Definitions and Calculations — Page 73, Lesson 99

Let’s start with the big picture, because this page is the heart of how an airline actually proves an aeroplane is safe to fly before every single departure. I’m walking you through the Load and Trim Sheet for the MRJT1 — that’s the EASA Medium Range Twin Jet, a modern twin-jet airliner used as the standard example in the CAP 696 loading manual. You may well be asked to complete part of this sheet in the EASA exam, so we’re going to treat it as a real working document, not just a diagram. 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 inside the aeroplane — that means the dry operating mass, the traffic load, and the 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 ZFM and the TOM — the zero fuel mass and the take-off mass — must both fall inside the relevant area of the graph envelope once you finish the mass calculations. If either one sits outside that envelope, the aircraft is unsafe to fly. That’s the whole point of this sheet — it’s a legal and physical check, not paperwork for its own sake. Let’s go through Part A, the loading summary, section by section. Section 1 is used to establish three things: the limiting take-off mass, the maximum allowable traffic load, and the underload — that’s the amount of payload capacity you haven’t used — all before last minute changes, which we abbreviate 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. So when you read a loading manifest, every entry is one of those codes. Section 3 summarizes the loading, and its job is to cross-check that the limiting values from Section 1 have not been exceeded. So Section 1 sets the limits, Section 2 distributes the load, Section 3 verifies you stayed inside the limits. Now Part B — the distribution and trim portion. The lower part of it is the CG envelope graph. The vertical scale is in terms of mass, and the horizontal axis is in terms of MAC — mean aerodynamic chord. So you’re plotting mass against centre-of-gravity position along the chord. Here’s the actual procedure, step by step, and this is the mechanical skill you’ll be tested on. Step 1: Using data from the loading summary, you enter the Dry Operating Index — the DOI — for the DOM, the dry operating mass. The DOI is a number that represents where the CG sits for the empty, ready-for-service aeroplane. Step 2: You move the index vertically downwards into the centre of the first row of horizontal boxes. Now, here’s the key detail: each box within that row has a pitch — a spacing — which represents either a defined mass or a number of persons. And there’s an arrow indicating the direction in which the pitch is to be read. So the boxes aren’t just decoration; each one is a calibrated step. Step 3: You 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 the row to the left tells you how many passengers or how much mass you’re adding, and you shift the index along the boxes by that amount. That’s the core mechanism — you’re literally walking the CG index across the graph, one load item at a time, and at the end you check that the final ZFM and TOM positions sit inside the envelope. Take a look at the actual manifest on the page — shows the loading manifest for the MRJT, and shows the continuation with Table 3. Those are the real documents you’d be working from. So, to tie it together: Part A tells you what mass you have and where it’s distributed; Part B translates that into a CG position along the MAC; and the envelope on the graph is the hard limit that decides whether the aeroplane is legal to fly. That’s the Load and Trim Sheet in one sweep.