Definitions and Calculations — Page 27, Lesson 39

Let’s start with the big picture. The manufacturer of the structural parts of the aircraft is the one who decides how much stress the aeroplane will face, both on the ground and in the air. Based on that, they impose suitable mass limits so the structure’s integrity is guaranteed throughout the aircraft’s working life. Those limits are four: the maximum taxi mass, or MTM; the maximum zero fuel mass, or MZFM; the maximum structural take-off mass, or MSTOM; and the maximum structural landing mass, or MSLM. These values must never be exceeded in normal operation. Now, one important note before we go further: in Mass & Balance terms, mass and weight are synonymous. They’re used to express the same thing. So when we say weight, we mean mass, and vice versa. Why do these limits matter? Because increasing age, inappropriate use, and hostile environmental and climatic conditions all induce stress and fatigue into the aircraft’s structure. But the principal stress factor — the main one — is weight. Weight is what drives fatigue in the structure more than anything else. Weight also has pronounced effects on performance, handling, and aerodynamic properties. Let me walk you through what happens as weight increases. Performance is reduced. Take-off and landing distances increase. The V1 decision speed, the VR rotation speed, the V2 take-off safety speed, and the stopping distance all increase. The climb gradient, rate of climb, and ceiling height all reduce. The rate of descent increases. The stalling speed increases, and the maximum speed reduces. The safety margins and the effective speed range between low-speed and high-speed buffet reduce. Drag and fuel consumption increase. Range and endurance reduce. Wing root stresses increase. Manoeuvrability reduces — the aircraft becomes less responsive to control inputs and more difficult to fly. And finally, wing root stresses and undercarriage loads increase, as do tyre and brake wear. Now let’s move to the centre of gravity — the CG. The CG is defined in five ways. It is the point that the total weight of the aircraft is said to act through. It is the point of balance. It is that part of the aircraft that follows the flight path. It is the point that the aircraft manoeuvres about in the air. And it is the point that the three axes of the aircraft pass through. So when we talk about an out-of-limit CG position, we’re talking about a CG that falls outside the certified envelope. The effects of that are what we’ll dig into next.