Class A Aircraft - Take-off — Page 379, Lesson 460

I want to walk you through the take-off performance requirements for a Class A aeroplane — that's the certification category for large transport jets, the kind you'll fly commercially. We're going to start with the single most important concept in this whole chapter: the net take-off distance required. Here's the key idea. The take-off distance required is not one single distance. It is the greatest of three distances, and the regulator makes you plan for the worst case. Let me give you all three, because you need to know each one precisely. First, all engines operating. This is the horizontal distance the aeroplane travels, with every engine running, to reach a screen height of 35 feet — and then you multiply that distance by 1.15. So it's the all-engines-operating distance to 35 feet, increased by 15 percent. That 1.15 factor is a safety margin built into the certification. Second, one engine inoperative on a dry runway. This is the horizontal distance from the BRP — that's the brake release point, the start of the take-off roll — to the point where the aeroplane attains 35 feet. And the assumption here is that the critical power unit fails at VEF, on a dry, hard surface. I'll come back to VEF in a moment. Third, one engine inoperative on a wet runway. This is the horizontal distance from the BRP to the point where the aeroplane attains 15 feet — not 35 feet this time — again assuming the critical power unit fails at VEF, but now on a wet or contaminated hard surface. And there's an important qualifier: this must be achieved in a manner consistent with the achievement of V2 by 35 feet. So even though the screen height drops to 15 feet, you still have to be able to reach V2 by 35 feet. Now, why does the screen height drop from 35 feet to 15 feet on a wet runway? The note in the text tells you exactly why: it's to help reduce the take-off mass penalties that a wet runway will undoubtedly cause. A wet runway gives you less braking and less acceleration, so if you demanded 35 feet on a wet runway, you'd have to offload a lot of payload. Dropping the screen to 15 feet eases that penalty. Let me now define the speeds you've just met, because they're the backbone of everything. First, VMCG — the ground minimum control speed. CAP 698, which is the UK CAA's performance document, defines it at the bottom of page 3, section 4. VMCG is the minimum speed on the ground at which the take-off can be safely continued, when the critical engine suddenly becomes inoperative with the remaining engine or engines at take-off thrust. Let's unpack what that actually means physically. When an engine fails, the remaining live engine still generates thrust. That thrust acts along the aeroplane's centreline, but it's now unbalanced — one side is pushing, the other isn't. That asymmetry causes the aeroplane to yaw away from the live engine. The amount of yaw is a function of the amount of thrust the live engine is generating. More thrust from the live engine means more yaw. The only way to counteract that yaw is to use the ailerons and the rudder to steer the aeroplane back in the right direction. But here's the catch — when the aeroplane is on the ground, the rudder's effectiveness depends on the airflow over it, which depends on your speed. Below VMCG, you simply don't have enough rudder authority to hold the centreline against that asymmetric thrust. That's why VMCG is a minimum — it's the slowest you can be and still maintain directional control on the ground after a critical engine failure. Now, I mentioned VEF and V2. VEF is the engine failure speed — the speed at which the critical engine is assumed to fail in the certification calculations. V2 is the take-off safety speed, the speed you must be able to reach by 35 feet. These are the speeds that tie the whole take-off distance calculation together. Let me show you how V1 fits into all of this, because it's the decision speed. Look at the graph in Figure 14.1 — it shows the ideal position of V1. The graph plots the accelerate-stop distance and the accelerate-go distance against speed. The intersection point of those two curves is where V1 should ideally sit. That's the balance between "I can still stop safely" and "I can continue the take-off safely." So here's how it all fits together. You have three distances to satisfy — all engines to 35 feet times 1.15, one engine out to 35 feet on dry, one engine out to 15 feet on wet. The take-off distance required is the greatest of those three. And the speeds — VMCG, VEF, V1, V2 — are the tools that define when and how those distances are measured. VMCG tells you the minimum ground speed for control after an engine failure. VEF is the assumed failure speed. V1 is your decision speed, ideally at the intersection of the stop and go curves. And V2 is your safety speed by 35 feet. That's the foundation of Class A take-off performance. The next piece will build on these speeds and distances, so make sure you're comfortable with VMCG, VEF, V1, and V2 before we move on.