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

Right, let's pick this up. We've already covered the climb limit mass, so now I want to walk you through the second half of the Class A take-off climb requirements: obstacle clearance. First, let me tie off the climb limit mass idea, because it sets up everything. The climb limit mass deals with the most severe gradient requirement of the take-off flight path. For a twin engine jet aeroplane, that most severe gradient is in segment 2, and it's 2.4%. So being at or below the climb limit mass guarantees that, in the event of an engine failure, the aeroplane can still attain a 2.4% gradient or more. And a crucial point here: these climb gradients are air based, meaning they're independent of the effect of wind. That's why the climb limit mass graph has no wind component shown on it. Now, obstacle clearance. This is about ensuring the aeroplane physically clears things on the ground. 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 ft. That's the vertical clearance requirement. But what if the aeroplane can't achieve that vertical clearance? Then it must turn away from the obstacle and clear it by a horizontal distance. That horizontal distance is at least 90 m plus 0.125 × 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 m. In that case, you may use a horizontal obstacle clearance of half the aeroplane wingspan plus 60 m, plus 0.125 × D. So that's 60 m + ½ wing span + 0.125D. Note the difference: for smaller aircraft, the fixed 90 m is replaced by 60 m plus half the wingspan. Now, there's a limit to this. Obstacles further away than certain values need not be considered at all. We've seen this horizontal clearance information before when we discussed multi-engine Class B obstacle clearance. But 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, 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 the net gradient is the gross gradient diminished by a safety factor. That safety factor changes depending on the number of engines. Here are the exact reductions: for a 2-engine aircraft, the net gradient is the gross gradient reduced by 0.8%. For a 3-engine aircraft, it's reduced by 0.9%. And for a 4-engine aircraft, it's reduced by 1.0%. So the more engines you have, the larger the safety factor subtracted from the gross gradient to get the net gradient. That's the key distinction: the climb limit mass uses the gross gradient to ensure the aeroplane can physically achieve the 2.4% segment 2 gradient. But for obstacle clearance calculations, you must use the net gradient, which is the gross gradient minus that engine-dependent safety factor. So the net gradient is always lower than the gross gradient, and that's the conservative value you use to ensure the aeroplane clears obstacles with the required margin.