What Is Fuel Management?
Basically, fuel management is the combination of two related tasks, initial fuel planning and in-flight fuel monitoring. In the preflight planning stage, a navigation log is prepared along with a reasonable fuel plan. Then, during flight, the actual fuel burn is compared with the estimated fuel burn so that trends can be spotted early. If the trend shows that fuel quantity could become a concern, it can be addressed before it becomes a serious issue.
Training
During flight training, instructors tend to teach one of two methods to estimate total required fuel. In the first method, the student computes a separate groundspeed and estimated time en route (ETE) for each leg of the flight using an average true airspeed, heading, and winds aloft. Then, the average fuel burn per hour from the pilot's operating handbook (POH) is used with the ETE to determine the fuel burn for each leg. Finally, all of the leg fuel burns are added together to obtain total required fuel.
The second method builds upon the first but adds more a few more steps. With this method, detail-oriented instructors have their students compute separate climb, cruise, and descent fuel burns from POH data. These are summed and added to the engine start, taxi, run-up, and takeoff fuel to get the total required fuel.
Both of these methods work quite well to calculate required trip fuel. But when contemplating a flight near the maximum range of the aircraft, with less-than-ideal weather conditions and in busy airspace, we need both an accurate fuel estimate plus a management scheme to allocate fuel for contingencies.
The Fuel "Pie"
This is where the "fuel pie" concept can help. Imagine the total fuel onboard as a fuel "pie." We can cut that pie into different-sized slices, with each slice representing an allocation of fuel. Some slices are thick and contain lots of fuel, while other slices are thin and contain less fuel. For example, a student pilot may choose to partition his trainer's 24.5 gallons of usable fuel into three slices. The first slice is the fuel required to fly to the destination. Let's say it's a 2.5-hour flight, and the trainer burns six gallons per hour in cruise, so that slice equals 15 gallons. The second slice is for the day VFR (visual flight rules) reserve, and that slice represents 30 minutes at normal cruising speed, or three gallons. The amount left over is 24.5 minus 18, or 6.5 gallons, which is about one hour's flying time.
Similarly, airline pilots, corporate pilots, and other professional pilots employ navigation logs that typically partition their total fuel load into six or seven different allocations. The allocations are listed in a fuel table, each with a different name and purpose. Above is a sample navigation log fuel table for a corporate jet flight from Van Nuys, California (VNY) to Teterboro, New Jersey (TEB).
As you can see, the allocations are for fuel to the destination (DEST), IFR reserves (RESV), flying to the alternate (ALTN), holding (HOLD), subtotal of required fuel (REQD), extra (EXTRA), and taxi (TAXI). Note that all quantities are expressed in pounds, which is normal for jet fuel. Let's discuss the various allocations and their purposes.
Destination fuel is the fuel required (considering wind and forecast weather conditions) to fly from the departure airport to the destination airport. It is the sum of the fuel required for takeoff, climb, cruise, descent, approach, and landing. Destination fuel is generally the largest portion of the fuel pie, although on short flights it may not be. It generally doesn't include taxi fuel, which has its own allocation.
Reserve fuel (along with destination fuel) is the fuel required to satisfy the federal aviation regulations. The amount of reserve fuel required is based on the type of flight being conducted and the conditions. FAR 91.151, "Fuel requirements for flight in VFR conditions," states that:
(a) No person may begin a flight in an airplane under VFR conditions unless (considering wind and forecast weather conditions) there is enough fuel to fly to the first point of intended landing and, assuming normal cruising speed-
(1) During the day, to fly after that for at least 30 minutes; or
(2) At night, to fly after that for at least 45 minutes.
The reserve for flights under instrument flight rules (IFR) is 45 minutes as well.
The key point to remember about regulations is that they are absolute minimums, and just because something's legal doesn't mean it's safe. For example, if you were an instructor, would you allow your student to take off on a solo cross-country where he had planned to land at the destination with only 30 minutes of fuel remaining? Of course not. That's why many pilots double their personal fuel reserve numbers from 30 to 60 minutes for day VFR and from 45 to 90 minutes for night VFR. Note also that reserve fuel is required for dispatch purposes only. There is no requirement that it be on board at the destination or the alternate.
Alternate fuel is the fuel required to fly from the destination to an alternate airport. It includes the fuel required to go around or miss the approach, climb to an appropriate cruise altitude, cruise to the alternate, descend, and make an approach and landing.
