All right, let's pick this up right where the obstacle clearance margins… — Page 433, Lesson 533

All right, let's pick this up right where the obstacle clearance margins left off. We've just established that the basic vertical clearance is 35 feet, but the rules immediately layer on some conditions that change that number, and they all hinge on bank angle. Here's the first hard rule: up to 400 feet above the take-off surface, your bank angle may not be more than 15 degrees. Above 400 feet, the bank angle may not be more than 25 degrees. So the moment you cross that 400-foot threshold, the regulation allows you to steepen the turn, but only up to that 25-degree ceiling. Now, EU-OPS 1.495 does permit operators to exceed these bank angles, but it's not a free pass. There are two strict conditions. First, the operator must use special procedures, and second, those procedures must have been approved by the relevant authority. The special procedures must take account of the gradient loss from such bank angles, and these must be published in the aeroplane flight manual. So the authority has to sign off on the fact that the operator understands and accounts for the performance penalty. The maximum bank angles that these special procedures allow are up to 20 degrees between 200 and 400 feet, and up to 30 degrees between 400 and 1500 feet. Notice the structure: a steeper bank is permitted only in a higher band, and only with that approved procedure in place. Then we get the direct consequence for obstacle clearance. If any turn of more than 15 degrees is required at any point in the take-off flight path, then the vertical clearance is increased to 50 feet instead of 35 feet. So the 15-degree bank angle isn't just a limit—it's also the trigger that decides which clearance margin applies. Turn more than 15 degrees, and you need 50 feet of clearance, not 35. Now, here's the practical problem. Manually working out the obstacle clearance capability of the aeroplane could take a long time, because there are so many points to bear in mind and the calculation itself is quite lengthy. So thankfully, most operators and manufacturers have produced either rapid look-up tables or graphs to quickly enable the pilot to work out if an obstacle in the take-off climb will be cleared by the relevant vertical margins following engine failure. These tables or graphs will produce a mass. That mass is called the obstacle limit mass, and an example is shown in Figure 16.2, which you can also find in CAP 698 on pages 36 and 37 of section 4. It is the maximum mass that will allow the aeroplane, in the event of engine failure, to clear the obstacle by the relevant vertical margin. So you read the graph, and it gives you a mass—that's your ceiling. If you're at or below that mass, you clear the obstacle with the required margin; if you're above it, you don't. Now, notice that winds are included on the graph. This is important, because obstacle clearance calculations must use ground gradients, and these are dependent on wind. Let me make sure that distinction is clear: the aeroplane's performance in the air is one thing, but what matters for clearing a fixed obstacle on the ground is your ground gradient—how much height you gain per unit of horizontal distance over the ground. Wind directly changes that. And remember, EU-OPS had a rule about the wind. It stated that when using the winds to work out the ground gradient, only use 50% of headwinds and no less than 150% tailwinds. So a headwind only counts for half its reported value, and a tailwind counts for at least one and a half times its reported value. That's a conservative treatment—you don't get full credit for a helpful headwind, and you get penalised extra for a harmful tailwind. Now here's the elegant part. Notice the slope of the headwind and tailwind lines on the graph. This shows that the graph applies the wind rule for you. Therefore, if you enter the graph with the actual reported wind, the graph corrects it automatically, so you do not need to. You don't have to do the 50% and 150% arithmetic yourself—the graph has already built that rule into its lines. You just feed in the reported wind, and the obstacle limit mass you read off already reflects the corrected ground gradient. So to tie it all together: the bank angle rules set your limits and trigger the 50-foot margin, the obstacle limit mass is the maximum mass that clears the obstacle after engine failure, and the graph handles the wind correction for you because ground gradient depends on wind. That's the complete picture for this part of the take-off flight path.