Class A Aircraft - Take-off — Page 383, Lesson 478

Let’s pick this up right where the take-off speed story gets interesting. We’ve already looked at V1, VMCG, and VMBE. Now I want to walk you through what happens when the pilot rotates the aircraft — and why the certification rules are so strict about it. First, the effects of early and over-rotation. Imagine you rotate to the correct attitude — the right nose-up pitch — but you do it at too low a speed. Lift-off will not happen until the normal VLOF, the normal lift-off speed. But here’s the catch: because you’re holding that rotated attitude for longer, you’re generating more drag during that extra time. That increased drag gives you an increased distance to lift-off. So you haven’t gained anything — you’ve actually made the take-off longer. Now, over-rotation. If you rotate to an attitude greater than the normal lift-off attitude, you could bring the wing close to its ground stalling angle. That’s a serious condition — a ground stall. But here’s the safeguard: a ground stall should not be possible with the leading edge devices correctly set. That’s why it is of extreme importance that these devices are set to the take-off position. The leading edge devices — slats or similar — are what protect you from stalling at high angles near the ground. Now, the certification rule. CS-25.107 — that’s the European certification specification for large aeroplanes — requires two things. First, the take-off distance using a rotation speed of 5 knots less than VR shall not exceed the take-off distance using the established VR. So if you rotate 5 knots early, your take-off distance must not get worse than the scheduled one. Second, reasonable variations in procedures — such as over-rotation and out of trim conditions — must not result in marked increases in take-off distance. And here’s the definition of “marked increase” — it’s any amount in excess of 1% of the scheduled distance. So if your take-off distance grows by more than 1% of what’s scheduled, that’s a marked increase, and that’s not allowed. Now let’s define VLOF — the lift-off speed. VLOF is the calibrated airspeed at which the aeroplane first becomes airborne — that’s the moment when the main wheels have left the runway. So it’s not when the nose wheel lifts; it’s when the mains are off the ground. VLOF should be faster than the minimum unstick speed, VMU. The margin above VMU is determined by several factors. Here are the numbers. VLOF must not be less than 110% of VMU in the all engines operating condition, and 105% of VMU in the one engine inoperative condition. But there’s a special case. If the attitude of the aeroplane in obtaining VMU was limited by the geometry of the aeroplane — meaning tail contact with the runway — then VLOF must not be less than 108% of VMU in the all engines operating condition, and 104% of VMU in the one engine inoperative condition. So the geometry-limited case gives you slightly lower margins. Next, the tyre speed limit. Aeroplane tyres are designed to carry very high loads and operate at very high speeds. It’s common for a jet aeroplane tyre to carry loads as heavy as 27,000 kilograms while operating at ground speeds up to 235 miles per hour — or ground speeds of 204 knots. Tyres are carefully designed and tested to withstand operation up to, but not necessarily beyond, these ratings. So that’s a hard limit — you don’t plan to exceed it. Finally, V2MIN. This is the minimum take-off safety speed, with the critical engine inoperative. V2MIN may not be less than two things. First, 1.13 times VSR for 2 and 3 engine turboprops and all turbojets without provision for obtaining a significant reduction in the one engine inoperative power-on stalling speed. OR 1.08 times VSR for turboprops with more than 3 engines and turbojets with provision for obtaining a significant reduction in the one engine inoperative power-on stalling speed. So the distinction is whether you can reduce the stalling speed with one engine out. Second, V2MIN may not be less than 1.1 times VMC — the minimum control speed in the air. So to tie it together: VLOF is your actual lift-off speed, tied to VMU. V2MIN is your minimum safety speed with an engine failed, tied to VSR and VMC. And the rotation discipline — early or over-rotation — is governed by CS-25.107 with that 1% marked-increase limit. That’s the full picture for this part of the take-off speed story.