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

Right, let's pick this up. We're starting the take-off performance work now, and the very first thing we need to nail down is what we actually mean by the distance a take-off requires. I want you to think of the take-off as 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, and here's the key phrase: the airborne distance to a "screen" of defined height. So we're not just talking about getting airborne; we're talking about climbing to a specific height above the runway, and that height is our screen. 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, that total is called the Take-off Distance, and we abbreviate that as TOD. So TOD is the complete distance, ground run plus airborne climb, from brake release to the screen. Let's talk about the speeds involved, because we have two very specific ones here. The first is called VR. That's the speed for rotation. At VR, the pilot pulls back on the control column, and that action raises the nose wheel off the runway. That's the rotation. Now, pulling back on the column increases the lift, and eventually the main wheels themselves will lift off the runway. The speed at which that happens — when the main wheels actually leave the surface — is called VLOF, which stands for the speed for lift-off. So to be crystal clear: VR is when you start rotating, and VLOF is when the mains actually unstick. Now, calculating that take-off distance is important, but here's the crucial safety concept I want you to grasp. There is a chance — a remote chance, but a real one — that the actual take-off distance you calculated will not be achieved. The aeroplane might not perform as well as predicted. So the aviation authorities have created a set of safety regulations to account for that poor performance. These regulations require factors to be applied to the estimated distances, to give us a satisfactory safety margin. And here's where the terminology changes, and you need to remember this precisely. The calculated take-off distance — the raw, unadjusted figure — is called the Gross Take-off Distance. But when we apply those safety factors, it becomes the Net Take-off Distance. So gross is the raw calculation, net is the safety-adjusted figure. The regulations and their associated safety factors, we'll discuss those later in the relevant chapters on the different classes of aeroplanes. For now, just hold onto that distinction: gross versus net.