Definitions and Calculations — Page 33, Lesson 48

I want to walk you through the heart of mass and balance — the definitions that everything else in this subject hangs on. We're going to build these up one by one, because each one is a foundation stone for the next. Let's start with something you'll see on every Load and Trim Sheet: the Dry Operating Index, or DOI. In the example we're looking at, the moment for the Dry Operating Mass is a huge number — 31,150,574 — but the DOI is just 31. That's the whole point of an index. Instead of carrying around a seven-figure moment, we compress it into a small, manageable number. The DOI is that compressed value for the Dry Operating Mass. Later in the course, you'll use the DOI together with another index called the Fuel Index to complete a Load and Trim Sheet. So remember: the index is the shorthand version of the moment. Now, the first mass definition: Basic Empty Mass, or BEM. All light aircraft use the BEM and its centre of gravity position as the foundation from which to calculate all relevant masses and CG positions. So for a light aircraft, this is your starting point — the empty aircraft, and where its CG sits. Next, the Dry Operating Mass, or DOM. All large aircraft use the DOM as the foundation instead. And here's the critical operational point: the Load and Trim Sheet cannot be completed until the DOM and its CG position are known. So on a large aircraft, you can't even start the paperwork until you have the DOM and its CG. Then we have the Operating Mass, or OM. This is also used when completing the Load and Trim Sheet. So you've got BEM for light aircraft, DOM for large aircraft, and OM as another figure that feeds into the sheet. Now, the Traffic Load. This was originally known as the 'payload'. It's the revenue-generating load — the thing that pays the salaries and, hopefully, produces a profit for the operator. This one matters a lot for the exam: the definitions and calculations of the traffic load constitute a sizeable part of it. There are six ways to define the traffic load, and you need to be familiar with all of them. We'll get to those six definitions as we go. Next, the Useful Load. This is simply the sum of the traffic load and the take-off fuel load. So useful load equals traffic load plus take-off fuel. Now, the Maximum Zero Fuel Mass, or MZFM. This one has a real physical reason behind it. The maximum stress in the wing roots occurs when the wing fuel tanks are empty. Think about that: when the tanks are full, the fuel weight actually helps relieve the bending stress on the wing. As the fuel is consumed, that relief disappears. To ensure the wings do not fold up permanently above the aircraft as the fuel is consumed, the manufacturer imposes a maximum zero fuel mass on the structure. That's the MZFM — a structural limit, not an operational choice. Finally, the Maximum Structural Taxi Mass, or MSTM. Every litre of fuel on board is essential for safe operations, but aircraft are allowed to carry additional fuel for engine starting and ground taxiing. This additional fuel is limited to a maximum value, and it's allowed to take the weight of the aircraft above the MSTOM — the Maximum Structural Take-Off Mass — during ground operations only. The key point: this extra fuel should be consumed by the time the aircraft is ready to commence the take-off run. So by the time you start rolling down the runway, you should be back at or below the take-off limit. The MSTM may also be referred to as the Ramp Mass or the Block Mass. And to round it out, you'll also need the definitions of MSTOM, PLTOM, MSLM, and PLLM — that's Maximum Structural Take-Off Mass, Performance-Limited Take-Off Mass, Maximum Structural Landing Mass, and Performance-Limited Landing Mass. Those are the remaining structural and performance limits you'll be working with. So the picture is this: light aircraft build from BEM, large aircraft build from DOM, the traffic load is your revenue, the useful load adds the fuel, the MZFM protects the wing structure, and the MSTM lets you carry a little extra fuel on the ground — but only until the take-off run begins.