuming a gradient of descent that is worse than the aeroplane can actually achieve. For a two-engine aeroplane with one engine inoperative, the gross gradient of descent is increased by 1.1%. That increases to 1.4% for three-engine aeroplanes, and 1.6% for four-engine aeroplanes. So the more engines you have, the larger the penalty we apply to the gradient. The net flight path is therefore the gross path, made worse by that percentage, and it’s this net path that we use for obstacle clearance. Now, the regulations. EU-OPS states, in part, 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): the gradient of the net flight path must be positive at at least 1000 ft above all terrain and obstructions along the route within 5 NM on either side of the intended track. So we need a positive gradient — the path must be climbing — and it must clear everything by at least 1000 ft, within a corridor 5 nautical miles either side of track. But if the aeroplane is unable to satisfy that restriction, or when it would be too limiting in terms of weight, then we work out a drift down procedure, as detailed in 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 the net flight path must clear vertically, by at least 2000 ft, all terrain and obstructions along the route within the prescribed corridor. So notice the difference — under (b) we’re allowed to descend all the way to a suitable aerodrome, but the clearance requirement is higher: 2000 ft instead of 1000 ft. There are two additional requirements. First, the net flight path must have a positive gradient at 1500 ft above the aerodrome where the landing is assumed to be made after engine failure. So even at the destination, we need a positive gradient at that height. Second, fuel jettisoning is permitted — but only to an extent consistent with reaching the aerodrome with the required fuel reserves. So we can dump fuel to lighten the aeroplane, but we must still arrive with the reserves we need. Now, how do we actually find out if the aeroplane is able to level off at 1000 ft above an obstacle? We use a graph — it’s in CAP 698, on page 40 of section 4, and it’s reproduced here as Figure 17.14. That graph lets us calculate the maximum mass for a given net level-off altitude. So we enter with the altitude we need to clear, and we read off the maximum mass the aeroplane can have to achieve that level-off on the net flight path. Let me also point you to the figures that illustrate this. Figure 17.12 shows the drift down profiles — the net flight path. And Figure 17.13 shows the net and gross descent profiles for a typical twin-engine medium range jet, so you can see visually how the net path sits below the gross path by that safety margin. So the whole logic here is: engine fails, we drift down, but we plan on the net path — made worse by 1.1%, 1.4%, or 1.6% depending on engine count — and we must clear terrain by 1000 ft with a positive gradient, or if that’s not possible, we plan a drift down to an aerodrome clearing everything by 2000 ft, with a positive gradient at 1500 ft above that aerodrome, and we may jettison fuel only down to the required reserves. That’s the complete obstacle clearance requirement for the en-route drift down.