A fuel management system can help you make the fuel calculations needed for in-flight decisions about potential routing, fuel stops, and diversions. A fuel management system offers the advantage of precise fuel calculations based on time, distance, winds, and fuel flow measured by other aircraft systems. When a route has been programmed into the FMS, the fuel management function is capable of displaying currently available fuel and aircraft endurance and providing an estimate of fuel remaining as the aircraft crosses each waypoint in the programmed route. A fuel management function is useful not only for making primary fuel calculations, but also for backing up calculations performed by the pilot. If there are leaks, plumbing malfunctions, or inadequate leaning, the fuel display can be deceptive. You must always land at the earliest gauge indication of low fuel in the tanks, time of normal landing, or any sign of fuel value disagreement with the flight planning. Errors can be determined when the aircraft is safely on the ground.
Initial Fuel Estimate
Many fuel management functions lack a fuel quantity sensor. Without access to this raw data of fuel quantity, fuel management functions perform calculations using an initial fuel estimate that was provided by the pilot prior to departure. Figure 5-20 illustrates how an initial fuel estimate is given for one manufacturer’s fuel management unit.
It is important to make accurate estimates of initial fuel because the fuel management function uses this estimate in making predictions about fuel levels at future times during the flight. For example, if you overestimate the initial fuel by eight gallons and plan to land with seven gallons of reserve fuel, you could observe normal fuel indications from the fuel management system, yet experience fuel exhaustion before the end of the flight. The accuracy of the fuel calculations made by the fuel management function is only as good as the accuracy of the initial fuel estimate.
You must know the capacity of the aircraft fuel tanks and amount of fuel required to fill the tanks to any measured intermediate capacity (e.g., “tabs”). When full fuel capacity is entered into the fuel management system, the tanks must be filled to the filler caps. For some aircraft, even a fraction of an inch of space between the filler cap and the fuel can mean that the tanks have been filled only to several gallons under maximum capacity. Objects can plug lines, preventing the fuel from flowing to the pickup point. Some aircraft have bladders and dividers in the fuel system. A bladder can move within the tank area and not actually hold the quantity of fuel specified. Always check to ensure that the fuel servicing total matches the quantity needed to fill the tank(s) to the specified level.
Estimating Amount of Fuel on Board
Since the fuel management function’s predictions are often based on the initial quantity entered, it is important to monitor the fuel gauges to ensure agreement with the fuel management function of the FMS as the flight progresses. It is always prudent to use the most conservative of these measures when estimating fuel on board.
Predicting Fuel at a Later Point in the Flight
A primary function of the fuel management function or system is to allow you to predict fuel remaining at a future time in the flight. The fuel management system uses a combination of the currently available fuel and the current rate of fuel consumption to arrive at the measures. Some units require the current or estimated fuel burn rate to be entered. Some units have optional sensors for fuel flow and/ or quantity. Be absolutely sure of what equipment is installed in your specific aircraft and how to use it. Since the rate of fuel consumption instantly changes when power or mixture is adjusted, (usually with altitude) the fuel management function or system should continually update its predictions. It is common for the fuel management system to calculate fuel remaining at the arrival of the active waypoint, and the last waypoint in the route programmed into the FMS/RNAV. These measures are shown on the MFD in Figure 5-21.
When no route is programmed into the FMS/RNAV, the fuel management function may not display information due to lack of data.
Most fuel management function or systems display the amount of fuel remaining, as well as the endurance of the aircraft given the current fuel flow. Most systems display the aircraft endurance in hours and minutes, as shown in Figure 5-21.
Some units show a fuel range ring on the MFD that indicates the distance the aircraft can fly given current fuel and fuel flow. This feature, illustrated in Figure 5-22, is useful for making fuel stop or alternate airport planning decisions. It may or may not include allowances for winds. Many units allow you to specify personal minimum fuel reserves. In this case, the fuel range ring indicates the point at which the aircraft will reach reserve fuel minimums.
Risk: Stretching Fuel Reserves
The availability of predictive information about fuel burn and fuel availability introduces the possibility of flying closer and closer to fuel minimums or stretching fuel holdings farther than would be appropriate with “back of the envelope” calculations in a traditional aircraft. You must be aware of this tendency and discipline yourself to using fuel management systems to increase safety rather than stretch the limits. Refueling the aircraft offers a good opportunity to compare the amount of fuel burned with that predicted by the fuel management system and your own calculations. It is always a good exercise to determine why the fuel management function or system’s numbers differ from what is actually pumped into the tank(s). Was it improper leaning? More/ less winds? Are the EGT/CHT gauges indicating properly?