Definitions and Calculations — Page 38, Lesson 55

Let’s pick this up right where the fuel picture starts to take shape, because this is where the definitions really start to matter for your job as the commander. First, a quick link to what came just before: the MEL, the Minimum Equipment List. That document 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 you’re determining the BEM and the DOM of an aeroplane — the Basic Empty Mass and the Dry Operating Mass. We’ll get to those in detail shortly, but for now, just hold that connection: the MEL helps you figure out what’s actually on the aircraft when you start weighing it. Now, the core of this excerpt: fuel mass. And the first thing I want you to understand is who carries the responsibility. 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 this isn’t a suggestion; it’s a legal and operational duty placed squarely on you. How do you satisfy that requirement? By filling the tanks in a very specific, regulated order. Let me walk you through each component of the fuel tank contents, because each one has a precise definition. First, 2% 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 the tank needs to vent. So two percent of the tank capacity 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 think of it this way: the fuel you burn just getting the engines started and taxiing out is accounted for separately, so it doesn’t eat into the fuel you’ve calculated for the actual flight. Next is Trip Fuel. This is the amount of fuel required to complete the planned flight from Airfield A to Airfield B. That’s your primary flight leg — the fuel to get you 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 kilograms, your contingency would typically be between 300 and 500 kilograms — a buffer for weather deviations or holding time. After that comes 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 the fuel that gets you to your backup field. 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 line of defence — fuel you don’t plan to use, but must have on board. And finally, Captain’s Discretion. This is fuel taken up for economic or other operational reasons. So if you, as captain, decide you want extra fuel for a commercial reason — maybe to avoid a fuel stop, or for some operational flexibility — that’s your discretion fuel. Now, here’s the key summing point. All of these components together — the venting allowance, start and taxi, trip, contingency, alternate, final reserve, and captain’s discretion — add up to what’s called the Ramp Fuel, or Block Fuel, as it’s sometimes called. That’s the sum of all the fuel in the tanks. So when you see “ramp fuel” or “block fuel,” that’s the total fuel loaded before you start taxiing. Now, 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. Once you’ve determined the mass of the trip fuel, you may need to convert that mass value into a quantity value, for the benefit of the refuel operator. Because 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, of the fuel. The specific gravity, often abbreviated SG, is the ratio of the density of the fuel to the density of water. Normally, the delivery note gives you this information — it tells you the density or SG of the fuel being pumped into your tanks. So let me tie that together. You’ve got a chain of fuel components, each with a precise regulatory definition, summing to the block fuel. And then you’ve got a conversion problem: mass to volume, using density or specific gravity, with the delivery note as your source of that value. I want to make sure you’ve got the full picture of that fuel tank contents diagram, because it’s the backbone of everything we’ll do in mass and balance. Let me show you the flow. That figure lays out the definitions and the flow diagram — you can see how each fuel component stacks up to give you the ramp or block fuel. Now, one thing I want to flag before we move on: the excerpt ends mid-sentence about the delivery note. So we’ll pick up there next — how you actually use that density or SG value in the conversion. But for now, make sure you can recite each fuel component in order, and understand that the commander owns the responsibility for ensuring that final reserve is never compromised.