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. The good news is that these are exactly the same as what you've already learned for the single-engine aeroplane, so this is a reinforcement of familiar ground. You'll also find these same rules in CAP 698, under paragraph 2.1.1, on pages 1 and 2 of section 3 — that's your reference document for the UK. Now, the core idea here is that we take the gross take-off distance — that's the distance the aeroplane actually needs to get airborne, before we apply any safety factors — and we multiply it by certain factors to make sure we have a safe margin. The factors depend on what's available at the aerodrome. First case: when there is no stopway or clearway available. A stopway is an area beyond the runway that can support the aeroplane during an aborted take-off, and a clearway is an area beyond the runway over which the aeroplane can climb initially. If neither is available, then the take-off distance, when multiplied by 1.25, must not exceed the TORA. TORA stands for Take-Off Run Available — that's the length of runway declared available for the ground run. So the rule is: Gross TOD × 1.25 must not exceed the TORA. Second case: when a stopway and/or clearway is available. Then we have three separate checks. First, the take-off distance must not exceed the TORA — so Gross TOD must not exceed the TORA, no factor applied there. Second, when multiplied by 1.3, it must not exceed the ASDA. ASDA is the Accelerate-Stop Distance Available — that's the runway plus stopway, the distance available to accelerate and then stop if you abort. So Gross TOD × 1.3 must not exceed the ASDA. Third, when multiplied by 1.15, it must not exceed the TODA. TODA is the Take-Off Distance Available — that's the runway plus clearway. So Gross TOD × 1.15 must not exceed the TODA. Now, before we even apply those factors, the regulations in CS-23 — that's the certification standard for this class of aeroplane — state that when calculating the gross take-off distance, certain details must be accounted for. Let me walk you through each one. First, the mass of the aeroplane at the start of the take-off run. That's simply how heavy the aeroplane is when you begin rolling. Second, the pressure altitude at the aerodrome — that's the altitude corrected for atmospheric pressure, which affects air density and therefore performance. Third, the ambient temperature at the aerodrome — warmer air is less dense, so it hurts performance. Fourth, the runway surface conditions and the type of runway surface — is it dry, wet, grass, paved? Fifth, the runway slope — is it uphill or downhill? And sixth, a very specific 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 can 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 approach to wind. Now, from that list, the regulations specifically stipulate that due account must be made of the runway condition. Here's the key point: most performance data in the aeroplane flight manual assumes a level, dry, and hard runway. That's the baseline. So when the runway conditions are different — say it's grass, or wet, or sloped — correction factors must be applied to the gross take-off distance. These factors are detailed in CAP 698, at the top of page 2, section 2, under paragraph c). It states that if the runway is other than dry and paved, the following correction factors must be used when determining the take-off distance. And those are shown for you in Figure 11.1. So to tie it all together: you start with the gross take-off distance, you account for mass, pressure altitude, temperature, runway surface, slope, and wind — using those headwind and tailwind limits — then you apply the 1.25, 1.3, and 1.15 factors against TORA, ASDA, and TODA respectively, and finally you apply the surface condition correction factors if the runway isn't dry and paved. That's the complete picture for multi-engine Class B take-off field length.