Firewall Forward: Fuel Systems

Fuel systems for most Experimental/Amateur-Built airplanes are fairly simple, but fuel problems account for a fair share of Experimental airplane accidents, especially on first flights and during the Phase I flight-test period (25 or 40 hours). Perhaps the simplicity of the systems engenders a casual attitude, or maybe some builders can’t resist the urge to “improve” things. Even though our focus is firewall forward, a broader look at fuel systems seems appropriate, especially in light of the recent NTSB report on Experimental/Amateur-Built aircraft. (See a complete analysis of this report by Ron Wanttaja).

Good fuel systems are simple fuel systems. Some bad fuel systems are also simple, but no good fuel systems are complex, at least not on the types of airplanes we are concerned with here. Complexity and reliability seldom go hand in hand. Here are a few good general rules:

• Keep the number of fittings and bends to a minimum. In this case, less is more.

• Remember that water will seek the low points of any fuel system, and it must be eliminated wherever it collects. Low points in fuel tanks and fuel lines must have drains. For planes that can be built as trigears or taildraggers, builders need to consider that taildragger versions may have low points in the fuel system that trigears do not.

• Vent lines should have no low points other than the fuel tank and the outlet for the vent line. No other low points are permitted, ever.

• Your fuel system should be controlled with one, and only one, fuel valve. It can be a right/left/ both/off valve, but there should never be more than one. The only exception would be if you needed to install ferry tanks for a very long flight.

• Size your fuel lines so that you can get 150% of maximum fuel flow even at a 15˚ angle of attack (up or down). If you need a fuel pump to achieve that, then you need a backup or boost pump in case your primary pump fails.

Fuel Lines

Fuel lines for four-cylinder Lycoming engine installations are pretty much standardized at 3/8-inch diameter. The IO-390 engine needs just about the full capacity of a 3/8-inch fuel line. Larger powerplants may require half-inch diameter fuel lines, at least for gravity-fed systems.

Most airplane fuel lines are made of 3/8-inch aluminum tubing, the most popular being 5052-0 tubing with a wall thickness of 0.035 inch. It is light and strong and makes for a professional-looking installation. It is easily fabricated with standard tubing tools, but it does not hand-bend easily, except for small angle bends. However, a tube bender works well with 5052 tubing to produce bends up to 180˚. The softer 3003-0 aluminum tubing can also be used, but it will typically not produce as nice an installation. Its main advantage is that it is much easier to bend, and it is also less expensive. The problem with the 3003 tubing is not that it is hard to bend, but that it is hard to straighten. So making neat, straight runs is difficult.

Aluminum tube fittings are standard AN items, readily available from aircraft supply stores. You will need an aircraft flaring tool to use these fittings. Please note that automotive flaring tools will not work with AN fittings because they flare to 45˚, versus the 37˚ required for aviation applications. Swagelok fittings provide an alternative to the flared AN fittings, and they work well. Their advantages are that they are easy to work with and require no flares. Their downsides are that they are more expensive and a bit heavier than AN fittings. They are available from McMaster-Carr and other industrial supply retailers. All aluminum fuel lines should be secured with cushioned Adel clamps.

All fuel lines need to be protected from heat and fire as much as possible. Any fuel line forward of the firewall should be covered with Aeroquip Firesleeve or its equivalent. This is mainly to protect it from fire but also to reduce the likelihood of vapor lock in the fuel lines. Fuel-line penetrations through the firewall should be made with steel bulkhead fittings.

Be sure to route fuel lines as far away from exhaust pipes and mufflers as possible, with an absolute minimum clearance of 1½ inches. Secure them with cushioned Adel clamps just as you would oil lines, and don’t forget to allow some slack for engine movement. For 3/8-inch lines, the maximum recommended unsupported length is 16 inches.

Fuel systems typically run at low pressures, so it is less troubling to use push-on hoses and barbed fittings for fuel lines than it is to use them for oil lines. Less troubling is not the same as best practice. If you decide to go with these low-cost hoses and fittings, you still need to protect them with Aeroquip Firesleeve.

Flexible fuel lines can be made using Aeroquip or similar hoses. In the article on oil lines in the November issue of KITPLANES®, we covered this process, which is the same for oil or fuel lines, the only difference being that the firesleeve protection is required for fuel lines, but optional for oil lines. All fuel and oil lines should be carefully cleaned out after assembly.

Fuel Pumps

For builders of high-wing airplanes with carbureted engines, the standard engine-driven fuel pump, putting out 4-6 psi, is just fine. In fact, with smaller engines, you may decide to eliminate the fuel pump completely. Such a system will not need an auxiliary pump. However, for a low-wing airplane, an engine-driven pump and an auxiliary pump will be required. If the engine uses a carburetor, then the standard engine-driven pump, teamed with a low-pressure auxiliary pump, will provide for all of your fuel-pumping needs. If you have fuel injection, your fuel-pump requirements will be dictated by the fuel-injection system chosen.

