Class A - En Route — Page 457, Lesson 568

Let’s pick this up right where the drift-down graphs leave off. I want to walk you through what happens after you’ve used those graphs, and then into a brand-new rule that governs your whole route planning. So, first, the drift-down graph itself. You enter the graph with the level-off altitude that is required. And that altitude is calculated as the obstacle height AMSL — that’s Above Mean Sea Level — plus 1000 feet. The graph then shows you the mass the aeroplane must be at in order to level off at that altitude. In the worked example on the graph, to level off at 18,000 feet in an ISA + 20 atmosphere — that’s International Standard Atmosphere, 20 degrees warmer than standard — the aeroplane would need a mass of just less than 48,000 kg. Now, if that mass isn’t achievable, you can’t just give up. You turn to other graphs, which are on pages 41 to 44 of CAP 698. CAP 698 is the CAA’s performance manual — the regulatory document that contains all these charts. Those graphs are more complicated, but they do something crucial: they let you work out whether the aeroplane can clear any obstacle in the flight path by 2000 feet. That’s the key clearance margin for this procedure. When you use those graphs, be careful. You must adjust the weight of the aeroplane for any non-standard conditions, and for anti-ice use. Anti-ice bleeds engine power and adds drag, so it changes the performance picture. And notice one important trend: the heavier the aeroplane, the longer and lower the drift-down procedure is. Heavier means it sinks more and takes longer to stabilise. Now let’s move to a completely different topic — range limit following engine failure. This is a big one. After an engine fails, the aeroplane has to operate at a lower altitude. That lower altitude significantly decreases the engine’s efficiency. In fact, it’s so severe that the fuel flow on the remaining engine is almost as much as the fuel flow with both engines operating at high altitude. So you’re burning nearly the same fuel, but on one engine. Add to that the reduced true airspeed — TAS — and the result is that the specific range is dramatically decreased. Specific range is the distance you can fly per unit of fuel. So with one engine, you simply cannot go as far. Because of that reduced range, it may not be possible to reach the destination airfield. The priority now shifts — it’s no longer about getting to your planned destination. The priority is to find an alternate airfield to land before the fuel runs out. This issue is so important that it had to be regulated. The authorities set a safety standard: in the event of engine failure, the aeroplane must have the capability of reaching a suitable airfield within a certain time period. That regulation is EU-OPS 1.245. Let me give you the exact rule. For twin-engine aeroplanes beyond a certain size, the aeroplane must be no further away from a suitable aerodrome than the distance flown in 60 minutes, using the one-engine-operative cruise speed as TAS, in still air. So at every point on the route, a twin must be within 60 minutes of an alternate. For aeroplanes with 3 or more engines, that time is increased to 90 minutes. This rule has a huge impact on route planning, especially over the sea. Think about it — if you’re flying a twin across the Atlantic, you can’t just draw a straight line. You have to stay within that 60-minute range limit of a suitable alternate at all times. In the example in Figure 17.15, you can see that to comply with the 60-minute rule, the aeroplane’s track must always be within the 60-minute range limit of a suitable alternate. And from that diagram, a direct track from Europe to North America is simply not possible. You have to route around, staying within reach of an alternate the whole way. So to tie it together: the drift-down graphs tell you if you can clear obstacles after an engine failure, and the EU-OPS 1.245 rule tells you how far you can be from an alternate in the first place. Both are about surviving that engine failure safely.