Class A - En Route — Page 464, Lesson 570

We've just covered the 60-minute rule for twin-engine aeroplanes, and now I want to show you how that rule has been extended. As more reliable and efficient aeroplanes are produced, an extension to the 60-minute rule has been introduced, and that extension is called ETOPS. ETOPS stands for Extended-range Twin-engine Operational Performance Standards. It hugely increases the operational capability of twin-engine aeroplanes, where previously only aeroplanes with three or more engines could operate. But here's the key point: ETOPS must be applied for by the airlines concerned, and approval must be gained from the appropriate aviation authority. It's not automatic. To gain ETOPS approval, a greater range of performance parameters must be known. These accompany the application and are eventually published in the operating manual. The extra data includes three specific areas: Area of Operation, Critical Fuel Reserves, and Net Level-off Altitudes. Let me unpack each of those. Area of Operation defines the geographic region the aeroplane is approved to fly in. Critical Fuel Reserves is the fuel you must carry to guarantee you can reach a suitable alternate if an engine fails. Net Level-off Altitudes is the altitude the aeroplane can maintain with one engine inoperative, after accounting for performance degradation. Now, why does this matter in practice? Gaining an ETOPS approval of 120 minutes, for example, will greatly benefit flight tracks across the Atlantic Ocean, as shown in Figure 17.15. The route from Paris to New York, for example, can now be flown direct by a twin jet aeroplane. Currently, the longest ETOPS approval is given to the Boeing 777. It holds an approval of 180 minutes, with contingencies for 207 minutes over the Pacific. In the future, ETOPS may be evolving into a newer system called LROPS. LROPS stands for Long Range Operational Performance Standards, and it will affect all aircraft, not just those with a twin-engine configuration. That's a crucial distinction — ETOPS is specifically for twins, but LROPS will apply to every aircraft type. Now let's move to the next part of this section, which deals with optimum altitude and the aerodynamic ceiling. These are two separate concepts, and I want to make sure you understand the difference. First, optimum altitude. There's a common misconception that an aeroplane always flies at the optimum altitude because it's economically the most attractive. But that's not correct. The optimum altitude increases continuously during flight, because as fuel burns off, the aeroplane gets lighter and can fly higher more efficiently. So the aeroplane sometimes flies above or below the optimum altitude, because the optimum altitude is a moving target — it keeps rising as weight decreases. It doesn't stay fixed. Now, the aerodynamic ceiling. This is a specific altitude with a precise definition. The aerodynamic ceiling is the altitude at which the speeds for low speed buffet and for high speed buffet are the same. Let me explain what that means. Low speed buffet is the buffet you get when you're flying too slowly and the wing is approaching its stalling angle of attack. High speed buffet is the buffet you get when you're flying too fast and shock waves form on the wing. At the aerodynamic ceiling, these two buffet speeds converge — they become the same speed. Above that altitude, there's no speed at which you can fly without experiencing one buffet or the other. So the aerodynamic ceiling is the absolute maximum altitude the aeroplane can reach. Now, I want to be clear about what the aerodynamic ceiling is not. It is not the altitude at which the aeroplane reaches 50 ft/min. It does not depend upon thrust setting and increase with increasing thrust. And it is not the altitude at which the best rate of climb theoretically is zero. Those are all incorrect descriptions. The correct definition is the one I gave you: it's the altitude where the low speed buffet speed and the high speed buffet speed are the same. So to tie it all together: ETOPS extends the 60-minute rule for twins, with specific performance data required for approval, and it's evolving into LROPS which will cover all aircraft. Then, separately, we have optimum altitude which continuously increases during flight, and the aerodynamic ceiling which is defined by the convergence of low and high speed buffet speeds.