Thinking Pilot‘s Guide To Fuel Management

Examine the fuel systems on most general aviation airplanes, and you might logically assume that fuel management shouldn't be much of a problem. Most of the time, "management" is simply a matter of filling one or two tanks, selecting the one you wish to use first and launching into thin air.

Unfortunately, it turns out there's considerably more to it than that. First, do you know how much fuel each tank will hold? Well, of course, it's usually written right there on the selector: Left Tank -- 32 gallons, Right Tank -- 32 gallons. What could be simpler than that?

Think again. My personal airplane is a Mooney with wet wings, so I'll use that as the example. When the airplane begins to age and has endured flexing of the wings from thousands of landings, the tanks may loosen up, shift slightly and begin to leak. When they do, you'll have two choices: Reseal the wings---a messy, extremely time-consuming and expensive job; or install rubber bladders---usually cheaper, less time consuming and a more or less permanent fix.

Problem is, rubber bladders must fit the available space between the ribs and spars, and they may not always fit flush in the enclosure. The result is that bladders usually reduce fuel capacity, partially to accommodate the neoprene itself. In the Mooney's case, you'll lose about two gallons per side. Now, you're down to 30 gallons per tank, and that should be duly noted in the aircraft logbook. You can only hope someone has put new decals of tank capacity near the filler necks, as well.

Wet wings can pose another problem. If the airplane has been damaged in an accident and repaired, fuel capacity once again can be compromised if the tank itself has been slightly dented, but still deemed serviceable.

In addition to these inconsistencies, few pilots have ever checked to see exactly how much the tanks on their airplane hold. A few years ago, I had a mis-fueling problem, and a local shop was forced to drain both tanks completely dry. Before we refilled the tanks, we checked the ramp in front of the shop to find the most level area, then, parked the airplane on that spot and measured the height of each wingtip above the asphalt to make certain the load on each gear was symmetrical and the tanks were exactly level. Mooneys use the infamous rubber doughnut gear system that's rarely asymmetrical, but a standard oleo airplane may suffer from unevenly inflated gear struts and throw off your calculations. (Ask any Aerostar owner.)

Then, we called the truck and had the fueler fill the tanks 10 gallons at a time, alternating left and right. The good news was that the airplane actually held 66.4 gallons, partial compensation for all the monkey motion and expense of determining the airplane's "real" fuel capacity.

The shape and location of fuel tanks on different models may make our search for actual capacity seem a little fanatical, but I now know exactly how much fuel is onboard my airplane when the tanks are once and truly topped.

Incidentally, don't assume your tanks are full just because the fuel level is right up to the lip when you pop the cap, especially on an aircraft with high dihedral. I once had a new Piper Malibu headed for Germany that didn't require as much fuel in Iceland as I thought it should after the hop across from Greenland. I removed the left cap, shook the wingtip fairly vigorously, heard the tank make an incriminating "burp" and immediately saw the fuel level drop well below the filler neck. There obviously had been a major air bubble that needed to be free. I called the truck back, and the lineman added an additional five gallons.

Similarly, don't fall into the habit of taking the fueler's word that the tanks are full, just because you fly a high-wing airplane and it's raining. Fuelers are good people, but their mistake can become your problem. High wings deserve love, too, and so do you.

Okay, so now you know exactly how much fuel your airplane can hold under optimum conditions. How much should you load aboard? Many pilots subscribe to the philosophy that the only time you can have too much fuel is if you're on fire, and that means topping the tanks every time. Nothing wrong with that philosophy, as long as you accept the reality that you're burning fuel to carry fuel. Every extra pound you have aboard, whether it's people, baggage or additional fuel, costs money to tanker from one place to another.

Checking fuel for water should be an automatic reflex for every pilot, but don't assume everything is fine if the sample comes back clear. Give it a sniff to make certain you're not looking at a full sample cup of water.

In my home state of Alaska, pilots not only carry full fuel most of the time, they'll often secrete additional barrels of 100LL or Jet A in the bush to help them fly the "back" portion of an out-and-back trip.

If you're more inclined to operate the way airlines and corporate jets fly, however, you may want to simply calculate the amount you'll need for a given trip, then add an appropriate fudge factor and reserve for contingencies. In that instance, you'll need to know exactly what you're starting with. If you don't have a fuel computer/totalizer, you can buy one of those semi-clear universal fuel gauges that allows you to stick the tanks and come up with a very accurate estimate of fuel onboard. Sporty's sells a perfectly serviceable one for $12.95. Don't even think about using a fuel gauge to approximate FOB (fuel on board).

