Class A - Take-off Climb — Page 433, Lesson 532

We're now into the heart of the Class A take-off climb, and I want to walk you through the two big safety concepts that govern how we actually construct and fly this flight path. First, let's talk about the difference between the gross and the net take-off flight path. The gross gradient is the raw, unadjusted climb performance of the aeroplane. But when we apply the net gradient—which is the gross gradient reduced by the regulatory margin—we get the net take-off flight path. And this is the critical one: the net take-off flight path must clear all obstacles by 35 feet. That 35-foot clearance is the regulatory floor, and it's measured from the net path, not the gross one. So even if the aeroplane physically climbs better, we plan and certify against the net path. Now, the second point is the effect of wind on the ground gradient. This is about how the aeroplane's climb path looks relative to the ground, not the air. A headwind increases the ground gradient—it steepens the path over the ground and improves obstacle clearance. A tailwind does the opposite: it decreases the ground gradient and deteriorates obstacle clearance. EU-OPS gives us a specific rule for adjusting for wind when we calculate the ground gradient. We may use no more than 50% of the reported headwind, and no less than 150% of the reported tailwind. Think about what that does: it's deliberately conservative. We only credit half the headwind that's helping us, but we penalise ourselves with one and a half times the tailwind that's hurting us. That rule simply adds another safety margin into the calculation. Now let's move to the turn. We said earlier that if the aeroplane can't clear an obstacle vertically, it can turn away and clear it horizontally. But there are strict restrictions on how much we're allowed to turn. It's clearly not safe to bank sharply just to scrape past an obstacle by the regulatory margins. Here's the physics: turning increases the effective weight by imposing extra g loads. That means the climb gradient is reduced, and stall speeds are increased. So we must make an allowance for the effect of the turn on both the climb gradient and the speed. The flight manual usually gives us a gradient decrement for a 15° banked turn at V2. That's our baseline. For greater bank angles, we scale it up. For a 20° bank, we use 2 times the gradient decrement, and we fly at V2 plus 5 knots. For a 25° bank, we use 3 times the gradient decrement, and we fly at V2 plus 10 knots. Notice the pattern: more bank means a bigger gradient penalty, and we also add speed to protect against the increased stall speed. Finally, there are hard limits on where and how much we can turn on the flight path. Turns are not allowed below a height of half the wingspan or 50 feet, whichever is greater. So if your wingspan is 80 feet, half is 40, so the limit is 50 feet. If your wingspan is 120 feet, half is 60, so the limit is 60 feet. Up to 400 feet, the bank angle may not be more than 15°. Above 400 feet, the bank angle may not be more than 25°. So the higher you get, the more bank you're allowed, but never beyond 25°. Let me tie this together. The net take-off flight path is our certified obstacle-clearance path. We adjust it for wind using the EU-OPS 50% headwind, 150% tailwind rule. And if we need to turn, we apply the gradient decrement and speed increase for the bank angle, respecting the height and bank limits. Every one of these is a safety margin stacked on top of the others.