Definitions and Calculations — Page 33, Lesson 48

Let’s start with the big picture. In this part of the course, we’re building the vocabulary and the arithmetic you’ll use on every Load and Trim Sheet. I’m going to walk you through the definitions one by one, because each one is a foundation stone for the calculations that follow. First, a quick note on the numbers you see at the top. The example shows a moment for the DOM of 31 150 574, but the Dry Operating Index, or DOI, is only 31. That’s the whole point of an index system. Instead of carrying around a seven-figure moment, we scale it down to a manageable number. 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 DOI is the scaled-down version of the Dry Operating Moment. Now, the first definition: Basic Empty Mass, or BEM. I’ll point you to CAP 696 for the formal definition, but here’s what matters for our work. All light aircraft use the BEM and its CG position as the foundation from which to calculate all relevant masses and CG positions. So if you’re working on a light aircraft, the BEM is your starting point. Everything else — fuel, passengers, cargo — gets added on top of that foundation. Next, Dry Operating Mass, or DOM. Again, CAP 696 has the formal definition, but the key point is this: all large aircraft use the DOM as the foundation from which to calculate all relevant masses and CG positions. And here’s a critical operational fact — the Load and Trim Sheet cannot be completed until the DOM and its CG position are known. So the DOM is not optional; it’s a prerequisite. You can’t even start the sheet without it. Then we have Operating Mass, or OM. CAP 696 defines it, and the OM is also used when completing the Load and Trim Sheet. So we have three masses so far — BEM for light aircraft, DOM for large aircraft, and OM which also comes into play on the sheet. Now, Traffic Load. This one used to be called the ‘payload’. It’s the revenue generating load — the thing that pays the salaries and hopefully produces a profit for the operator. And I want to flag this for you: the definitions of and calculations of the traffic load constitute a sizeable part of the exam. 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, but for now, hold onto the core idea — traffic load is the revenue-generating load. Next, Useful Load. This one is a simple sum. The useful load is the sum of the traffic load and the take-off fuel load. So it’s the revenue load plus the fuel you’re taking off with. Now we come to the Maximum Zero Fuel Mass, or MZFM. CAP 696 defines it, but let me explain why it exists, because the reason is structural. 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 that 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. So the MZFM is a structural limit — it caps the mass of the aircraft with no usable fuel in the tanks, to protect the wing roots from permanent deformation. Finally, 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 purposes. This additional fuel is limited to a maximum value, and it’s allowed to take the weight of the aircraft above the MSTOM — that’s the Maximum Structural Take-Off Mass — during ground operations only. The key phrase there is “ground operations only.” The additional 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, that extra taxi fuel is gone, and you’re back within the take-off limit. The MSTM may also be referred to as the Ramp Mass or the Block Mass. So if you hear “ramp mass” or “block mass,” that’s the same thing. And then the excerpt closes by pointing you to CAP 696 for the definitions of four more terms: MSTOM, PLTOM, MSLM, and PLLM. Those are the Maximum Structural Take-Off Mass, the Performance-Limited Take-Off Mass, the Maximum Structural Landing Mass, and the Performance-Limited Landing Mass. We’ll cover those in detail as we move forward. So let me pull it together. We have the BEM and DOM as the two foundations — light aircraft use BEM, large aircraft use DOM. The OM also feeds the Load and Trim Sheet. The traffic load is the revenue generator, and the useful load is traffic load plus take-off fuel. The MZFM protects the wing structure when tanks are empty, and the MSTM lets you carry extra taxi fuel on the ground, as long as it’s burned off before take-off. That’s the framework. Now you know what each mass is and why it exists.