Definitions and Calculations — Page 68, Lesson 90

Let’s start with what the captain is actually trying to find out. Before departure, he needs to know whether he can accept any last‑minute additions to the load — a VIP, an extra passenger, emergency evacuation cases. That spare capacity is called the underload. And here’s the trap I want you to notice right away: the underload is not simply the difference between the regulated take‑off mass and the actual take‑off mass. You have to bring in the MZFM — the Maximum Zero Fuel Mass — and the Traffic Load as well. Let me define the terms we’re juggling, because they’re the building blocks of the three formulae. First, the MZFM, Maximum Zero Fuel Mass. That’s the maximum mass of the aeroplane with no usable fuel on board — the structure, the crew, the payload, everything except fuel. If the aircraft is already sitting at its MZFM, it has no underload at all. Even if it’s below the MZFM, any apparent underload may already have been consumed by extra fuel uptake — so you can’t just look at the mass and assume there’s room. Next, the Traffic Load. That’s the mass of the passengers, baggage, and freight — the revenue‑producing payload. When you allow a last‑minute addition, you are increasing the traffic load. Now, the key idea: the traffic load that can actually be carried is the lowest of three separate limits. You must know these three formulae — they’re exam‑standard. Structural Limited Traffic Load = MZFM − DOM. Here DOM is the Dry Operating Mass — the mass of the aeroplane ready for service, including crew, catering, and everything that isn’t traffic load or fuel. So this formula says: the most traffic load the structure will allow is the difference between the zero‑fuel limit and the dry operating mass. Take‑off Limited Traffic Load = RTOM − DOM − Take‑off fuel. RTOM is the Regulated Take‑off Mass — the maximum take‑off mass permitted by the regulations. So you take that regulated take‑off limit, subtract the dry operating mass, and subtract the take‑off fuel. What’s left is the traffic load you can carry at take‑off. Landing Limited Traffic Load = RLM − DOM − Fuel remaining. RLM is the Regulated Landing Mass — the maximum landing mass permitted. Same logic: regulated landing limit, minus dry operating mass, minus the fuel that will remain at landing. So the allowed traffic load is the lowest of those three values. Why lowest? Because the aeroplane must satisfy all three constraints simultaneously — it must not exceed the structural zero‑fuel limit, it must not exceed the take‑off limit, and it must not exceed the landing limit. The most restrictive one governs. And that’s how the captain finds his underload: he compares the allowed traffic load against what he already has loaded, and the difference is his spare capacity for last‑minute changes. One more thing to keep straight: the excerpt lists the items that feed into these calculations — the mass of consumables other than fuel, the traffic load items, the take‑off mass, landing mass, zero fuel mass, the load distribution, the aeroplane CG positions, and the limiting mass and CG values. Those are the inputs the captain works with. So, in short: underload is spare traffic‑load capacity, found by taking the lowest of the structural, take‑off, and landing limited traffic loads, and comparing it to what’s already on board.