The next question may be determining range and the amount of fuel necessary to make the trip safely with a reasonable reserve. If I'm not familiar with the specific airplane I'll be flying, but I know it's full, I'll automatically subtract 5% from max fuel capacity. That means a 100-gallon airplane is assumed to have 95 gallons onboard. On most trips, I'll also subtract my fuel reserve right up front, an hour and a half at normal cruise if I'm flying IFR, one hour if I'm VFR. When I'm traveling at max cruise on a moderately long trip, I'll add 50% to fuel burn for the first hour to cover taxi, takeoff and climb to cruise altitude. Whatever is remaining will be my pure cruise requirement, the amount of fuel I'll expect to burn during straight and level flight.

On a VFR trip in a Saratoga HP, for example, I'd round the airplane's 102 gallons down to 96 as a starting point (roughly 5%), subtract another 25 gallons for the first hour, reduce fuel by another 17 gallons for VFR reserve at the end of the flight and wind up with 54 gallons for the pure cruise portion of the trip. In the case above, I'll have about 4 + 10 before I'd need to be on the ground without cutting into my reserve. (The numbers above are for max cruise. You obviously could do much better using long-range power.)

Once you've determined how much fuel you'll need, loaded it aboard and made certain the caps are tight, the next step is to guarantee you have access to all the fuel you have. That means running the engine on all tanks for a few minutes on the ground to make certain they're all feeding properly. If you're flying the aforementioned Saratoga HP, you'll need to check all four tanks for a max range trip. While you're at it, make certain the fuel selector is tight and fits snugly into all positions. A fuel selector that comes off in your hand isn't very reassuring.

For that very reason, I have a firm rule against running any tank dry in flight. I'll usually run the tank down to what I calculate is three or four gallons, then switch tanks while fuel pressure is still good. I also make it a habit to carry a pair of vice grips in one of the front seatbacks. If a fuel selector does break off (yes, it has happened), I need to have a way to get at the remaining fuel.

Also, make sure you're totally checked out on the system. Some airplanes have only two main tanks, almost stupid simple. Others offer one or two auxiliary tanks that feed transfer fuel into the mains once they've burned down sufficiently. Keep in mind, if you get nervous and turn on the transfer pumps too soon, you may start to pump fuel overboard when the mains become full.

Don't automatically take the book fuel burns as gospel, by the way. Contrary to what some pilots believe, the manufacturers don't make up those numbers for fuel burn, climb, cruise, range and endurance. They do, however, optimize burn rates, EGT and miscellaneous performance numbers using highly experienced test pilots, often with thousands of hours in type, flying behind flight test instrumentation far more precise and exotic than your engine analyzer.

Most manufacturers take whatever measures are necessary to optimize performance and the most out of every drop of petrol. Practically everyone lists cruise numbers at mid-cruise weights (otherwise, they'd need to depart over gross).

The CG may be deliberately located as far aft as possible to minimize download on the tail and maximize speed---just make certain it's still within limits for the landing. All vents probably will be closed, the engine and prop will be new and perfectly tuned/balanced, and the test airplane may even have antennas removed or flushed to the fuselage to reduce parasite drag. On composite designs, you'll sometimes find antennas buried inside the airframe or wing, since composite structures usually don't attenuate VHF/UHF radio signals. If you have a fuselage aux tank back in the baggage compartment, you can sometimes take a tip from the manufacturers by burning that fuel later in the flight rather than up front. Again, this will keep the CG slightly farther aft and improve cruise speed.

(A few years back, I was doing a pilot report at the Sun 'n Fun Airshow in Florida on an especially fast single-engine retractable and recorded an excellent cruise speed at high altitude, not quite up to the advertised spec, but very close. The company pilot in the right seat seemed disappointed, though I was impressed. Without making a special effort, it's almost impossible to optimize cruise conditions on a one-shot speed test. Two days after returning home to California, the manufacturer called and asked if I'd like to take one more quick flight in a different airplane that they guaranteed would meet the spec. They flew the airplane out to Long Beach, I jumped in, rocketed up to FL250, and that particular demonstrator came within one knot of the advertised cruise. Of course, I did notice the boarding step was missing.)

When you're doing fuel planning, I've also found it pays to be a pessimist with regards to winds aloft. I've been bedeviled by winds more times than I can remember, so much so that I tend to disbelieve or at least undervalue optimistic forecasts for tailwinds. A forecast 10-knot or less tailwind becomes a 5-knot push in my calculations. An alleged 20-knot tailwind deserves only about 10 knots of respect in the flight plan. Similarly, I always assume any headwinds will be slightly worse than forecast.

The most likely consequence of such assumptions is that I arrive at my destination slightly sooner and with extra fuel onboard, not such a bad thing.

There's really nothing magical about fuel management, as long as you know what you're starting with and have a realistic idea of how much you'll need for the trip. While it's true, there's no excuse for running out of fuel in any airplane, proper flight planning and an accurate assessment of how much fuel you have onboard should insulate any pilot from the dangers of fuel exhaustion.