Definitions and Calculations — Page 73, Lesson 96

Let’s start with the definition that ties this whole page together: underload. Underload equals the maximum traffic load minus the actual traffic load. In the example here, that’s 12 800 kg minus 12 400 kg, which gives you 400 kg. So the underload is simply the payload capacity you didn’t use — the difference between what the aeroplane could carry and what you actually loaded. Now, the good news: the MRJT1 Load and Trim Sheet does most of this arithmetic for you. I want you to practise with it, because in the exam and in the real world, that sheet is your friend. Let’s walk through a full sample calculation. We’ll do it two ways: first with the Loading Manifest — that’s the data sheet in Figure 4.10 — together with the CG limits envelope from Figure 4.11. Then we’ll check the same result with the Load and Trim Sheet from Figure 4.12. All of these are in CAP 696, Chapter 4, for the MRJT1. Here are the values we’re starting with. The DOM — that’s the dry operating mass — is 34 300 kg, and its CG position is at 15% MAC. MAC stands for mean aerodynamic chord — it’s the reference line for the centre of gravity position, expressed as a percentage. Then the passengers: total 116, and we use the standard weight of 84 kg each. They’re distributed across zones: 10 each in zones A and G, 12 each in zones B and F, and 24 each in zones C, D and E. Let’s check that adds up: 10+10 is 20, plus 12+12 is 24, that’s 44, plus 24+24+24 is 72, so 44 plus 72 is 116. Good. Cargo: 600 kg in hold 1, and 1500 kg in hold 4 — and that hold 4 figure includes checked baggage. Fuel: 15 000 kg at take-off, 260 kg for start and taxi, and 10 000 kg trip fuel. Now, the method. We use the CAP 696 MRJT1 data sheets to find the balance arm for the MAC, and vice versa. The moment/1000 is calculated from the arm/1000. The balance arm itself is found by dividing the total moment by the total weight. And the fuel balance arm and the quantity in each tank also come from the data sheets. One important note about the fuel: the centre fuel tank content is used before the wing tank fuel content, and the centre tank includes 24 kg of unusable fuel. Unusable fuel is fuel you can’t burn — it sits in the tank and can’t reach the engines, but it still counts in the mass and balance. Let’s do the first calculation together. If the DOM CG is at 15% MAC, that’s 15% of 134.5 inches — and that comes from CAP 696, MRJT1, Chapter 4, Page 2. 15% of 134.5 is 20.175 inches. So the CG balance arm becomes 20.175 plus 625.6, which gives you 645.8 inches aft of the datum. The datum is the reference point from which all balance arms are measured. For the fuel, we extract the balance arm from Figures 4.5 and 4.6 of the loading manual. For example, the maximum contents of tanks one and two is 9084 kg, with a balance arm of 650.7. Now, you fill in the blank sheet with all this information, and then you check the aeroplane against the limits in the CG envelope. I’ve done the calculation myself — I estimated changes to the fuel tank CG position and accounted for the unusable fuel in the centre tank. But here’s a key exam point: in the EASA exams, the fuel CG position will be fixed, and there will be no unusable fuel to account for. So don’t worry about that extra step in the exam. If you want to check your mass values and CG positions using the Load and Trim Sheet for the MRJT1, use a DOI of 40.5. DOI is the dry operating index — it’s a shorthand number that represents the DOM’s moment contribution, and it lets you enter the sheet quickly. Finally, the page lists the maximum permissible aeroplane mass values you must respect: taxi mass, zero fuel mass, and take-off mass. Taxi mass is the mass at the start of taxi, including the fuel you’ll burn before take-off. Zero fuel mass is the mass of the aeroplane with all payload but no usable fuel. Take-off mass is the mass at the moment of take-off. Each has its own limit, and the CG envelope check ensures you’re inside all of them. Here’s the loading manifest figure so you can see the sheet we’re filling in: And here’s the Load and Trim Sheet table for the MRJT1: So the whole process is: gather your masses and arms, compute moments, sum them, divide to get the CG, then verify every limit — underload, taxi mass, zero fuel mass, take-off mass, and the CG envelope. That’s the complete picture for this sample calculation.