ATPL · Navigation · Flight Performance Planning 2 Flight… lesson — Page 38, Lesson 48

I want to walk you through the fuel graph and how we use it during flight monitoring. Let's start with the shape of the graph itself. You'll notice that the slope of the fuel consumption line changes shortly after the start. This isn't a mistake — it's because the aircraft usually climbs at a slower speed than cruise, but the engines are at or near max continuous power during the climb, whereas in the cruise they're at cruise power. So the fuel burn rate is higher in the climb, giving a steeper slope initially, then it shallows out once we level off. Now, let's put numbers to this. We are expecting to use 5000 kg of fuel for the entire en-route portion of the flight. That's our trip fuel — the fuel we plan to burn from take-off to landing, excluding any reserves. Our contingency fuel will be 5% of the remaining trip fuel. At the start of the trip, that's 5% of 5000 kg, which is 250 kg. As we fly and burn fuel, the remaining trip fuel reduces, so the contingency fuel amount reduces proportionally, reaching zero at the end of the trip. So our minimum take-off fuel — the total fuel we must have on board before we start engines — is the trip fuel plus the contingency fuel, which comes to 6950 kg. Here's an important operational point: although we must have that contingency fuel on board, very often we do not actually use it. The trip fuel is supposed to be based on a realistic figure — it's what we genuinely expect to burn. The contingency fuel is only there to cover unforeseen fuel consumption deviations, incorrect meteorological forecasts, and unexpected ATC re-routing. On the majority of trips, these things should not occur. So in those cases, the fuel will track down what we call the 'probable fuel consumption' line, and we will arrive with the contingency fuel still unused. During the flight, we take fuel checks every half hour — or at another interval as specified in the company's Flight Operations Manual. From these checks, we build up a history of the fuel consumption and establish a trend. By extrapolating the slope of that trend line forward, we can indicate to us the expected arrival fuel if that trend continues. Let me show you what that looks on the graph. In Figure 2.3, the trend line shows we are going to arrive with sufficient fuel. In Figure 2.4, the trend line shows we are not — we're going to run short. In that case, appropriate action would have to be considered, such as returning to the departure airfield or diverting to a suitable en-route airfield to up-lift fuel. So the key takeaway here is that we use these regular fuel checks and the trend line they create to monitor whether our actual fuel consumption matches the plan, and to give us early warning if we need to take action.