General Principles - Climb — Page 195, Lesson 230

Let’s pick this up right where the climb geometry left off. We’ve just worked out how to find the height gain over a horizontal distance using that 100:10 ratio, and now we’re finishing the obstacle-clearance calculation before we move into rate of climb. So, in our example, the obstacle is 900 metres high, and the distance from the end of the TODA to the obstacle is 9740 metres. We divide that distance by the horizontal ratio of 100, which gives us 97.4. That means the horizontal distance is 97.4 times greater than the base ratio, so the height gain will also be 97.4 times greater. We multiply the vertical ratio of 10 by 97.4, and that gives us 974 metres of height gain. But here’s the catch — the climb segment doesn’t start at Reference Zero. It starts at 15 metres, which is 50 feet, above Reference Zero. That’s the screen height. So we have to add that screen height to the height gain: 974 plus 15 gives us 989 metres. That’s the actual height the aircraft will have reached at the obstacle’s position. Since the obstacle is 900 metres, the aircraft clears it by 89 metres. That’s the margin we’re looking for. Now let’s move on to rate of climb. I want to give you a clear overview first, because a lot of students confuse angle of climb with rate of climb. They’re related, but they’re not the same thing, and you need to know when to use each one. The same thinking applies later to rate of descent, so get this foundation solid now. The key idea is this: power is the rate of doing work. Associate the word RATE with the word POWER. Work equals force times distance. So power equals force times distance, divided by time. When we’re considering rate of climb, we need to do the maximum amount of work on the aeroplane in a given time. So ask yourself — when climbing, what force must be balanced? The answer is drag. That’s the force we’re working against. Now, the remaining product from that formula is distance divided by time. That’s a speed — for example, nautical miles per hour, which we call knots, abbreviated kt. So rate of climb is essentially about how quickly we can convert power into vertical speed, working against drag. Let me make sure you’ve got the distinction. Angle of climb is about the steepness of the path — how many degrees you’re going up. Rate of climb is about how fast you’re gaining altitude — feet per minute, or metres per second. In our obstacle example, we were dealing with angle of climb, because we needed to know whether the aircraft’s path would clear a specific obstacle at a specific distance. Rate of climb, on the other hand, tells us how quickly we’re gaining height over time, which matters for things like reaching a cruising altitude within a certain time. So remember: power is the rate of doing work, work is force times distance, and when climbing, the force we must balance is drag. The distance over time part of the formula gives us a speed, like knots. That’s the foundation for rate of climb.