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 talked about V1 and VR, so now I want to walk you through what happens when the rotation is done badly, and then we’ll define the speeds that come after rotation — VLOF, the tyre speed limit, and V2MIN. First, the effects of early and over-rotation. Imagine you rotate to the correct pitch attitude, but you do it at too low a speed. The aircraft won’t become airborne until it reaches the normal lift-off speed, VLOF. But here’s the catch — during that extra time you’re sitting in the rotated attitude, you’re generating more drag. That higher drag over a longer time means the distance to lift-off increases. So an early rotation doesn’t get you off sooner; it actually stretches your take-off distance. Now, over-rotation — that’s rotating to an attitude greater than the normal lift-off attitude. If you pitch up too far, you could bring the wing close to its ground stalling angle. That’s the angle at which the wing would stall while you’re still on or near the runway. The good news is that a ground stall should not be possible if the leading edge devices are correctly set. That’s why it is of extreme importance that these devices are set to the take-off position before you start. The leading edge devices — slats or similar — change the wing’s behaviour so it can’t stall at the ground attitude. Now, the certification requirement. CS-25.107 — that’s the European airworthiness code 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 distance must not be worse than the scheduled distance. 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 the note defines what “marked increase” means: any amount in excess of 1% of the scheduled distance. So if your take-off distance grows by more than 1% because of an over-rotation or an out-of-trim condition, that’s a marked increase and it’s not acceptable. Let’s move to VLOF — the lift-off speed. VLOF is the calibrated airspeed at which the aeroplane first becomes airborne, which is 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. VMU is the minimum speed at which the aeroplane can be made to leave the ground — the unstick speed. The margin above VMU is determined by several factors, and the regulation gives you specific numbers. Here’s the key set of limits. 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 the numbers change. In that case, 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 smaller margins — 108% and 104% instead of 110% and 105%. That’s because if the tail is the limiting factor, the aeroplane can’t pitch as far, so the unstick speed is achieved differently. Now 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 — that’s 27 tonnes on a single tyre — 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 the tyre speed limit is a real operational constraint — you can’t just keep accelerating on the ground forever; the tyre has a rated maximum ground speed. Finally, V2MIN. This is the minimum take-off safety speed, with the critical engine inoperative. The critical engine is the one whose failure is most adverse to the aeroplane’s performance. V2MIN may not be less than two things. First, it may not be less than 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. VSR is the reference stall speed. Alternatively, it may not be less than 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 1.13 factor applies to the simpler cases, and the 1.08 factor applies when you have more engines or a system that significantly reduces the stall speed with one engine out. Second, V2MIN may not be less than 1.1 times VMC — and VMC is the minimum control speed, the speed at which you can maintain directional control with the critical engine inoperative. So to tie it together: VLOF is your lift-off speed, and it must stay above VMU by those percentage margins. The tyre has a hard speed limit on the ground. And V2MIN is your minimum safety speed after take-off with an engine failed, set by those VSR and VMC factors. These are all certification constraints that define the envelope you operate within.