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 an aeroplane gains over a given horizontal distance using that 100:10 ratio. Now I want to finish that obstacle-clearance example, because it introduces a very important correction you must never forget. In the example, the obstacle is 900 metres high, and the distance from the end of the TODA — that’s the Take-Off Distance Available — to the obstacle is 9740 metres. We take that horizontal distance and divide it by the horizontal ratio of 100. So 9740 divided by 100 gives 97.4. That tells us the horizontal distance is 97.4 times greater than the basic 100-metre unit. Because the height gain is proportional, the height gain is also 97.4 times greater. So we multiply the basic 10-metre height gain by 97.4, which gives 974 metres. But here’s the correction I mentioned. The climb segment does not start at Reference Zero. It starts at 15 metres, which is 50 feet, above Reference Zero. That 15 metres is called the screen height, and you must add it to the height gain. So 974 plus 15 gives 989 metres. That is the actual height of the aeroplane when it reaches the obstacle’s horizontal position. The obstacle is 900 metres, so the aeroplane clears it by 89 metres. That margin — 89 metres — is your obstacle clearance. Now I want to move on to a completely different concept: rate of climb. And before we get into the maths, I want to clear up a confusion that trips up a lot of students. Angle of climb and rate of climb are not the same thing. Angle of climb is about the steepness of the path through the air. Rate of climb is about how quickly the aeroplane gains height over time. The same idea will apply later to rate of descent, so get this foundation right now. Let’s start with the definition of power. Power is the rate of doing work. So whenever you see the word rate, associate it with the word power. Work is force multiplied by distance. So power is force times distance, divided by time. Let me say that again slowly. Power equals force times distance, over time. Now, when we consider rate of climb, what we are really trying to do is 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 is the force we are working against. The remaining part of the formula, distance divided by time, is simply a speed — for example, nautical miles per hour, which we abbreviate as kt, knots. So hold on to this: power is the rate of doing work, work is force times distance, and in a climb the force we must overcome is drag. The distance over time part is just your speed. That is the foundation for rate of climb, and we’ll build the full picture from here.