You may have to divert to an alternate if your original destination becomes unsuitable or unusable for landing. For example, what if the forecast winds you were expecting to be straight down the runway turned out to be nasty crosswinds? What if a disabled aircraft were to block the only runway? Or, if you were flying IFR, what if you tried to land but missed the approach because the weather went below minimums? For these and other reasons, it's always a good idea to have enough fuel to fly to an alternate airport.
Holding fuel is the fuel set aside to hold the aircraft at normal holding speed. Although there is no specific regulation regarding holding fuel, most navigation logs allocate 30 minutes of fuel for holding. Holds frequently occur when flying under IFR, in busy airspace, and in lousy weather. Holding when flying under VFR is rare, but there are some situations in which you might have to hold for at least a few minutes. For example, you could be asked to hold while awaiting clearance to enter busy Class B or C airspace. Or a tower controller may ask you to hold outside Class D airspace if special VFR operations are being conducted, or if the tower is handling an aircraft in distress.
Required fuel is simply the total of destination, alternate, reserve, and holding fuel.
Extra fuel is fuel set aside over and above reserve requirements for any unforeseen contingencies. For example, let's say you're using more fuel than planned because of stronger than forecast headwinds. Or you find yourself circumnavigating towering cumulus buildups, military airspace, or thunderstorms along your route. In these situations, knowing that you have some extra fuel on board can be a very comforting thought.
Taxi fuel is simply the fuel required to taxi out to the active runway. In many training aircraft, this is usually combined with the engine start, run-up and takeoff fuel, and it seldom amounts to more than one gallon.
In-Flight Monitoring
The second task of fuel management is in-flight monitoring. By keeping a close eye on the estimated fuel burn versus the actual fuel burn, trends can be identified early.
For example, if you find that your estimated times en route for each leg are longer than expected, you should see an increasing difference between estimated fuel burn and actual fuel burn. You can use this information to extrapolate revised time and fuel estimates for the navigation log. If it appears that fuel remaining could become a concern, an unplanned fuel stop may be in order.
If you find that your navigation log fuel estimates are consistently off, perhaps the performance data you are using doesn't accurately reflect your particular aircraft. Remember that the handbook data are based on a brand-new aircraft, leaned per the engine manufacturer's recommendations, and flown by an experienced test pilot. There are a few good reasons why you may not be getting the same performance.
First, your trainer's engine may be a little tired, resulting in fuel burns that are somewhat higher than those in the POH. Second, you may not be leaning the mixture as aggressively as the test pilot did during performance testing. If you don't lean the same amount, you won't obtain the same performance and fuel burns. If you are unclear about how to properly lean the mixture, ask your instructor to explain and demonstrate it to you. Finally, your aircraft may not be configured the same as when it left the factory. For example, some trainers have their wheel pants removed to facilitate maintenance of the tires and brakes. If the performance data in the POH are for an aircraft with wheel pants and your aircraft doesn't have them, you'll probably notice reduced performance and increased fuel burns. Be sure to check the POH. Some POHs have two sets of performance charts, one for aircraft with wheel pants and one for aircraft without. Last, occasionally some owners change the propeller on their aircraft to enhance climb or cruise performance. These are called either climb props or cruise props. If this has been done, appropriate performance data should have been added to the POH.
One of the best ways to get accurate performance data for your aircraft is to measure it yourself. And it's very easy to do. It's just like checking how many miles to the gallon you get in your car. First, have the aircraft's fuel tanks topped off. Then go for a flight. (It's best if it's more than an hour in length.) Upon landing, top off the tanks again. Take the number of gallons the fuel truck pumped into your tanks (fuel used) and divide by the flight time to obtain your fuel burn per hour. You can do this over a few flights, at different altitudes, to obtain an average fuel burn per hour. Of course, if you don't want to top off the tanks, you can use a commercially available fuel dipstick designed for your specific aircraft to take the fuel measurements.
Besides a good plan and a cool head, fuel is probably a pilot's most vital resource. By having a good plan and checking it during flight, you can avoid learning the most embarrassing fuel management lesson there is: Those imprecise fuel gauges are notoriously accurate when they're reading "empty."
Christopher Parker is an active CFI as well as an aviation author, speaker, and FAA remedial training specialist. He pilots a California-based business jet.
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