Single-engine Class B Aircraft - Take-off — Page 293, Lesson 374

Let’s pick this up right at the take-off rule for a single-engine Class B aeroplane, because this is where the regulation turns into a hard number you’ll use. The rule is this: once the aeroplane has completed the take-off — and by “completed,” we mean it has reached the screen height — there must be at least 25% of the runway still remaining behind you. So the regulation isn’t just about getting airborne; it’s about getting airborne with a built-in safety margin. That 25% of runway left over is the margin the authorities demand. Now, here’s how you interpret that in practice. Say the actual take-off distance — the gross performance — is 3000 ft. The authorities are effectively saying that the worst case in a million take-offs would be one that runs 3750 ft long. That 3750 ft is the net performance. So you can see the difference between net and gross performance right here: gross performance is the 3000 ft, the better, ideal figure; net performance is always the worse figure, 3750 ft, the one with the safety factor built in. The regulation is written so that even in that worst-case scenario, you still have 25% of the runway left. Now, before we even apply those factors, CS-23 tells us what must be accounted for when calculating the gross take-off distance. These are the inputs you cannot ignore. First, the mass of the aeroplane at the start of the take-off run. Second, the pressure altitude at the aerodrome. Third, the ambient temperature at the aerodrome. Fourth, the runway surface conditions and the type of runway surface. Fifth, the runway slope. And sixth — this one is a specific wind rule — not more than 50% of the reported headwind component, or not less than 150% of the reported tailwind component. So if you have a headwind, you may only credit half of what’s reported; if you have a tailwind, you must assume it’s one and a half times worse than reported. That’s a conservative assumption built into the regulation. Let’s focus on the runway surface and slope, because that’s where the correction factors come in. Most performance data in the aeroplane flight manual assumes a level, dry, hard runway. So if your runway is anything other than that, you must apply correction factors to the gross take-off distance. These factors come from CAP 698, at the top of page 2, section 2, under section c). It states that if the runway is other than dry and paved, you use the following factors when determining take-off distance. Here they are. For grass on firm soil, up to 20 cm long, dry — the factor is × 1.2. For that same grass, wet — the factor is × 1.3. For a paved runway that is wet — the factor is × 1.0. So a wet paved runway gets no penalty, a factor of 1.0, meaning no change. Dry grass increases your take-off distance by a factor of 1.2, and wet grass by 1.3. And that matches what you’ve already learned: grass runways will increase the take-off distance compared to paved runways. Dry grass costs you 20% more distance; wet grass costs you 30% more. So when you’re planning, you multiply your gross take-off distance by these factors, and that gives you the distance you must plan for on that surface. That’s the take-off rule and the surface correction factors. The key takeaway: gross performance is the ideal, net performance is the worse case, and the surface factors are how you adjust the gross figure for the real runway beneath you.