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

We're now into the Class A take-off climb, and I want to pick up with the net take-off flight path. Remember, we have a gross gradient, which is the raw climb performance of the aeroplane. When we apply the net gradient — that's the gross gradient reduced by the regulatory margin — we get the net take-off flight path. And the rule is simple: this net take-off flight path must clear all obstacles by 35 ft. That 35 ft is the vertical clearance margin we must guarantee. Now, the second point is the effect of wind on the ground gradient. A headwind will increase the ground gradient and improve obstacle clearance. A tailwind will decrease the ground gradient and deteriorate obstacle clearance. So wind directly changes how steeply the aeroplane's path climbs relative to the ground. But here's where EU-OPS adds a safety margin. When we adjust for wind to 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. If we have a headwind, we only credit half of it, so we don't overestimate our climb. If we have a tailwind, we assume it's 1.5 times worse than reported, so we don't underestimate the penalty. Either way, we're building an extra safety margin into the calculation. Now, what if the aeroplane can't clear the obstacle vertically? We said earlier it can turn away and clear it horizontally. But there are restrictions on how much we're allowed to turn. It's clearly not safe to bank sharply just to clear 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. V2, remember, is the take-off safety speed. For greater bank angles, we scale it up. For a 20° bank, use 2 times the gradient decrement and fly at V2 plus 5 knots. For a 25° bank, use 3 times the gradient decrement and fly at V2 plus 10 knots. So the steeper the bank, the bigger the gradient penalty and the higher the speed we must carry. Finally, there are hard limits on turns on the flight path. Turns are not allowed below a height of half the wingspan or 50 ft, whichever is greater. Up to 400 ft, the bank angle may not be more than 15°. Above 400 ft, the bank angle may not be more than 25°. So we have a three-tier structure: no turns at all very low, limited to 15° of bank below 400 ft, and up to 25° of bank above that. That figure shows the horizontal clearance information we saw when we discussed multi-engine obstacle clearance — it's the same concept applied here. So to tie it together: net gradient for the 35 ft vertical margin, the 50%/150% wind rule for the ground gradient, the bank-angle gradient decrements with their speed increases, and the height and bank limits for turns. That's the complete Class A take-off climb picture.