Definitions and Calculations — Page 38, Lesson 55

I want to walk you through the fuel requirements that govern how we load an aeroplane for a flight. This is a core part of mass and balance, and it's built on a chain of definitions that all stack together. Let's take them in order. First, a quick note on where this fits. The MEL — that's the Minimum Equipment List — lists the basic equipment requirements for each aircraft, and also the optional specialist equipment that can be fitted for a particular role. It's very useful when determining the BEM and the DOM of an aeroplane. BEM is the Basic Empty Mass, and DOM is the Dry Operating Mass. The MEL helps you figure out what's actually on board when you start your mass calculations. Now, the heart of this passage: fuel. It is the commander of the aeroplane's responsibility to ensure there is sufficient fuel on board to safely complete the intended flight, and to land with not less than a specified level of fuel remaining in the tanks — irrespective of delays and diversions. So the captain owns this, not the dispatcher, not the fueller. The commander signs off that the fuel is adequate. The safe operating fuel requirements are satisfied by filling the tanks in a specific order, shown in Figure 2.6. Let me walk you through each component of that fuel load, because each one has a precise definition. First, the fuel tank contents. Two percent of the fuel tank is required for venting. That's a fixed allowance — the tank can't be filled to the absolute brim because fuel needs space to expand and vent. So 2% of the tank volume is reserved for that. Then we have Start and Taxi Fuel. This is a specified amount of fuel which is additional to the Regulated Take-off Mass, to allow the aircraft to start up and transit to the runway without consuming any of the take-off fuel. So this is the fuel burned while the engines are starting and while you taxi from the stand to the runway threshold. It's extra, on top of what you need for the actual take-off. Next is Trip Fuel. This is the amount of fuel required to complete the planned flight from Airfield A to Airfield B. That's the basic fuel for the sector itself — from departure to destination. Then Contingency Fuel. This is the amount required to enable the aircraft to circumnavigate bad weather between Airfield A and Airfield B, and/or to remain in the hold at Airfield B until a landing slot is available. Usually it's 3% to 5% of the trip fuel. So if your trip fuel is, say, 10,000 kg, contingency is typically 300 to 500 kg — that's your buffer for weather deviations or holding. After that, Alternate Fuel. This is the calculated amount of fuel required to divert from Airfield A — or Airfield B — to an alternate airfield C, due to an emergency. So if you can't land at your destination, this is what gets you to the alternate. Then Final Reserve. This is a reserve of fuel over and above the fuel requirements defined so far, to cater for any other unpredicted emergency. It's your last safety net, beyond everything already planned. Finally, Captain's Discretion. This is fuel taken up for economic or other operational reasons. The captain can choose to carry extra fuel — maybe to avoid a fuel stop, or for a commercial reason. Now, the sum of all of that — all the fuel in the tanks — is called the Ramp or Block Fuel, as it's sometimes called. That's the total fuel loaded before you start taxiing. It's the sum of every component we just listed: start and taxi, trip, contingency, alternate, final reserve, and captain's discretion. Taking off at airfield A and landing at airfield B is classed as a trip or a sector. That's the terminology — one departure and one arrival is one sector. Now, once the commander has determined the mass of the trip fuel, they may need to convert that mass value into a quantity value, for the benefit of the refuel operator. Fuel is sometimes dispensed in gallons or litres — that's a volume, not a mass. So to convert quantity — gallons or litres — into mass — pounds or kilograms — and vice versa, you need to know the density or the specific gravity — the SG — of the fuel. Normally, the delivery note gives you that density or SG value. So the key relationship here: mass equals volume times density. If you know the SG of the fuel and the volume in litres or gallons, you can compute the mass in kilograms or pounds. And the delivery note is where you get that density figure from. Let me show you the flow diagram that ties all this together — it's Figure 2.4, which lays out the definitions and the flow of how these masses relate. And Figure 2.5 shows a weighing schedule, simplified for training purposes — that's how you'd actually record these masses in practice. So the whole picture is: you start with the trip fuel for the sector, add contingency, alternate, final reserve, and captain's discretion, plus start and taxi fuel, and the 2% venting allowance — and the total is your block fuel. That's what goes in the tanks, and that's what the commander is responsible for ensuring is sufficient.