September 2004Features

No fuelin' around

Learning the operation of this critical system is more than a dalliance in plumber's science


Fuel system Some say engine oil is the lifeblood of a small airplane, while others argue that without electricity nothing gets done. But there's an equally strong argument for fuel as the most critical element in taking a machine into the sky. Without it, you're literally going nowhere.

Get into any detailed discussion of aircraft fuel systems and you might find yourself transported to some off-axis plumber's school. There are tanks and valves and lines and what not. Like airplanes themselves, fuel systems are all different, ranging from the simplest single-tank arrangements to multitank, some-here/some-there setups that consume a significant portion of the pilot-brain bandwidth just to keep straight.

Have you ever wondered why the manufacturers would put fuel into the wings? Wouldn't it make more sense to have it closer to the engine, or maybe at least on the centerline of the airplane so that fuel couldn't move from one tank to the next and cause an imbalance? Airframe manufacturers like to put fuel out in the wings for two main reasons: safety and packaging. Placing fuel in the wings, particularly in a simple training aircraft, uses real estate that would otherwise be undeveloped, whereas cabin space is always at a premium. (In fact, many early airplanes had fuel in so-called header tanks, usually located right on the firewall, often slung directly above the pilot's feet. This design is simplicity itself: Short fuel lines lead directly to the carburetor with a minimum of fuss.)

Wind effects
Wind effects
Although you may not launch into a 60-kt headwind, it provides a dramatic example of the effect that wind can have on range.

Placing fuel aft of the cabin invokes even greater compromises, including a tendency to move the center of gravity (CG) aft -- remember that an airplane is most stable at its forward CG limit -- and long hoses to the engine compartment. It's desirable to keep the fuel lines as direct as possible, ideally avoiding the cabin altogether.

But by far the greatest concern is safety. Loading fuel in the wings makes it possible for the fuel to go somewhere other than directly on the pilot and passengers in the event of a crash. In fact, early research in post-crash fires led Cessna to build the twin-engine 310 with fuel tanks at the wing tips, in the shape of giant tuna. The theory was that an accident would cause the tip tanks to come off the airplane early in the crash sequence, leaving the rest of the airplane high and dry (of fuel.) Ultimately, the 310 needed more fuel capacity, so auxiliary tanks were placed in the wings, and later twin Cessnas did away with the tip tanks entirely. By and large, modern aircraft follow the fuel-in-the-wings orthodoxy, with the exceptions being early Pipers and some Diamond training aircraft where the fuel is carried behind the pilots in what can only be described as a structural cell, betraying the airplane's roots as a motorglider with removable wings.

So while most fuel tanks are in the wings, the actual vessel varies considerably from airplane to airplane. Most Cessnas, for example, have aluminum tanks that are dropped into open wing bays during construction of the aircraft. The tank walls are separate from the wing structure, like a water bottle inside your lunch cooler. This design has some advantages in construction and repair but adds a bit of weight compared to other designs. Indeed, saving weight -- always a critical issue in aircraft, particularly the smaller ones -- dictated a different kind of tank in which the wing skins themselves form part or all of the tank. These are called wet wings, used most often in Mooneys. A nasty-looking pliable, fuel-resistant material is used to seal the ends of the tanks and around rivets and other structures; the advantage of this design is that a lot of fuel can be carried efficiently by structural volume, but the sealer is known to wear out and cause leakage. Piper Cherokees and the various derivatives of the PA-28 line use a variation on this theme, with a section of the wing's leading edge built as a fuel tank. The primary advantage is that the tank can be removed easily for inspection, repair, or replacement.

Still other ways to carry fuel involve rubber bladders, something you'd find in, say, the early Cessna Skylane and all Beech Bonanzas. Rubber bladders simplify the wing structure needed to carry them, but they wear out as well and, in the worst cases, can trap water behind folds in the floor of the cell.

Speaking of water, each fuel tank is required to have a means of draining a sample of fuel from the lowest point, exactly where water would be expected to stand. (Sounds strange, yes, but water is heavier than fuel, so it will settle to the bottom of any container.) Your instructor will show you not only the drains for each fuel tank, but also those for the lowest point in the fuel system, often at the fuel selector or something called a gascolator. This device is a small sump positioned at the lowest point, usually on the engine side of the firewall, designed to trap water before it can get to the carburetor or fuel-injection system.

