Multi-engine Class B - Take-off — Page 341, Lesson 421

Let's get into the take-off performance for a multi-engine Class B aeroplane. I want to start with the surface type, because that's the first correction we apply to the basic take-off distance. We have a small table here, and it gives us factors to multiply the take-off distance by, depending on the surface. For grass on firm soil, up to 20 centimetres long — that's the condition of the grass — if it's dry, the factor is 1.2. If that same grass is wet, the factor goes up to 1.3. For a paved runway that is wet, the factor is 1.0. Now, what does a factor of 1.2 actually mean? It means the take-off distance is multiplied by 1.2, so it's 20% longer than the basic paved, dry distance. The wet grass at 1.3 means 30% longer. And the paved wet runway at 1.0 means no change — the distance is the same as the basic figure. The key idea here is that grass, and especially wet grass, increases the rolling resistance and reduces the acceleration, so the aeroplane needs more runway to get airborne. That's why we apply these factors. Next, we have the slope correction. This is a separate correction, applied on top of the surface factor. The rule is: for every 1% of upslope, the pilot must increase the take-off distance by 5%, which is a factor of 1.05. So a 2% upslope would mean a factor of 1.10, and so on. But here's the important limitation: no factorization is permitted for a downslope. If the runway slopes downwards, you simply apply no correction factor at all. Why? Because a downslope decreases the take-off distance — gravity helps you accelerate. By ignoring that benefit, we add a little extra safety margin to the calculation. We deliberately don't credit the downslope, so the computed distance is conservative. Now let's talk about how this data is presented. These take-off distance graphs come from CAP 698, which is the UK CAA document for performance. In section 3, on page 3, there's the "normal take-off" graph, and on page 7 there's the "maximum effort" take-off — in other words, a short field take-off. These are two different techniques, and you use the appropriate graph depending on the situation. There are worked examples in CAP 698, one at the bottom of page 2 and another at the top of page 6 of section 3, and you should work through those to learn when to apply each factor. Now, the accelerate-stop distance. This is a different concept from take-off distance. For most aircraft, there is no regulatory requirement for accelerate-stop distance — but the data may still be given. You'll find the accelerate-stop distance graphs in CAP 698 on pages 5 and 8 of section 3, for a typical multi-engine Class B aeroplane. Again, there are examples to work through. However, for commuter category aircraft, the accelerate-stop distance is a defined requirement, and it's the sum of three distances. First, the distance to accelerate the aircraft to VEF with all engines operating. VEF is the critical engine failure speed — the speed at which the critical engine is assumed to fail. Second, the distance to accelerate from VEF to V1, assuming the critical engine fails at VEF. V1 is the decision speed — the speed beyond which you commit to take-off. And third, the distance to come to a full stop from the point at which V1 is reached. So the accelerate-stop distance is the total runway needed to either stop safely if the engine fails before V1, or continue the take-off if it fails after. Finally, let's define the gross take-off distance required. This is the distance from the start of take-off to a point 50 feet above the take-off surface, with take-off power on each engine, rotating at VR — that's the rotation speed — and achieving the specified speed at the screen. The "screen" is the imaginary vertical plane at the end of the take-off distance, 50 feet high. So the gross take-off distance is the full distance to climb to that 50-foot screen height with all engines operating. Let me just tie this together. We start with a basic take-off distance, then we apply the surface factor for grass or wet pavement, then we apply the slope factor for upslope only — never downslope. And we have two different graphs: normal and maximum effort. The accelerate-stop distance is a separate calculation, mandatory for commuter category, and it's the sum of those three segments I described. And the gross take-off distance is defined by that 50-foot screen height.