Class A Aircraft - Take-off — Page 383, Lesson 471

Right, let’s pick this up where the speeds themselves left off. We’ve already met VMBE and VMCG, so now I want to show you how they actually control V1 — and why that matters for the distances you have to plan. First, the rule that ties them together. V1 must not be less than VMCG. Look at Figure 14.2 and you’ll see that relationship drawn out. Why the hard floor? Because if the engine fails at a speed below VMCG, the aeroplane is uncontrollable on the ground — the rudder simply can’t hold the centreline against the asymmetric thrust. And here’s the key: the very definition of V1 is that the take-off can be continued following engine failure. If you’re below VMCG, you can’t continue, so V1 can’t live down there. That’s the lower limit. Now the upper limit. The rule also says V1 must not be greater than VMBE. Think about what V1 has to do: at that speed, you must be able to either stop the aeroplane or continue the take-off. But above VMBE, it is impossible to bring the aeroplane safely to a stop — the brakes and reverse thrust simply can’t absorb the energy. So V1 is boxed in: not below VMCG, not above VMBE. Now let’s make it real with a scenario. Suppose you’re operating at high density altitude — high density, as in the air is thin. That pushes VMCG up, higher than the idealised V1 you’d otherwise choose. What happens? Take-off is prohibited — you can’t legally depart with that combination. But the problem is solvable. The fix is to increase the chosen V1 until it is equal to or greater than VMCG. That moves V1 back inside the box. But — and this is the part I really want you to feel — moving V1 costs you. Watch what happens to the distances. The accelerate-stop distance increases. The take-off distance decreases. And more importantly, the total field length required increases. So you’ve traded one problem for another: you’ve made the aeroplane controllable, but you now need a longer runway to do it. The good news is the logic is clean. So long as the runway is as long as the total field length required, then moving V1 to that higher point is not a problem. The runway just has to be long enough to absorb the new, longer total. So here’s the mental picture I want you to keep. V1 sits in a corridor — VMCG below it, VMBE above it. When density altitude pushes VMCG up, you slide V1 up with it. That slide changes your distances: accelerate-stop grows, take-off shrinks, and the total field length grows. And the whole thing is governed by that simple test — is the runway at least as long as the total field length required? If yes, you’re legal. If not, you’re not going. That’s the relationship between VMCG, V1 and VMBE, and the distance consequences of moving V1 from its balanced position.