MASS (kg) — Page 38, Lesson 52

Let’s start with the weighing schedule in front of you, because it ties together everything we’ve been doing with mass, arm, and moment. Look at the table. We have three weighing points on the aeroplane: the nose wheel and the two main wheels, left and right. Each row gives you three numbers: the mass in kilograms, the arm, and the moment in kilogram-inches. The arm is the distance of that weighing point from the datum, and the moment is mass times arm. So for the nose wheel, 134 kilograms at an arm of minus 2 gives a moment of minus 268. The minus sign just tells us the nose wheel is ahead of the datum. The left main and right main are each 550 kilograms at an arm of 90, giving 49,500 kilogram-inches each. Now add the three masses: 134 plus 550 plus 550 is 1,234 kilograms. That is the Basic Empty Mass, the BEM. And the total moment, 98,732 kilogram-inches, divided by that mass gives you the CG position of the basic empty aeroplane. That’s the whole point of a weighing schedule — it gives you the BEM and its CG, and from those two numbers you can compute every other mass and CG position you’ll ever need. The schedule is signed and dated, and here’s the important part: any change to the basic equipment between weighings — say you install a new radio or remove a seat — is recorded in the aeroplane’s technical log. When that change differs from what’s listed on the previous weighing schedule, it has to be accounted for separately at the next weigh. So the schedule is a snapshot, and the technical log keeps track of what’s changed since that snapshot. Now, how do we actually get those three masses? There are three principal methods, and which one you use depends on the size of the aeroplane. First, weigh-bridge scales. These are used for light aeroplanes. You have a separate electronic weighing platform under the nose or tail wheel and under each main wheel assembly. The mass at each platform is read directly on the balance arm or an electronic display, and you add the three masses together to get the BEM. Second, hydrostatic units, for larger, heavier aircraft. These 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 point on the aircraft. Again, you add the mass values from each unit to get the BEM. Third, electronic equipment, also for the larger, heavier aircraft. Here you fit strain gauges at each jacking point, and they work on the principle that electrical resistance varies with the load applied. The readings are added together to give the BEM. Notice the pattern: in every method, you weigh at each support point and sum the readings to get the Basic Empty Mass. Once you have the BEM and its CG position, everything else follows by simple addition and multiplication — the DOM, the OM, the TOM, and so on. And two more things you can compute arithmetically: the mass of the fuel load, provided you know the quantity and the specific gravity of the fuel. And for passengers and baggage, you can either use the actual masses or the standard masses given in the table. That’s the weighing schedule and the three weighing methods. The key takeaway: the BEM and its CG are the foundation, and every other mass and moment in the aeroplane builds on them.