MASS (kg) — Page 38, Lesson 52

Let’s start with that weighing schedule on the screen, because it ties together everything we’ve been doing with mass, arm, and moment. Look at the table. We have four rows of actual measurements. The nose wheel shows a mass of 134 kilograms, at an arm of minus 2 inches, which gives a moment of minus 268 kilogram-inches. Then the left main wheel: 550 kilograms at an arm of plus 90 inches, moment 49,500. The right main wheel is identical — 550 kilograms, arm 90, moment 49,500. Add those three together and you get the Basic Empty Mass, the BEM: 1,234 kilograms, at an arm of 80 inches, giving a total moment of 98,732 kilogram-inches. Notice the nose wheel arm is negative. That’s because the datum — the reference point — sits behind the nose wheel, so anything ahead of the datum carries a negative arm. The main wheels are behind the datum, so they’re positive. That sign convention is exactly why the moments add up the way they do. Now, that schedule is a snapshot. Between weighings, any change to the basic equipment — say you install a new radio or remove a seat — gets recorded in the aeroplane’s technical log. And because that change varies from what’s listed on the previous weighing schedule, it has to be accounted for separately at the next weigh. So the log is your running record of what’s actually on the aircraft. Let’s move to the weighing equipment itself. There are three principal methods, and which one you use depends on the size and weight of the aeroplane. First, weigh-bridge scales. These are generally used for light aeroplanes. You have a separate electronic weighing platform for the nose or tail wheel and for each main wheel assembly. The mass at each platform is recorded directly on the balance arm or on an electronic display, and you simply add the masses together to get the BEM. Second, hydrostatic units. These are for larger, heavier aircraft. They work on Pascal’s Law — the principle that the pressure of a liquid in a closed container is proportional to the load applied. The units are fitted at each jacking point, interposed between the lifting jack and the jacking points on the aircraft. Again, you add the mass values on each unit together to get the BEM. Third, electronic equipment. Also for the larger, heavier aircraft. Here you have strain gauges fitted at each jacking point, and they use the principle that electrical resistance varies with the load applied. The readings are added together to give the BEM. So in all three cases, the core idea is the same: measure the load at each support point, add them up, and that sum is your Basic Empty Mass. Once you have the BEM and its CG position established, everything else follows by simple addition and multiplication — the DOM, the OM, the TOM, and so on. They’re all just built up from that foundation. Two more calculation points. The mass of the fuel load can be calculated arithmetically, provided you know the quantity and the specific gravity of the fuel. Specific gravity is the ratio of the fuel’s density to that of water — that’s what lets you convert a volume of fuel into a mass. And for passengers and baggage, you have a choice: use the actual mass, or use the standard masses given in the table. That table is your regulatory fallback when you don’t weigh each person individually. So the whole chapter hangs together like this: weigh the aircraft, record the BEM and its CG, then every other mass — fuel, passengers, baggage, the operating mass, the take-off mass — is calculated from that starting point using arms and moments.