Class A - Take-off Climb — Page 433, Lesson 530

Let’s pick this up right where the climb limit mass left off, because this excerpt is the payoff of that idea. I want to walk you through the Class A take-off climb, and the first thing we need to nail down is what the climb limit mass actually guarantees. You’ll remember we talked about two requirements at the start of the chapter. The climb limit mass deals with the first one — point a. Being at or below the climb limit mass guarantees that the aeroplane attains the most severe gradient requirement of the take-off flight path. Now, what is that most severe gradient? It’s in segment 2 of the take-off flight path, and for a twin-engine jet aeroplane that gradient is 2.4%. So the climb limit mass ensures that, in the event of an engine failure, the aeroplane can still achieve a 2.4% gradient or better. Here’s a subtle but crucial point: these climb gradients are air-based. That means they are independent of the effect of wind. If you look at the climb limit mass graph, you’ll see there’s no wind component shown — that confirms it. The gradient is measured relative to the air mass, not the ground, so wind doesn’t change the climb capability. Now we move to obstacle clearance. EU-OPS states that an operator must ensure the net take-off flight path clears all obstacles by a vertical margin of at least 35 feet. That’s the vertical clearance requirement. But if the aeroplane cannot achieve that vertical clearance, it must turn away from the obstacle and clear it by a horizontal distance. That horizontal distance is at least 90 metres plus 0.125 times D, where D is the horizontal distance the aeroplane has travelled from the end of the take-off distance available. So the formula is 90 m + 0.125D. There’s an alternative for aeroplanes with a wingspan of less than 60 metres. In that case, the horizontal obstacle clearance may be half the aeroplane’s wingspan plus 60 metres, plus 0.125 times D. So the formula becomes 60 m + ½ wingspan + 0.125D. Now, obstacles further away than the values shown below need not be considered — there’s a limit to how far out you have to check. We’ve actually seen this horizontal clearance information before, when we discussed multi-engine Class B obstacle clearance. But for Class A, there are two crucial points to consider when working out the vertical clearance. The first relates to climb gradient. Remember, climb gradient is a ground-based gradient. To work out the obstacle clearance of the aeroplane, you need to know the climb gradient. But EU-OPS states that the climb gradient to use for calculating obstacle clearance must be the net climb gradient. And remember, the net gradient is the gross gradient diminished by a safety factor. In this case, that safety factor changes depending on the number of engines. So here are the exact reductions: the net gradient is the gross gradient reduced by 0.8% for a 2-engine aircraft, 0.9% for a 3-engine aircraft, and 1.0% for a 4-engine aircraft. That’s the safety factor applied to the gross gradient to get the net gradient you use for obstacle clearance. So to tie it all together: the climb limit mass gives you the 2.4% gradient in segment 2 for a twin, and that’s air-based. Then for obstacle clearance, you take the gross gradient, reduce it by the engine-count safety factor to get the net gradient, and use that net gradient to check that the net take-off flight path clears obstacles by at least 35 feet vertically, or if it can’t, by the horizontal distances we just covered.