So this isn't just a take-off check — Page 33, Lesson 45

I want to walk you through the very heart of mass and balance — the centre of gravity and why the regulations treat it as a hard, non-negotiable limit. Let's start with the rule that governs everything. EU-OPS 1 states that the CG position must remain within the range limits at all times — whether in the air, taking off, landing, or loading and unloading on the ground. So this isn't just a take-off check. It's a continuous requirement across every phase of operation. And here's the key reason: changes to the load distribution that may occur during any stage of the intended flight — for example fuel burn, or passenger or crew movements — will affect the CG position. Those changes must be properly accounted for prior to take-off. In other words, you don't just check the CG once at the gate; you have to anticipate how it will move during the flight and make sure it stays inside the limits the whole way. Now, the definitions. These are meant to clarify and enhance the definitions found in CAP 696, pages 3 and 4. So let's take them one by one. First, the Centre of Gravity itself. It is the point through which the force of gravity is said to act on a mass. In aircraft terms, it's the point on the aircraft through which the total mass is said to act in a vertically downward manner. So imagine the entire weight of the aeroplane concentrated at a single point, pulling straight down. That point is the CG. But it's more than just a theoretical point — it is also the point of balance. And because it's the balance point, it directly affects the stability of the aircraft, both on the ground and in the air. That's why we care about it so much. Next, the Centre of Gravity Limits. This is a crucial concept. The CG is not a fixed point. It has a range of movement between a maximum forward position and a maximum rearward position. Those two extremes are set by the aircraft manufacturer and cannot be exceeded. The CG must be on or within that limit range at all times. The limits are given in the flight manual, and they are defined relative to the datum — which we'll get to in a moment. They may also be given as a percentage of the mean chord of the wing. Now, the wing mean chord was formerly called the Standard Mean Chord, but it is now known as the Mean Aerodynamic Chord, or more simply, the MAC. So when you see a CG limit expressed as a percentage, it's a percentage of that mean aerodynamic chord. Finally, the Datum. The datum is a point along the longitudinal axis — that's the centre line — of the aeroplane, or its extension, designated by the manufacturer as the zero or reference point from which all balance arms, meaning distances, begin. So every distance you measure for balance purposes is measured from this datum. And here's the practical use: by taking moments about the datum, the CG position of the aircraft can be determined. That's the whole calculation — you take moments about the datum to find where the CG lies. One more assumption for this phase of study: the lateral displacement of the CG from the longitudinal axis is assumed to be zero. That means we're only worrying about the fore-and-aft position of the CG along the aircraft's length, not any sideways offset. For now, we treat the CG as sitting right on the centre line. So let me tie it together. The datum is your reference zero. You measure all arms from it. You take moments about it to find the CG. The CG must stay within the manufacturer's forward and rearward limits at all times — in the air, taking off, landing, and even while loading and unloading on the ground. And you must account for how fuel burn and passenger or crew movement will shift that CG during the flight, before you ever take off. That's the complete picture of what we're working with.