Multi-engine Class B - Take-off — Page 341, Lesson 418

Let's start with the field length requirements for a multi-engine Class B aeroplane. I want you to understand that these are exactly the same rules you learned for the single-engine aeroplane, and you'll find them in CAP 698 under paragraph 2.1.1 of pages 1 and 2 of section 3. Here's the core idea. We work with the gross take-off distance, which is the distance the aeroplane actually needs to get airborne before we apply any safety factors. Then we apply multipliers to that gross distance, and the result must not exceed the declared runway distances. First case: when no stopway or clearway is available. The take-off distance, when multiplied by 1.25, must not exceed the TORA. So the gross take-off distance times 1.25 must be less than or equal to the take-off run available. Second case: when a stopway and/or clearway is available, we have three separate checks. The take-off distance must not exceed the TORA — that's the gross take-off distance without any multiplier. Then, when multiplied by 1.3, it must not exceed the ASDA, the accelerate-stop distance available. And when multiplied by 1.15, it must not exceed the TODA, the take-off distance available. So remember those three factors: 1.25 when there's no stopway or clearway, and then 1.0 against TORA, 1.3 against ASDA, and 1.15 against TODA when there is a stopway or clearway. Now, before we even apply those factors, CS-23 regulations say the gross take-off distance itself must account for certain details. Let me list them for you. 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 then a very specific wind rule: not more than 50% of the reported headwind component, or not less than 150% of the reported tailwind component. That wind rule is important. We only credit half the headwind, and we penalise ourselves with one and a half times the tailwind. Now, from that list, the regulations specifically require us to account for runway condition. Most performance data in the aeroplane flight manual assumes a level, dry, and hard runway. So when the runway is different, we must apply correction factors to the gross take-off distance. There are various correction factors — for grass runways, for wet runways, and for runways which are sloped. CAP 698, at the top of page 2, section 2, under paragraph c), states that if the runway is other than dry and paved, we must use these correction factors when determining the take-off distance. They're shown for you in Figure 11.1. So the key takeaway: the gross take-off distance is computed with those six factors accounted for, then we apply the 1.25, 1.3, or 1.15 multipliers against TORA, ASDA, and TODA, and then we correct for the actual runway surface using the factors in that figure.