Fuel Policy and Fuel Monitoring — Page 38, Lesson 47

I want to walk you through fuel monitoring — how we keep track of fuel consumption during a flight, especially on longer trips. Even if we start with adequate reserves, if the actual fuel consumption rate turns out higher than what we forecast, we could end up below the minimum requirement later in the flight. So we need early warning of both the fuel flow rate and the total fuel quantity remaining. On sophisticated modern aircraft, this monitoring is done by the Flight Management System. The fuel contents and the fuel flow-meter readings are fed directly into the Flight Management Computer — that's the FMC. The FMC also knows the route distance still to go, the current ground speed, and the anticipated descent profile. From all that data, it can calculate the expected fuel on arrival at the destination. The pilots can check this figure at any time. The FMC also compares that expected arrival fuel against the sum of the alternate fuel plus the final reserve fuel. If the expected arrival fuel drops below that sum, a warning is displayed to the pilots on the Control and Display Unit — the CDU. For aircraft that don't have an FMS, the usual method is the 'Howgozit' fuel graph. Let me explain how that works. A graph is drawn with 'Fuel Remaining' on the vertical axis — the y-axis — and 'Distance to Go' on the horizontal axis — the x-axis. You can see an example at Figure 2.2. Now, a note: questions on the 'Howgozit' graph are not set in the EASA exam. I'm covering it simply to help your understanding of fuel monitoring. Let's walk through the example. We're assuming a flight of 1000 nautical ground miles. We have to land with 1000 kg — that's our final reserve fuel. The fuel required to fly to the alternate airport is 700 kg. So our minimum fuel on arrival at the destination is the sum of those two: 1000 kg plus 700 kg, which gives us 1700 kg. Just out of interest, notice that the slope of the graph changes shortly after the start. That's because aircraft usually climb at a slower speed, which affects the fuel consumption rate during that phase.