Definitions and Calculations — Page 73, Lesson 99

Right, let's get into the heart of the loading paperwork. We're looking at the Load and Trim Sheet for the MRJT1 — that's the EASA Medium Range Twin Jet, a modern twin-jet airliner. This is the combined document you'll use to prove the aeroplane is safe to fly before every departure. The sheet is split into two halves. The left-hand side, Part A, is the loading document. It itemizes the mass and the mass distribution within the aeroplane — that's your dry operating mass, your traffic load, and your fuel load. The right-hand side, Part B, shows how each of those masses, in turn, affects the position of the centre of gravity relative to the mean aerodynamic chord — the MAC. Now, the critical safety gate: the Zero Fuel Mass, the ZFM, and the Take-Off Mass, the TOM, must both end up inside the relevant area of the graph envelope once you've finished the mass calculations. If either falls outside, the aircraft is unsafe to fly. That's the whole point of this sheet. Let's walk through Part A, the loading summary. It's completed in three sections. Section 1 establishes the limiting take-off mass, the maximum allowable traffic load, and the underload — that's the spare capacity you have — all before last minute changes, which we abbreviate as LMC. Section 2 shows the distribution of the traffic load, and it uses a set of abbreviations you must know cold. TR is Transit. B is Baggage. C is Cargo. M is Mail. Pax is Passengers. Then the passenger classes: Pax F is First Class, Pax C is Club or Business, and Pax Y is Economy. Section 3 summarizes the loading, and its job is to cross-check that the limiting values from Section 1 have not been exceeded. It's your final verification within Part A. Now Part B — the distribution and trim portion. The lower part of it is the CG envelope graph. The vertical scale is mass, and the horizontal axis is in terms of MAC — mean aerodynamic chord. Here's the step-by-step method for using Part B. Step one: using data from the loading summary, you enter the Dry Operating Index — the DOI — for the Dry Operating Mass, the DOM. That's your starting index point on the graph. Step two: move the index vertically downwards into the centre of the first row of horizontal boxes. Now, each box within that row has a pitch — a spacing — and that pitch represents either a defined mass or a number of persons. There's also an arrow indicating the direction in which that pitch is to be read. Step three: move the index horizontally, in the direction of that arrow, to a pitch value that corresponds to the value in the Mass/No box immediately to the left of the row. So you read the number from the box on the left, then shift the index along the row by that many pitches, in the direction the arrow points. That's the core mechanism — you're sequentially shifting the CG index for each load item, and where it ends up on the envelope tells you whether the ZFM and TOM are within limits.