It is important to be sure that your auxiliary fuel pump is the type that flows freely when it is shut off. Some pumps do and some don’t. Facet, a popular manufacturer of fuel pumps for Experimental aircraft, makes both kinds, so be sure about which one you have. For example, Van’s recommends the Facet model 40108 fuel pump for the RV-series airplanes, all of which are low wing. This pump has 3/8-inch (-6) flare fittings and allows for the free flow of fuel though the pump when it is off. The Facet model 40109 fuel pump has a check valve in it that shuts off fuel flow when the pump is not running. This feature is ideal for a transfer pump that moves fuel from an auxiliary tank to the main tank, such as is seen with GlaStars and the Sportsman, but it would be a disaster for a backup to the engine-driven fuel pump. If you insist on using a fuel pump that will not flow freely when off, you will need to provide a bypass line with a check valve in it to allow fuel to flow through the system in the event of a pump failure.

Fuel Injection

There are three basic types of fuel injection available for homebuilders: mechanical-port systems like those made by Precision Airmotive and Airflow Performance, electronic-port fuel injection such we see on the new Rotax 912 iS and the Aerosance system found on some newer Continentals and a few Experimental Lycomings, and throttle-body systems by companies such as Rotec and Ellison. The mechanical-port system is by far the most common.

For fuel-injected engines, the auxiliary fuel-pump situation is determined by the fuel-injection system. These systems run at much higher pressure and require completely different pumps—ones that you cannot buy for $50 or less. Fuel injection such as the Precision Airmotive (Bendix) system found on most Lycoming engines requires an engine-driven pump that puts out about 25 psi or more and an electric auxiliary pump with similar pressure output. The auxiliary pump and related filter cost about $800, not counting the rest of the fuel-injection system.

In addition to the certified fuel-injection systems that Precision Airmotive makes and supports, they make an Experimental-only system under the brand name of Silver Hawk EX. It is a mechanical system much like the certified system, but it is tailored and priced for the amateur builder. There are other good mechanical fuel-injection systems available for Experimentals, for example the Airflow Performance system, but the Silver Hawk EX system is the most common.

Fuel injection offers one distinct advantage over a carburetor, which is the ability to closely control the fuel flow to each cylinder and thereby achieve greater fuel efficiency. Carburetors are simple and effective, but they rely on the intake system to distribute the fuel-air mixture from a central point. This works, but it produces an uneven distribution of fuel that can vary with the position of the throttle. Thus one cylinder can be leaned to the optimum fuel-air ratio, but the other cylinders are left to fall where they may. With some fiddling, a reasonable compromise can usually be reached, but it is common for exhaust-gas temperatures (EGTs) to vary by 100˚ or more with a carbureted engine. That means that some cylinders are running efficiently, but others are wasting gasoline by running too rich. It is common to see wasted fuel flow on the order of 1 gallon per hour or more when using a carburetor.

Not to be overlooked is the need for fuel injection if you intend to fly upside down or perform negative-G maneuvers in your airplane. Carburetors do not run well upside down, making many aerobatic stunts impossible to perform without fuel injection and, of course, fuel pickups that also work inverted.

Gascolators, Filters and Valves

The gascolator serves as the last-chance fuel strainer and drain before gasoline goes to the engine-driven fuel pump and then on to the carburetor or fuel-injection system. It should be mounted low on the engine side of the firewall in most airplanes. A drain needs to extend from the gascolator out of the bottom of the fuselage to allow easy access during preflight. The Andair gascolator sets the standard in the industry for these units, but there are also numerous other suitable alternatives available from the usual aviation-supply sources. The Andair unit is available with -6 male fittings, which will save you the trouble of installing two AN816 nipples. Most of the other units come with ¼-inch NPT pipe threads.

While the gascolator functions as a fuel filter with its fuel-strainer screen, many airplane builders like the added security of a separate automotive-type inline fuel filter. With fuel injection, a special filter will be required, which is usually installed in conjunction with the high-pressure boost pump. In most cases, it is better to install the fuel filter inside the cabin, away from the heat of the engine compartment and away from any possible engine fire. Be sure to secure it well, and put it someplace where it can be accessed for easy maintenance.

Fuel Valves

The fuel system should have a single valve that is easily accessible to the pilot in flight. This seems so basic as to not need stating, but a number of accidents have resulted from placing the fuel valve where it is difficult to reach. Extra fuel valves, accessible or not, can also cause confusion and should be avoided. The point is that a single, easily accessible fuel valve is what you want.

The simplest fuel valve is the single on/off valve. This shuts off all fuel flow to the engine but allows fuel to be drawn from wherever it is available. Some people prefer to be able to balance fuel from side to side and elect to use a right/left/both/off valve. These valves are nice, if somewhat more expensive. Andair has a whole family of fuel valves that offer just about every possible combination, including simultaneously switching feed and return lines for fuel-injection systems.