Between the tanks and the engine will be a collection of plumbing and, at least, an on/off valve. Such a shutoff is a safety precaution in the unlikely event of an engine fire. In the simpler airplanes, that's all you'll have. For example, the Diamond DA20's one tank requires no switching, but Cessna managed to imbue the 150 with a no-touch fuel system as well -- verify that the fuel shutoff is open or On and you're good to go.

Selector Boy
Fuel selector switch

I have a confession. Through most of my flying career, I've been lucky enough to pilot airplanes with sane fuel systems. Everything from the ubiquitous Cessna 150, with left and right tanks but nothing more than an On/Off switch, through the Skyhawk (make sure it's on Both and leave it there) and on up to various low-wing airplanes with left and right tanks. My own Beech Bonanza had two large wing tanks whose main challenge was making sure I landed with enough fuel for the load in the cabin, as the CG moves aft in this Beech as the fuel is consumed.

Now I fly a Bellanca Viking, one of the early ones with four wing tanks-two considered mains and two auxiliaries. The mains hold 17 gallons each, the auxes 19; at a typical cruise consumption of 15 gph that's somewhat more than an hour each tank. To add to the potential confusion, there are two fuel selectors. One determines which main tank is in use (and provides the Off position) in addition to selecting one of the auxiliaries. Which aux tank depends on the position of a second selector, way down on the floor between the pilots, deep in the shadows. Finally, there are the two fuel gauges-one for the mains and one for the auxes. When a main tank is in use, the gauge marked Main shows the level in that tank. The gauge marked Aux shows the level in the aux tank selected. But switch to an aux and the Main gauge goes to E. Let me tell you, the first time this happens it makes your heart skip.

It took me hours to get comfortable with the system, helped by two things. One is a J.P. Instruments EDM-800 engine analyzer with fuel computer. By taking notes when I change tanks, I always know how much fuel is where in the airplane. But the most important aspect of competency is discipline. For example, I always start the day on the left main tank, and normally burn a set amount before changing to an aux tank and then following a set procedure. Normally, it's left main to right aux to left aux to right main, theoretically leaving my reserve between the right main, which I expect to land on, and the left main, which is my don't-touch reserve.

To maximize the availability of fuel, this process requires running tanks dry, normally just the two auxes. It sounds a lot worse than it is, and the trick is to anticipate when the tank will run dry-a timer set to go off two or three minutes before the tank is expected to run dry, backed up by the fuel-computer readout. It always gives lots of warning-a quiver of the fuel-pressure needle, a slight bump of the engine. And, always, a quick flick to a full tank restores pressure immediately.

If I've learned anything from flying the Viking it's that the ability to keep a couple of balls in the air really cuts the tendency to sit there, complacent, waiting for the next part of the flight to take place. Plus it makes getting back in the Skyhawk downright relaxing.-- MEC

Fuel management gets slightly more complicated in larger aircraft, but not unduly so. Cessna's evergreen Skyhawk has a four-way fuel selector valve-Left, Both, Right, and Off -- that most pilots merely leave on Both. In this position, the engine draws fuel from both tanks evenly, and it works great 99 percent of the time. (It's also a boon to safety. The Skyhawk suffers about one-third the number of fuel-mismanagement accidents of similarly sized aircraft with tanks that must be switched.)

So why have individual tank selection? It's possible that a leaky fuel cap or an improperly aligned fuel vent can cause uneven consumption left to right. And when there's a significantly greater amount of fuel in one tank than the other, you'll notice the airplane becoming "wing heavy." (Aircraft designers face compromises at every turn. Place the fuel tanks close to the cabin and the tanks can be significantly imbalanced before affecting the handling of the airplane, but they're also closer to the occupants. Nothing's easy in aircraft design.) Also, if you're forced to park on a slope (relative to the wingspan of the aircraft) it may be useful to switch to the uphill tank to prevent spillage through the vent system after refueling) -- pilots of newer Cessnas are encouraged to turn the fuel selector from Both for just this reason.

