Definitions and Calculations — Page 68, Lesson 91

Let's pick this up with the traffic load calculations, because that's where the numbers really start to bite. I want to walk you through the worked example for the MRJT1 aircraft, and then we'll go straight into the fuel load definitions, because you can't do these calculations properly without them. Here's the setup. We have an MRJT1, and we're given three key masses. The Dry Operating Mass, or 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 whole point of this exercise is to find the allowable traffic load — that's the mass of payload, the passengers and cargo, that the aircraft can actually carry. And the way we do it is by checking three separate limits, and the lowest one wins. First, the Structural Limited Traffic Load. This is the limit set by the aircraft's structure, and it's simply the maximum structural take-off mass minus the DOM. So we take 51,300 kg, subtract the 34,300 kg DOM, and we get 17,000 kg. That's the most payload the structure can physically hold. Second, the Take-off Limited Traffic Load. This one accounts for the fact that at take-off, the aircraft is carrying both the payload and the full take-off fuel. So we take the maximum take-off mass, 62,800 kg, subtract the DOM of 34,300 kg, and then subtract the take-off fuel of 12,000 kg. That gives us 16,500 kg. This is the payload limit imposed by the take-off condition. Third, the Landing Limited Traffic Load. At landing, the aircraft is lighter because it has burned off fuel. So we use the maximum landing mass, 54,900 kg, subtract the DOM of 34,300 kg, and subtract the fuel remaining at landing, which is 4,000 kg. That gives us 16,600 kg. Now here's the critical rule. The allowed traffic load is the lowest of these three values. So we have 17,000, 16,500, and 16,600. The lowest is 16,500 kg, which comes from the take-off limit. So the allowed traffic load is 16,500 kg. Now, what if the actual traffic load you're carrying is only 16,000 kg? Well, 16,500 minus 16,000 is 500 kg. That means you have a 500 kg underload. That's the difference between the allowable traffic load and what you're actually carrying. You need to know how to determine the allowable traffic load using these formulae, so make sure you can reproduce each of those three calculations and then pick the minimum. Now, before we go further, I want to make sure you understand the fuel load definitions, because these are the building blocks. Let's go through them one by one. First, Start and Taxi Fuel. This is the mass of fuel used in starting and operating the APU — that's the Auxiliary Power Unit, the small engine that provides power on the ground — and the main engines, and in taxiing to the runway threshold for take-off. The key 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. So the start and taxi fuel is what gets you from the gate to the point where you're ready to roll. 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. Second, 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 the start of the take-off roll to touchdown at your destination. Third, Contingency Fuel. This is fuel carried in addition to the trip fuel, for unforeseen eventualities. Things like avoiding bad weather or having an extended hold duration at the destination airport. In calculations, contingency fuel is 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 here's a crucial point — don't forget that the contingency fuel is part of the landing mass if it is not actually used during the trip. So if you don't burn it, it's still sitting in the tanks when you land, and it counts toward your landing mass. Fourth, Alternate Fuel. This is also called Diversion Fuel. It's 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 if you can't land at your destination, this is the fuel that gets you to your backup airport. Fifth, Final Reserve Fuel. This is the minimum fuel that should be in the tanks on landing. Essentially, it's a final reserve for unplanned eventualities. And here's the specific requirement — 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. So for our jet aircraft, that's 30 minutes of holding fuel as the absolute minimum on landing. Sixth, 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. That's the regulation covering instrument landings and power unit failures which are not required for calculations. So additional fuel is the top-up you add when the standard fuel categories don't quite cover what the regulations demand. So there you have it. The traffic load is the minimum of the three structural, take-off, and landing limits. And the fuel load is built from these six categories — start and taxi, trip, contingency, alternate, final reserve, and additional. Each one has a specific job, and together they define how much fuel you must carry for a safe and legal flight.