General Principles - Take-off — Page 148, Lesson 176

Right, let's get into the take-off performance chapter. We're starting with the fundamental geometry of a take-off, and I want to build this up from the ground up. The distance required for take-off is split into two distinct segments. The first is the take-off roll, which we also call the ground run. That's exactly what it sounds like — the aeroplane accelerating along the runway with its wheels on the surface. The second segment is the airborne distance — the distance travelled in the air after the wheels leave the runway, up to a specific point. That point is a "screen" of defined height. Think of a screen as an imaginary vertical plane at a set height above the ground that the aeroplane must clear. Now, the total distance from the brake release point — that's the moment you release the brakes and start rolling — all the way to that screen height, is called the Take-off Distance, abbreviated TOD. That's the headline number for the whole manoeuvre. Let's now look at the speeds involved, because they define the phases. The speed at which the pilot attempts to raise the nose wheel off the ground is called VR — that's the speed for rotation. At VR, you pull back on the control column, and the nose wheel lifts off the runway. That action increases the lift being generated, and eventually the main wheels themselves lift off the runway. The speed at which that happens — when the main wheels actually leave the surface — is called VLOF, which is the speed for lift-off. So to summarise the sequence: you release the brakes, accelerate along the ground run, reach VR, rotate the nose up, and then at VLOF the main wheels leave the runway. From there you continue accelerating in the air until you clear the screen height. The total of all that is the TOD. Now, here's the crucial safety concept. Calculating the take-off distance is important, but you must understand there is a chance — albeit a remote one — that the actual calculated take-off distance will not be achieved. The aeroplane might not perform exactly as predicted. To account for this, the aviation authorities have created a set of safety regulations. These regulations require factors to be applied to the estimated distances, to give a satisfactory safety margin. And here's the key terminology change. The calculated take-off distance — the raw, unadjusted figure — is called the Gross Take-off Distance. But when you apply those safety factors, the terminology changes. The adjusted figure becomes the Net Take-off Distance. So the gross is the theoretical ideal, and the net is the certified, safety-margined figure you actually plan against. The regulations and their associated safety factors are discussed later in the relevant chapters on the different classes of aeroplanes, so we'll get into the specifics then. Let me show you this on the diagram — it lays out the take-off distance from brake release to screen height clearly. That's the core framework: the two segments, the three speeds — VR and VLOF — and the gross versus net distinction.