Definitions and Calculations — Page 27, Lesson 39

Let’s start with the manufacturer’s job, because that’s where all the limits come from. The manufacturer of the structural parts of the aircraft is responsible for determining the stresses the aeroplane will be subject to, both on the ground and in the air, and for imposing suitable mass limits so that the integrity of the structure is guaranteed throughout the aircraft’s working life. Those limits are four specific values, and I want you to know each one by its exact name. First, the maximum taxi mass, abbreviated MTM. Second, the maximum zero fuel mass, MZFM. Third, the maximum structural take-off mass, MSTOM. And fourth, the maximum structural landing mass, MSLM. These values must never be exceeded in normal operation. That’s a hard rule — they are structural ceilings, not suggestions. Now, an important note before we go further. In Mass and Balance terms, mass and weight are synonymous — they are used to express the same thing. So when you see “mass” and when you see “weight” in this subject, treat them as interchangeable. Why do these limits exist? Because stress and fatigue attack the structure. Increasing age, inappropriate use, and hostile environmental and climatic conditions are all factors that induce stress and fatigue into the aircraft’s structure. But here’s the key point: weight is the principal stress factor for inducing fatigue into aircraft structure. Not age, not weather — weight is the main driver. And weight doesn’t just fatigue the structure. It has pronounced effects on the aircraft’s performance, handling, and aerodynamic properties. Let me walk you through what happens with an increase in weight, because this is a long list and you need each effect. Performance is reduced. Take-off and landing distances will increase. The V1 decision speed, the VR rotation speed, the V2 take-off safety speed, and the stopping distance will all increase. The climb gradient, the rate of climb, and the ceiling height will all reduce. The rate of descent will increase. The stalling speed will increase, and the maximum speed will reduce. The safety margins and the effective speed range between low-speed and high-speed buffet will reduce. Drag and fuel consumption will increase. Range and endurance will reduce. Wing root stresses will increase. Manoeuvrability will reduce — the aircraft becomes less responsive to control inputs and more difficult to fly. And finally, wing root stresses and undercarriage loads will increase, as will tyre and brake wear. Let me make sure you’ve got the logic of that list. Heavier aircraft need more speed to get airborne and more distance to stop, so the decision speed, rotation speed, and take-off safety speed all climb. Climbing performance suffers — less gradient, less rate of climb, lower ceiling. Descent is faster. The stall speed rises because a heavier wing needs more angle of attack to generate the same lift. The buffet margins shrink, so you have less room between the low-speed and high-speed buffet boundaries. Drag and fuel burn go up, so range and endurance drop. The wing root takes more bending stress, the undercarriage and tyres and brakes take more load and wear. And the aircraft feels sluggish — less responsive to your control inputs. Now let’s move to the centre of gravity, the CG. This is a definition you must have exactly right. The centre of gravity is: the point that the total weight of the aircraft is said to act through; the point of balance; that part of the aircraft that follows the flight path; the point that the aircraft manoeuvres about in the air; and the point that the three axes of the aircraft pass through. So think of the CG as the single point where all the weight is concentrated, the balance point, the point that traces the flight path, the point the aircraft rotates around in flight, and the point where all three axes — the longitudinal, lateral, and vertical axes — intersect. Everything about mass and balance, and about how the aircraft behaves, hangs on where that CG sits and whether it stays within its limits. That’s the foundation. Next we’ll look at how we actually calculate the CG position and what happens when it goes out of limits.