Single-engine Class B Aircraft - Take-off — Page 302, Lesson 380

Let’s pick this up right where the take-off distance calculation left off. We had a dry, paved runway giving us a take-off distance of 2300 ft. Now I want you to imagine that runway is actually grass, and that grass is wet. Both of those variables — the surface type and the moisture — increase the take-off distance. Since our graph was built for a paved, dry runway, we have to correct the distance for these real-world conditions. Here’s the correction factor you need to remember: wet grass increases the take-off distance by 30%, which is a factor of 1.3. So we take our 2300 ft and multiply it by 1.3. That gives us 2990 ft. That’s our take-off distance on wet grass. Now let’s add another variable. Suppose the runway has an upslope of 1%. An upslope increases the take-off distance by 5%, which is a factor of 1.05. So we take our 2990 ft and multiply it by 1.05. That brings us to a total gross take-off distance of 3140 ft. Now, that word “gross” is important. To comply with field length requirements and to obtain a net take-off distance, we must compare this gross take-off distance against the available distances at the airfield. The rule is that our take-off distance must not exceed the limits laid down by the authorities. Those regulations are in CAP 698, page 1, section 2 — that’s the reference you’d turn to for the exact field length requirements. Now let’s move to the certification requirements. These come from a different document — EASA’s CS-23. For single-engine Class B aeroplanes, there are only two main certification specifications that apply to the take-off phase, and both concern take-off speeds. The first specification concerns VR. You may recall that VR is the rotation speed — the speed at which the pilot makes a control input with the intention of getting the aeroplane out of contact with the runway. The certification specification states that for a single-engine aeroplane, VR must not be less than VS1. VS1 is the stall speed, or more precisely, the minimum steady flight speed of the aeroplane obtained in a specified configuration. And the configuration concerned here is the one used for take-off. The second certification specification concerns the speed of the aeroplane at the screen height. The screen height is 15 metres, or 50 feet, above the take-off surface. The specification states that the speed at that height must be more than the higher of two values: first, a speed that is safe under all reasonably expected conditions, and second, 1.2 times VS1. That speed at the screen height — you’ll recall from an earlier lesson — is commonly referred to as the take-off safety speed. One last thing to note: these certification regulations about VR and the take-off safety speed are not found in CAP 698. They must be committed to memory. So let me recap the key numbers and factors: wet grass is a 1.3 factor, a 1% upslope is a 1.05 factor, VR must not be less than VS1, and the take-off safety speed at 50 feet must exceed the higher of a safe speed and 1.2 times VS1.