Class A - En Route — Page 457, Lesson 568

Let’s pick up right where the drift-down graphs left off, because the next idea builds directly on them. I want to walk you through what happens after you’ve used those graphs and you’re actually flying the en-route phase with an engine failed. First, a quick recap of the graph we just used. You enter it with the level-off altitude you require — that’s the obstacle height AMSL, meaning above mean sea level, plus 1000 feet. The graph then gives you the mass the aeroplane must be at to level off at that altitude. In the example, to level off at 18,000 feet in an ISA + 20 atmosphere — that’s International Standard Atmosphere plus 20 degrees — the aeroplane would need a mass of just less than 48,000 kg. If that mass isn’t achievable, you turn to the more complicated graphs on pages 41 to 44 of CAP 698. Those let you work out whether the aeroplane can clear any obstacle in the flight path by 2000 feet. And a key caution: when you use those graphs, adjust the weight for any non-standard conditions and for anti-ice use. Also notice the trend — the heavier the aeroplane, the longer and lower the drift-down procedure is. Now, the real problem after an engine failure isn’t just altitude — it’s range. Here’s the chain of cause and effect. After engine failure, the lower operating altitude significantly decreases the engine’s efficiency. 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. Combine that with the reduced true airspeed, and the specific range — that’s the distance flown per unit of fuel — is dramatically decreased. So the aeroplane simply cannot fly as far on the fuel it has. Because of that reduced range, you may not be able to reach the destination airfield. The priority now shifts: you need to find an alternate airfield to land before the fuel runs out. This is so important that the authorities regulate it. They 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 true airspeed, in still air. For aeroplanes with three or more engines, that time increases to 90 minutes. So at all points on the route, a twin-engine aeroplane must be within 60 minutes of an alternate airfield. This has a huge impact on route planning, especially over the sea. Look at Figure 17.15 — to comply with the 60-minute rule, the aeroplane’s track must at all times be within the 60-minute range limit of a suitable alternate airfield. And from that diagram, you can see a direct track from Europe to North America is simply not possible — you’d be outside the 60-minute limit for too much of the crossing. So the takeaway: engine failure doesn’t just cost you altitude and speed — it fundamentally changes your fuel economy and therefore your range, and the regulations force you to plan your entire route so that you’re always within striking distance of an alternate. That’s the en-route Class A picture.