Definitions and Calculations — Page 73, Lesson 101

Let’s pick up right where the load sheet procedure leaves off. We’ve already worked through the index calculations for the rows of boxes, and now I want to walk you through the final steps of completing the loading manifest, and then we’ll go through a full worked example. So, step 4: after you’ve done the index calculation for each row of boxes, down to and including row 0g, you drop a line vertically down from that index value until it bisects a mass on the vertical scale of the envelope — and that mass corresponds to the Zero Fuel Mass. The point of bisection must occur within the MZFM envelope. That’s your check that the zero fuel mass is within limits. Step 5: you go to the horizontal row of boxes marked ‘Fuel index’ and add the take-off fuel index. That’s a separate index value for the fuel you’ll have on board at take-off. Step 6: after completing that fuel index calculation, you drop a line vertically down from it until it bisects a mass on the vertical scale of the envelope corresponding to the Take-off Mass. The point of bisection must occur within the TOM envelope. So you’re checking the take-off mass against its own envelope. Step 7: providing both the ZFM and the TOM are within their respective envelopes as described, the aircraft is safe for the intended flight — including any permitted diversions. That’s the whole point of the exercise: both masses must sit inside their envelopes. Now let’s go through the example. We have a set of data for a Medium Range Twin Jet, EASA limitations. The DOM — that’s the Dry Operating Mass — is 34,300 kg. The DOI — Dry Operating Index — is 45.0. RTOM is 62,800 kg — that’s the Regulated Take-off Mass. MZFM is 51,300 kg — Maximum Zero Fuel Mass. RLM is 54,900 kg — Regulated Landing Mass. Then the traffic: 130 passengers at an average mass of 84 kg each, 130 pieces of baggage at 14 kg per piece, and 630 kg of cargo. Take-off fuel is 14,500 kg, and trip fuel is 8,500 kg. Now the limitations for this EASA Medium Range Twin Jet: Maximum Structural Taxi Mass is 63,060 kg. Maximum Structural Take-off Mass is 62,800 kg. Maximum Structural Landing Mass is 54,900 kg. Maximum Structural Zero Fuel Mass is 51,300 kg. Maximum number of passengers is 141. Then the cargo holds: Hold 1 has a max volume of 607 cubic feet and a max load of 3,305 kg. Hold 2 has a max volume of 766 cubic feet and a max load of 4,187 kg. The standard crew is already allowed for in the DOM: 2 on the flight deck, 2 cabin crew forward, and 1 cabin crew aft. Now here’s the key logic. Section 1 of the load sheet is completed first, and its purpose is to find the three potential take-off masses, labelled (a), (b), and (c). Value (a) is the take-off mass you would achieve if you loaded the aeroplane to the MZFM and then added your intended fuel load. Value (b) is the regulated take-off mass for the take-off airfield conditions as they exist. Value (c) is the take-off mass you would achieve if you were to land at the regulated landing mass and then added back the mass of the trip fuel. The lowest of (a), (b), and (c) is the limiting take-off mass for traffic load calculations. That’s the governing value — the one that constrains how much traffic you can carry. From there, the maximum allowable traffic load for the trip is determined by subtracting the operating mass from the limiting take-off mass. Then any underload is calculated by subtracting the actual traffic load from the allowable traffic load. In the example, that’s 14,000 minus 13,370, which equals 630. That underload sets the limiting mass for any last minute changes — the LMC. So if something changes at the last minute, you have 630 kg of margin to work with. Finally, sections 2 and 3 of the load sheet detail the mass and distribution of the traffic load, and they give the actual values of take-off and landing masses. So section 1 gives you the limits and the allowable load; sections 2 and 3 give you the real, actual numbers once the load is distributed. Let me show you the loading manifest itself so you can see how all these boxes and envelopes fit together on the actual form. And here’s the continuation with Table 3 for the MRJT1. So the whole procedure is: compute your indexes, check ZFM and TOM against their envelopes, find the three potential take-off masses, take the lowest as your limit, subtract operating mass to get allowable traffic load, and then the underload gives you your last-minute-change margin. That’s the complete logic of the load sheet.