Definitions and Calculations — Page 68, Lesson 88

We're moving into a new area now — the detailed calculations behind mass and balance, and I want to start by making sure you understand a couple of load limitations that govern how cargo can be placed on an aircraft. First, let's talk about running load. This is the mass per unit length, and it's expressed in kilograms per inch. The idea is that the aircraft floor structure can only support so much weight over a given span of its length. So if you have a box weighing 100 kg, and it's 3 feet long in the fore/aft direction — that's 3 feet running along the aircraft's length — then the running load is 100 divided by 3, divided by 12, because there are 12 inches in a foot. That gives you 4.16 kg per inch. But here's the key point: if that same 100 kg box has different dimensions, you can orient it differently. If it's 3 ft × 2 ft × 3 ft, you could turn it so the 2 ft side runs fore/aft, and then the running load becomes 100 ÷ 2 ÷ 12 = 4.16 kg per inch. Wait — let me re-read that. Yes, that's the same value in the example. The point is that the cargo may have to be orientated correctly to prevent exceeding the running load limitations. So the physical orientation of the box on the floor directly changes the load per inch. Now, the second limitation is area load, which is the load intensity — the mass per unit area, expressed in kg per square foot. The first box, standing on its base of 3 × 3 = 9 square feet, would have a load intensity of 100 ÷ 9 = 11.1 kg per square foot. But the second box, if placed on its side measuring 2 ft × 3 ft = 6 square feet, would have a load intensity of 100 ÷ 6 = 16.66 kg per sq ft. Again, the cargo may have to be orientated correctly to prevent exceeding the distribution load limitations. So you have two separate constraints: running load along the length, and area load on the floor surface. Now let's move to the bigger picture — calculation of the loaded mass and CG position for large aircraft. Here we introduce two important terms: TOM and ZFM. TOM is the Take-Off Mass, and ZFM is the Zero Fuel Mass. Their respective CG positions are determined by the same method used for light aircraft, except that the starting point is the DOM — the Dry Operating Mass — as opposed to the BEM, the Basic Empty Mass. So for large aircraft, you begin from the dry operating mass rather than the basic empty mass. The fuel reserves and the traffic load compilation are more complex for a large aeroplane than for a light one, and so a Load and Trim Sheet is used to coordinate the data and simplify the procedure. Now, here's an important operational point: many large aircraft operators do not bother to calculate the landing mass and CG position on the Trim Sheet, but instead calculate the ZFM and CG position. The reason is that should a large aircraft need to divert to another airfield, its actual landing mass could be many tonnes more, or less, than the estimated value, and its CG position could vary considerably from the projected value. Assuming that both TOM and ZFM and their respective CG positions are within limits prior to take-off, they will remain in limits throughout the flight. That's a critical safety assumption — if you're within limits at take-off for both take-off mass and zero fuel mass, you stay within limits for the whole flight. Now let's look at compiling the document — the Load Sheet. In its simplest form, a load sheet is a list showing the BEM/DOM and CG position. Added in tabular form are the elements of the traffic load and fuel, by their individual masses, arms, and moments. From this list, the take-off mass and CG position can be calculated. So you have the dry operating mass, then you add traffic load and fuel, each with its own mass, arm, and moment, and you sum them to get the take-off mass and CG. A load sheet is individual to each type of aircraft and must be compiled before each flight. And it's not just a good practice — it's a regulatory requirement. Under EU-OPS 1 Subpart J, a load sheet must contain certain mandatory information. Let me list those for you: the aeroplane registration and type; the flight number; the identity of the commander; the identity of the person who prepared the document; the dry operating mass and CG position; and the mass of take-off fuel and trip fuel. So that's the legal minimum content for the document. One more thing — don't forget there's a conversion chart on page 4 of the data sheets, which you'll need when working between units like feet, inches, and kilograms. Let me also point you to the figures — shows the stabilizer trim setting, which relates to how the aircraft's CG position affects the trim. And the text references Figure 2.23 and Figure 2.25 for examples of a completed Load Sheet and Trim Sheet respectively — those will help you see what a fully filled-out document looks like in practice.