General Principles - Climb — Page 195, Lesson 222

Let’s pick this up with the effect of bank angle on the climb, because that’s the first thing I want you to understand before we talk about wind. When an aircraft is banked, any increase in bank angle beyond approximately 15 degrees will significantly increase the amount of Lift that needs to be generated. Now, why does that happen? Because in a bank, part of the lift is used to turn the aircraft rather than to oppose weight, so to maintain the climb you have to generate more total lift. And here’s the chain of consequences: increased Lift will generate more Induced Drag. Induced drag is the drag created as a by-product of producing lift. So more lift means more induced drag. That extra drag reduces your Excess Thrust — the thrust you have available beyond what’s needed to overcome drag. And because excess thrust is what powers the climb, your maximum climb angle will be reduced. So the takeaway: bank beyond about 15 degrees, and your best climb angle suffers. Now let’s move to the effect of wind on climbing. The key idea is that wind is the motion of a body of air over the ground. And the effect wind has depends entirely on which climb gradient you’re talking about, because there are two types: Air gradient and Ground gradient. Let me define them carefully. Air gradient is the vertical distance gained in a body of air divided by the horizontal distance travelled through that same body of air. The crucial point is that the fact the body of air might be moving over the ground is NOT considered. So wind has no effect on Air gradient. This is the gradient that aviation authorities use to lay down minimum climb performance limits. For example, for a Class ‘A’ aeroplane, the regulation states: starting at the point at which the aeroplane reaches 400 ft — that’s 122 metres — above the take-off surface, the available gradient of climb may not be less than 1.2% for two-engined aeroplanes. So that 1.2% figure is an air gradient requirement. Let me show you what this looks like. Here we have an aeroplane in the bottom left corner of a body of air, directly above the control tower on the ground. When the body of air is stationary relative to the ground, we call that “Zero Wind” or “Still Air”. The aeroplane climbs to the top right corner of the body of air, and the Air gradient is shown as Gamma ‘a’ — that’s the Greek letter gamma, the symbol for the climb angle. There’s a useful simplification here. To simplify the study of climbing, for climb angles less than approximately 20 degrees, it is considered that doubling the climb angle will double the climb gradient. So below about 20 degrees, climb gradient is directly proportional to climb angle — double the angle, double the gradient. Now, the Ground climb gradient. This is also known as the Flight Path Angle, or FPA, and it’s influenced by wind. The Air gradient, by contrast, is also known as the Climb Angle. So you have two different angles: the air gradient, which is the climb angle, not affected by wind; and the ground gradient, the flight path angle, influenced by wind. Let’s look at a tailwind. Because the body of air is moving over the ground in the direction of flight, the Ground gradient is smaller than the Air gradient. So a tailwind does not change the Air gradient, but it decreases the Ground gradient. Think of it this way: the air is carrying you forward, so for the same vertical gain, you cover more ground distance, which flattens the ground path. Now a headwind. Because the body of air is moving over the ground opposite to the direction of flight, the Ground gradient is larger than the Air gradient. So a headwind does not affect the Air gradient, but it increases the Ground gradient. The air is pushing back, so you cover less ground for the same vertical gain, which steepens the ground path. And here’s the practical rule that ties it all together: the only time wind is used to calculate climb gradient is when obstacle clearance is being considered. In all other cases of climbing, still air is used, even if a wind value is supplied. So for performance calculations that aren’t about clearing obstacles, you ignore the wind and use still air. Only when you need to clear an obstacle do you bring the wind into the ground gradient calculation. So to summarise the whole picture: bank beyond 15 degrees and your climb angle drops because of induced drag. Wind never touches the air gradient — that’s the one the authorities regulate. Wind only changes the ground gradient, the flight path angle, and you only care about that for obstacle clearance.