Definitions and Calculations — Page 68, Lesson 91

Let's pick this up with the worked example on the MRJT1, because it ties the whole idea of traffic load together. We've got a specific aircraft, and we're given three numbers to start with. The Dry Operating Mass, the DOM, is 34,300 kg. The take-off fuel is 12,000 kg. And the fuel remaining at landing is 4,000 kg. Now, the point of this exercise is to find the allowable traffic load, and we do it three different ways, each limited by a different structural or regulatory constraint. The first is the Structural Limited Traffic Load. That's the maximum payload the structure of the aircraft can physically carry. We get it by taking the maximum structural take-off mass, which here is 51,300 kg, and subtracting the DOM. So 51,300 minus 34,300 gives us 17,000 kg. That's the structural limit. The second is the Take-off Limited Traffic Load. This one is limited by the regulated take-off mass for the conditions. Here that maximum is 62,800 kg. From that we subtract the DOM, 34,300, and also the take-off fuel, 12,000. So 62,800 minus 34,300 minus 12,000 equals 16,500 kg. The third is the Landing Limited Traffic Load. The maximum landing mass here is 54,900 kg. We subtract the DOM, 34,300, and the fuel remaining at landing, which is 4,000 kg. That gives us 54,900 minus 34,300 minus 4,000, which is 16,600 kg. Now here's the key point. The allowed traffic load is the lowest of these three limits. The structure can take 17,000, the take-off conditions allow 16,500, and landing allows 16,600. So the allowed traffic load is 16,500 kg, the take-off limited value, because it's the most restrictive. And if the actual traffic load you plan to carry is only 16,000 kg, then you have a 500 kg underload. That's the difference between what you're allowed and what you're actually carrying. You need to be able to work all three of these formulae out yourself. Now, before you can do traffic load calculations properly, you need a solid grasp of the fuel load definitions. Let me walk you through each one, because they each have a precise meaning. Start and Taxi Fuel. This is the mass of fuel used in starting and operating the APU and the main engines, and in taxiing to the runway threshold for take-off. The APU is the Auxiliary Power Unit, the small engine that provides power on the ground. The assumption here is that at the point of releasing the brakes for take-off, the aircraft is at or below the regulated take-off mass for the conditions prevailing. And in operations where fuel is critical, the start and taxi fuel must not be less than the amount expected to be consumed during those procedures. Trip Fuel. This is the mass of fuel required to complete the take-off run, the climb, the cruise, the descent, the expected arrival procedures, and the approach and landing at the designated airport. So it's the fuel for the entire planned journey from brake release to landing at your destination. Contingency Fuel. This is fuel carried in addition to the trip fuel, for unforeseen eventualities like avoiding bad weather or having an extended hold duration at the destination. In calculations, it's usually given as a percentage of the trip fuel. For example, if the trip fuel is 1,000 kg, then contingency fuel at 5% of the trip fuel would be 50 kg. And don't forget, the contingency fuel is part of the landing mass if it's not actually used during the trip. Alternate Fuel, also called Diversion Fuel. This is the mass of fuel required to carry out a missed approach at the destination airfield, and then the subsequent climb out, transit to, expected arrival procedures, approach, descent, and landing at an alternate airfield. So it's the fuel to get you from a go-around at your destination all the way to landing at your alternate. Final Reserve Fuel. This is the minimum fuel that should be in the tanks on landing. It's essentially a final reserve for unplanned eventualities. It should allow a piston engine aircraft to fly for a further 45 minutes, or a jet engine aeroplane to fly for a further 30 minutes, at a given height and holding speed. And finally, Additional Fuel. This is only required if the sum of the trip, contingency, alternate, and final reserve fuels are insufficient to cover the requirements of AMC OPS 1.255, which deals with instrument landings and power unit failures that are not required for calculations. So the whole picture is this: you have your structural, take-off, and landing limits on traffic load, and you take the lowest. And the fuel you carry is broken down into these defined categories, each serving a specific regulatory purpose. That's the foundation you need before you start crunching traffic load numbers.