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

Let's start with the surface type corrections, because that's where the take-off distance calculation begins for a multi-engine Class B aeroplane. I want you to picture the runway surface. We have a table here that gives you a multiplication factor for the surface condition. For grass on firm soil, up to 20 cm long, if the grass is 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 that factor actually do? As you've already learnt, grass runways will increase the take-off distance compared to paved runways. So the factor is a multiplier you apply to your calculated take-off distance. Dry grass multiplies your distance by 1.2, meaning a 20% increase. Wet grass multiplies it by 1.3, a 30% increase. And a wet paved runway is 1.0, meaning no change from the baseline. Next, we have the slope correction. This comes from section 3, point d) of the relevant document. If there is an upslope on the runway, you must increase the take-off distance by 5%, or multiply by a factor of 1.05, for every 1% of upslope. So a 2% upslope means you multiply by 1.10. But here's the critical safety point: no factorization is permitted for a downslope. If the aeroplane is taking off on a downwards sloping runway, you apply no correction factor at all. Why? Because a downslope will decrease the take-off distance. Ignoring that benefit adds a little extra safety margin to your calculation. You take the credit for the upslope penalty, but you never take credit for the downslope advantage. Now let's move to how the take-off data is presented. In CAP 698, on pages 3 and 7 of section 3, you'll find the take-off distance graphs for a typical multi-engine Class B aeroplane. The one on page 3 is for a "normal take-off." The one on page 7 is for a "maximum effort" take-off — in other words, a short field take-off. There are worked examples in CAP 698, one at the bottom of page 2 and another at the top of page 6 in section 3. You should go through those examples carefully, because they show you when to apply the various factors we've just discussed. Next, the accelerate-stop distance requirements. Here's an important distinction: other than for commuter category aircraft, there is no requirement for accelerate-stop distance. However, the data may still be given. In CAP 698, on pages 5 and 8 of section 3, you'll see the accelerate-stop distance graphs for a typical multi-engine Class B aeroplane. As with the other graphs, there are examples you should work through. For commuter category aircraft specifically, the accelerate-stop distance is defined as the sum of three distances. First, the distance to accelerate the aircraft to VEF with all engines operating. Second, the distance to accelerate from VEF to V1, assuming the critical engine fails at VEF. Third, the distance to come to a full stop from the point at which V1 is reached. Let me define those speeds for you. VEF is the critical engine failure speed — the speed at which the critical engine is assumed to fail. V1 is the take-off decision speed — the speed beyond which you commit to the take-off rather than aborting. So the accelerate-stop distance is the total runway you need if you decide to stop: accelerate normally, then lose an engine at VEF, continue accelerating to V1, and then brake to a full stop. Finally, let's define the gross take-off distance. This is the distance from the start of take-off to a point 50 ft above the take-off surface. It assumes 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 50 ft obstacle height you must clear. So to tie it all together: you start with your gross take-off distance, apply the surface factor for grass or wet pavement, apply the upslope factor if present, and that gives you the distance you need to plan for.