High-wing airplanes have some advantages in fuel-system design, primarily that the tanks are considerably higher than the carburetor, which means the fuel can "gravity feed" without the help of a boost pump. Low-wing airplanes have tanks much closer to the carburetor and need small electric boost pumps to ensure fuel flow when the engine compartment is hot or when the airplane is at a high angle of attack. Check the manual for recommendations on using the boost pump. Generally, in carbureted aircraft or those with fuel-injected Lycoming engines, the procedure is to use the pump during takeoff and landing (as a backup in the event the main pump fails during those higher-risk moments) and to restore pressure if you've run a tank dry. Moreover, low-wing designs are hard to build with a Both position in the fuel system -- although Commander managed to do it, few others have -- so pilots transitioning into, say, Piper Cherokees from Skyhawks are faced with not only a very different handling airplane but also the requirement to switch tanks from time to time.

With a few hours in type, this tank-switching routine becomes just that: routine. Work with your instructor on good practices, but keep in mind that more fussing isn't necessarily better fuel management. Most experienced low-wing pilots decide which tank will be the first one used in the flight. Often it's the left main, or the one closest to the pilot -- it doesn't have to be any particular tank, but make a plan and stick to it. Then these pilots decide how often they want to switch tanks. For low-powered aircraft, changing tanks every hour is often enough unless the resulting fuel imbalance can be felt in the controls. Set a timer for the desired interval and change tanks when it goes off. Take that moment to look at the fuel gauges and verify that what should be on board actually is. Make it part of your routine: Hear the beep, change tanks, look at the gauges again, reset the timer.

Fuel management should be high on your list of howgoesit thinking during flight. On a cross-country trip, continually assess your fuel status against the time to destination. Remember that airplanes consume fuel by the hour, not strictly by the mile. A far more accurate way to gauge consumption is to set power and mixture by the book and keep track of time. When deciding if you have enough fuel to make your destination, consider how long it will take you to get there, not how far away it is. Unexpected or stronger-than-forecast headwinds have left many pilots short of fuel by increasing the time required to reach their destination.

Don't let it happen to you: Be conservative, know your airplane's fuel system, and then you can relax and enjoy the flight.

How much does it hold?

Most fuel systems are listed with usable and unusable capacities, something along the lines of: total capacity, 55 gallons, usable capacity 52 gallons. Why is that?

Fuel system design dictates how much of the fuel in each tank is actually usable by the engine. For example, most tanks in high-wing aircraft have two pickups, or hoses from the tank to the rest of the system. This allows the fuel in these shallow tanks to continue to flow even at very high or very low angles of attack. If the pickup were only at the front, the pickup could suck only air during a nose-high climbout, and the engine would quit. With two pickups, both nose-high and nose-low attitudes can be used with comparatively low fuel levels.

Moreover, it's possible to get more fuel into the tank than the system can deliver to the engine under all flight conditions. Here, the certification rules are strict, ensuring that even the most ham-fisted handling of the airplane -- uncoordinated turns, for example -- won't result in a fuel-pickup problem. Sometimes the worst-case scenarios-where an uncoordinated turn forces the fuel away from both pickups -- make a gallon or more in each tank unusable in flight. More often than not, that fuel is usable in unaccelerated, coordinated flight. Then again, you're never going to trim your fuel margins so tight that you'll need to find out, right? -- MEC

Marc E. Cook has logged 3,000 hours in 16 years of flying a variety of light aircraft. A former senior editor for AOPA Pilot magazine, he now writes about aircraft, automobiles, and motorcycles. He is based in Long Beach, California.

Want to know more?

The AOPA Air Safety Foundation offers a comprehensive Seminar-in-a-Box (r) on fuel management, including detailed instructions on how to get the best mileage for today's expensive avgas. "Fuel Awareness" examines carburetor and injection fuel systems, along with an introduction to auxiliary fuel tanks. For more information, call 301/695-2184, or order online.

Links to additional resources about the topics discussed in this article are available at AOPA Online.


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