Remember that screen height of 50 ft above the take-off surface — Page 293, Lesson 370

I want to walk you through the take-off requirements for single-engine Class B aeroplanes, and I'll start by tying it back to the screen height concept you've just seen. Remember that screen height of 50 ft above the take-off surface? That's the point where the aeroplane must have achieved its specified speed, with take-off power set, having rotated at VR. Now, two critical speed constraints govern that rotation. First, the rotation speed VR must not be less than VS1 — that's the stalling speed in the take-off configuration. Second, the take-off safety speed, which is the screen height speed, must be not less than the greater of two values: either a speed that is safe under all reasonably expected conditions, or 1.2 times VS1. So you take whichever of those two is larger, and that becomes your minimum take-off safety speed. Now, for single-engine Class B aeroplanes, there is only one take-off requirement, and it's called the Field Length Requirement. The principle is simple: the mass of the aeroplane must be such that the take-off can be completed within the available distances — in other words, the take-off must be complete within the field length available. The Field Length Requirements are detailed in CAP 698, under paragraph 2.1.1 of pages 1 and 2 of section 2. There are two distinct cases. 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 not exceed the TORA. When a stopway and/or clearway is available, you have three simultaneous constraints: the take-off distance must not exceed the TORA — so gross TOD must not exceed TORA; when multiplied by 1.3, it must not exceed the ASDA — so gross TOD times 1.3 must not exceed ASDA; and when multiplied by 1.15, it must not exceed the TODA — so gross TOD times 1.15 must not exceed TODA. Let me define those acronyms clearly, because they're the heart of this. TORA is the Take-Off Run Available — that's the length of runway declared available and suitable for the ground run of an aeroplone taking off. ASDA is the Accelerate-Stop Distance Available — the runway plus stopway declared available for an aeroplane to accelerate to a specified speed and then stop. TODA is the Take-Off Distance Available — the TORA plus any clearway. And the gross take-off distance, or gross TOD, is the distance the aeroplane actually needs to reach the 50 ft screen height. Let me work through the example to make this concrete. Suppose the aeroplane flight manual gives the take-off distance as 3000 ft, and there is no stopway or clearway available at the airport. We must satisfy the requirement that the take-off distance multiplied by 1.25 must not exceed TORA. So we multiply 3000 ft by 1.25, which gives us 3750 ft. Essentially, this means the runway must be at least 3750 ft long — or, more correctly, the take-off run available, the TORA, must be at least 3750 ft long. That figure shows you the flight path of a typical aeroplane on the approach to land, which is the mirror image of what we're discussing — the same 50 ft screen height concept applies on the take-off side. The key takeaway is that the gross take-off distance multiplied by 1.25 must not exceed the TORA when you have no stopway or clearway. That 1.25 factor is your safety margin built into the field length requirement.