Primers

Pilots in moderate climates may never use a primer to start their planes, but the correct way to start a cold engine is to prime it first, and those who live where winter snows are common will especially appreciate this. Manual priming applies only to engines with carburetors, of course, as a fuel-injection system can perform the priming chores without extra equipment. Primers can be either manual or electric.

The old Essex primer is a familiar sight to any Cessna pilot and works well on Experimentals. ACS also makes a manual pump primer that is a bit less expensive. Many builders who want to avoid bringing another fuel line into the cabin use an electric pump for the priming chores. Either way gets the job done. Typically primer nozzles (AN4022-1) get installed into the heads of cylinders 1, 2 and 4 (on four-cylinder engines). The nicest primer systems are made up of stainless steel lines, but 1/8-inch aluminum tubing will also work. Just be sure that the line from the pump to the engine is flexible.

Sniffle Valve

A fuel-injected engine with a horizontal (forward- or rear-facing) intake should be equipped with a sniffle valve to allow excess fuel that can pool in the intake system to escape. A sniffle valve is a simple check valve located at the bottom of the engine. A line runs from the valve to a safe place to allow excess fuel to be dumped overboard. The engine will run without this valve, but excess fuel retained in the intake system can make the engine hard to start and can ignite in the case of a backfire. No need to expose yourself to these potential problems for want of a simple, inexpensive valve.

FADEC

Full Authority Digital Engine Control (FADEC) is not quite ready for prime time for most Experimental airplane builders. Such systems promise single-lever engine control for the pilot, with electronically controlled management of fuel injection, ignition and propeller in one unit. Aerosance (TCM) sold a few FADEC systems to Experimental builders but no longer offers them for Lycoming engines. Its energies have been focused on refining the system for TCM-powered Cirrus airplanes. Lycoming has a FADEC system on its new TEO-540 engine for the Lancair Evolution, but that technology has not been applied to smaller engines yet. Precision Airmotive has a system that manages fuel injection and ignition but not the propeller. Others are working on systems, and this is an area ripe for further development.

Fuel-Flow Tests

Once your fuel system is complete, it is time to test it and make sure it works well. It is worth noting that a fuel-system test has been a requirement for Experimental airplane builders for a long time (see AC 90-89A).

The ultimate test is to check the flow of the actual system in the airplane. Fuel flow should be checked by putting a minimal amount of fuel in the main tanks, say 2 gallons per tank, removing the fuel line from the inlet to the carburetor or fuel-injection servo, and checking how much fuel flows out of the line in 1 minute. From that number, a flow rate per hour can be calculated. The formula to use: Multiply the ounces of fuel drained in 1 minute by 60 to find ounces per hour. Then divide by 128 to get gallons per hour (or just multiply your ounces in 1 minute by 0.47). That number should be 1.5 times the full-power fuel flow of your engine.

Your engine manufacturer’s literature can tell you the full-power fuel flow of each engine, but a quick estimate can be made by taking the rated horsepower and dividing that number by 12. The result is in gph. So a 180-horsepower O-360 engine will use about 15 gph at maximum power. The fuel system feeding that engine needs to be able to deliver 1.5 times that, or 22.5 gph minimum. Using this number, you should drain at least 48 ounces of gasoline in 1 minute in your fuel test. This is the number for a high-wing airplane with a carburetor, where gravity provides the fuel pressure or is the secondary source of pressure, if the engine-driven pump fails. If you have a low-wing airplane or any airplane with fuel injection, you can use 1.25 for your safety factor instead of 1.5, but you will need a primary and a backup fuel pump to be sure you always have fuel pressure.

If you are building a high-wing airplane, the fuel-flow test should be run with the engine off and fuel flowing freely through an inactive pump. If the airplane is a low-wing design, use only the auxiliary fuel pump to achieve the required flow. On takeoff and landing, the auxiliary fuel pump must be on to guarantee adequate fuel flow in these critical situations.

To be sure the airplane will have fuel flow in all attitudes, it should be tested with the airplane in the level, 15˚-nose-up and 15˚-nose-down positions. In each configuration, you should measure fuel flow and unusable fuel. This means three separate tests. Otherwise, you may get surprised by how much fuel flow and/or how much unusable fuel you have in various attitudes. For example, the original GlaStar had to add header tanks to deal with the problem of excessive unusable fuel in the nose-down attitude, which was only discovered when someone ran the engine out of fuel on a long descent.

Enter the results of the fuel-flow test in your builder’s log. Your Designated Airworthiness Representative (DAR) may ask you about this before signing your airplane off and issuing an airworthiness certificate. In any case, you’ll want it for yourself.

Installing and testing your fuel system so that it will reliably and safely provide fuel to your engine is one of the most important things you will do while building your airplane, so give this important task the attention it deserves. Next time we will look at exhaust systems.