Definitions and Calculations — Page 68, Lesson 90

Let's pick this up right where the definitions leave off. We're now into the actual calculations for the MRJT1 — that's the McDonnell Douglas MD-80 series, the aircraft type this whole chapter is built around. The captain's job here is simple to state but tricky to do: he needs to know if he can accept any last-minute load changes — a VIP turning up, an emergency evacuation, that sort of thing. Before he can allow any of that, he has to know if he has any spare load capacity. That spare capacity is called the underload. Now here's the trap I want you to avoid. The underload is not just the difference between the regulated take-off mass and the actual take-off mass. That would be too easy. You have to bring in two more concepts: the MZFM — the Maximum Zero Fuel Mass — and the Traffic Load. Let me define those properly. The MZFM is the maximum mass the aeroplane is allowed to be at with no usable fuel on board — it's the structural limit on the airframe before fuel weight is added. The Traffic Load is the total mass of the payload: passengers, baggage, and freight — everything you're carrying for revenue. So why can't you just subtract? Because of two constraints. First, if the aircraft is already sitting at its MZFM, it has no underload at all — the structure is already at its limit. Second, even if you're below the MZFM, that apparent spare capacity may have already been eaten up by extra fuel you've taken on board. Fuel is heavy, and it counts against the same structural limits. So the rule is this: any last-minute load addition increases the traffic load. And the traffic load you can actually carry is the lowest of three separate limits. You need to know these three formulae cold. First, the Structural Limited Traffic Load. That equals MZFM minus DOM. DOM is the Dry Operating Mass — the mass of the aircraft ready for service but without traffic load and without fuel. So this formula tells you how much payload the structure itself can physically carry. Second, the Take-off Limited Traffic Load. That equals RTOM minus DOM minus Take-off fuel. RTOM is the Regulated Take-Off Mass — the maximum mass you're legally allowed to take off at. You subtract the dry operating mass, then subtract the fuel you'll burn on take-off. This tells you how much payload you can carry and still be within your take-off limit. Third, the Landing Limited Traffic Load. That equals RLM minus DOM minus Fuel remaining. RLM is the Regulated Landing Mass — the maximum mass you're allowed to land at. And here you subtract the fuel you'll have left at landing, not the take-off fuel. Now, why three different limits? Because each phase of flight has its own structural and regulatory constraint. The structure that holds the aircraft on the ground at take-off is stressed differently from the structure that absorbs the impact at landing. And the fuel you burn off between take-off and landing changes the mass. So the limiting factor could be any one of the three — and the allowed traffic load is the lowest of them all. That's the key takeaway: you calculate all three, and the smallest number wins. That's your underload — your spare capacity. And that's what the captain uses to decide whether he can accept that VIP or that emergency evacuation. Now, one thing I want to make sure you've got: the difference between the take-off fuel and the fuel remaining. Take-off fuel is what's in the tanks when you start the take-off roll. Fuel remaining is what's left when you land — which is take-off fuel minus everything you burned in flight. So the landing limit is always going to be more generous on fuel, but the landing mass limit itself is usually lower. That's why you check all three. So to summarise the logic: underload isn't a simple subtraction. It's the lowest of three structural and regulatory limits, each calculated against the dry operating mass, with the appropriate fuel figure subtracted. Get those three formulae memorised, because everything else in this chapter builds on them.