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

Let's get into the take-off performance rules for a single-engine Class B aeroplane. This is where the regulation turns raw performance numbers into a safe, legal take-off. The first thing I want you to grasp is the safety margin built into the rule. The regulation demands that once the aeroplane has completed its take-off — and by "completed" I mean it has reached the screen height, which is the height at the end of the take-off distance where the obstacle clearance plane begins — there must be at least 25% of the runway remaining behind it. So if the actual take-off distance is 3000 feet, the regulation effectively wants you to plan as if that take-off could stretch to 3750 feet. That extra 750 feet is the 25% margin. This is the difference between gross performance and net performance. The gross performance is the actual, measured take-off distance — here, 3000 feet. The net performance is the worse-case figure the authorities insist you use — here, 3750 feet. Net performance is always the worse of the two, because it's the one that builds in the safety margin. Now, before we even apply that 25% factor, the regulations in CS-23 tell us exactly what must be accounted for when calculating the gross take-off distance. Let me list them, because each one is a distinct input: - the mass of the aeroplane at the start of the take-off run - the pressure altitude at the aerodrome - the ambient temperature at the aerodrome - the runway surface conditions and the type of runway surface - the runway slope - and here's a subtle one — not more than 50% of the reported headwind component, or not less than 150% of the reported tailwind component. That last point is worth pausing on. A headwind helps you, so the regulation only lets you credit half of what's reported. A tailwind hurts you, so you must assume it's one and a half times stronger than reported. Both are conservative — they make your planning worse than reality, which is exactly what a safety margin should do. Now let's focus on the surface condition factors, because that's where the correction factors come in. Most performance data in the aeroplane flight manual assumes a level, dry and hard runway. So when the actual runway differs from that ideal, you must apply correction factors to the gross take-off distance. The source for these is CAP 698, specifically 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: - Grass on firm soil, up to 20 cm long, dry — multiply by × 1.2 - Grass on firm soil, up to 20 cm long, wet — multiply by × 1.3 - Paved, wet — multiply by × 1.0 So think about what these numbers mean. A factor of 1.0 means no correction — wet paved runway is treated the same as dry paved for take-off distance. A factor of 1.2 on dry grass means your take-off distance grows by 20%. And wet grass, at 1.3, grows it by 30%. That's the effect of grass increasing rolling resistance compared to a hard paved surface, and wet grass being even worse. So if your gross take-off distance was 3000 feet on dry pavement, on dry grass you'd plan for 3600 feet, and on wet grass, 3900 feet — before you even apply that 25% net-performance margin. That's the core of it: the gross distance is built from those six inputs, then corrected for surface, then the net margin is applied. Let's keep going if you're ready.