Single-engine Class B - Climb — Page 308, Lesson 388

Let’s start with the climb graph for the single-engine Class B aeroplane. I want you to understand that this graph has two main jobs, and the first one is the most straightforward: calculating the time to climb to your cruise altitude. To do that, you need two pieces of information — the cruise altitude you’re heading for, and the rate of climb. The graph gives you the rate of climb, you know your target altitude, and from those you can work out how long the climb will take. But here’s where it gets important. If you’re using the gradient from this graph — that is, the slope of the climb path — for obstacle clearance or for working out ground distance, then you must adjust that gradient for the effect of wind. This particular climb graph makes no correction for wind at all. It gives you an air gradient, not a ground gradient. Let me make that distinction crystal clear, because it’s the whole reason the wind correction exists. An air gradient is the climb path measured relative to the air mass around you. A ground gradient is the climb path measured relative to the ground. And here’s the key point: ground gradients are affected by wind, because the air mass itself is moving over the ground. If you have a headwind, your ground distance for a given height gain shrinks — you climb more steeply over the ground. If you have a tailwind, your ground distance stretches out. So obstacle clearance calculations and ground distance calculations must use ground gradients, and that’s exactly why you have to correct the graph’s gradient for wind before you use it for those purposes. If you ever need to refresh your memory on the difference between air gradients and ground gradients and how wind affects them, that’s covered in the general performance principles climb chapter — but for now, just hold onto this: the graph gives you an air gradient, and wind correction is your job. Now, CAP 698 — that’s the Civil Aviation Publication that contains the performance charts — has worked examples for you to practise with. On page 6 of section 2, there’s an example showing how to calculate the climb gradient using the graph. Just below that, there’s another example that works the other way: it determines the maximum permissible mass you can have in order to achieve a 4% climb gradient. That maximum permissible mass has a name you’ll hear a lot — it’s sometimes called the MAT or WAT limit. MAT stands for maximum allowable take-off mass, and WAT stands for weight, altitude, temperature — because those are the three factors that drive this limit. So when you see MAT or WAT limit, think: the heaviest you can be and still make that required climb gradient. Then, on page 8 of section 2, there’s a third example — this one shows how to calculate the horizontal ground distance required to climb to a given height. That’s the obstacle-clearance-style calculation, and it’s exactly where the wind correction we talked about comes into play. So the structure of what you’re learning here is: the climb graph gives you rate of climb and air gradient; you use it for time-to-climb directly, but for obstacle clearance and ground distance you must correct the gradient for wind; and CAP 698 walks you through all three calculation types, including the MAT or WAT limit for a 4% gradient. Now, the questions that follow are the book’s practice questions, and they reference CAP 698 SEP1 Figure 2.3 — the climb performance chart for the single-engine aeroplane. Let’s try those one at a time.