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08/08 - Fuel Tank Bonding and Grounding for Composite Aircraft
And some really good information for the rest of us

By David Gall
David Gall is an Air Traffic Controller and an EAA member.

Most things written on the subject of fuel tank bonding are good information, but do not address the issue from the designer/builder viewpoint of composite aircraft. We are responsible for constructing our plastic planes in such a manner that the ”normal” advice is applicable. To that end:

Static electricity builds up on the surface of fuel. Not throughout the bulk of the fuel, but on the SURFACE. The charge is a result of friction, either due to sloshing within a non-conductive tank, or due to traveling friction as when flowing through a pipe. When transferring fuel from one container to another (or from the pump or fuel truck), the fuel generates a tremendous electrical charge on the way out of the nozzle. The nozzle acts as a charge separator, dispensing fuel of one polarity while building a charge of opposite polarity on itself. Just like socks on carpet. If there is no conductive path for the resulting charge to make its way back to the nozzle by conduction through wires, it may make its own conductive path through the air: spark, just like fingers on a doorknob.

The plastic gas cans we buy at Wal-Mart are made of a conductive plastic. The fuel nozzles at the automobile gas station are grounded by wires encased in the hose itself. Placing the gas can on the ground completes the circuit and the can and nozzle are grounded and bonded to each other. Same for metal cans. Alternatively, placing the nozzle in direct contact with the can (metal or plastic) will complete the bonding circuit. However, there is a small risk of spark at the point of contact (just BEFORE actual contact is made). It is this contact that routinely grounds our vehicles via the metal-to-metal contact of the filler nozzle to the filler neck (don't you feel safe?).

The small incidence of refueling fires occurs mostly in older-style fillers where it is possible for the operator to unwittingly prevent the nozzle from contacting the filler neck. During the fueling operation, the static potential between the nozzle and neck increases (friction, remember) until the voltage is enough to jump the gap in the form of a spark. Likewise, the fool who fails to remove the plastic gas can from the back of his truck bed, (liner or not!) can create the same conditions by failing to make contact between the nozzle and gas can. Or, if there is a bed liner, the static charge on the gas can BEFORE the nozzle makes contact may make a spark sufficient to ignite the fuel-air vapors: boom!

The charge built up from refueling is not the only way to build up a static charge. The almost-non-conductive plastic that our airplanes are made from makes an excellent charge collector just by moving through the air - flying.  When we land, that charge stays on the airplane unless a path is provided to dissipate it. That charge will collect and concentrate in the metal parts of the plane. Similarly, on-board electrical equipment that is not properly bonded to a COMMON "ground" can set up charges on the airframe. If your airplane grounding/bonding point is in a flammable mixture (open fuel tank) at the instant you ground it, the inevitable spark may start a fire.

Make no mistake, there IS a spark when completing the grounding/bonding circuit, however small: the low incidence of fires is because there is not a flammable mixture at the spark location. That may be because the spark location is remote from the fuel, or because the fuel-air ratio is not suitable for ignition, or just that the spark is too small (not enough heat). The typical auto fuel filler neck has too much fuel vapor (too rich mixture) to ignite from the usually-microscopic grounding spark.

Humidity is a very poor choice of grounding/bonding conductor. Relying on the relative humidity of the air to dissipate a static charge may work for the small potentials built up by socks on carpet, but it is entirely insufficient for the charges built up by the refueling operation, or even just normal flying or driving. I live in central Florida and I can get a  shock almost every time that I get out of my vehicles if I choose (it depends on whether I hold onto the door as I get out); the humidity here is not sufficient to rapidly dissipate the charge that builds up on my car or truck just from normal driving. 

As I said before, static electricity builds up on the surface of fuel. Inside the fuel tank, this charge is distributed over the surface of the fuel. But dissimilar charges attract, and if an oppositely-charged item is brought close to the surface of the fuel, the charge on the fuel will rush toward it. If the resulting charge concentration is sufficient, a spark may occur. To prevent this, it is necessary to remove the charge from the SURFACE of the fuel. Commercial products do this by being made of conductive materials. Conductivity (among other things) is what differentiates an "approved" plastic gas can from a milk jug. The almost-non-conductive plastic that our airplanes are made from does not meet this criterion.  

To dissipate the charge on the fuel effectively in our plastic airplanes, it is necessary to have a conductor that contacts or penetrates the surface of the fuel regardless of fuel level. This conductor should also be connected to the fuel filler neck, fuel filler cap, fuel drain, fuel line, and any other metal or conductive item that comes in contact with the fuel, in order to prevent these items from acquiring differing levels of electric charge.  

The conductive circuit need not be entirely within the tank, but may consist of electrical connections to a common ground or bonding point. However, the conductor that penetrates the surface of the fuel absolutely MUST be in constant contact with the surface of the fuel at all normal operation attitudes and especially at ground parking attitudes regardless of fuel level (especially at near-empty when the explosion hazard is highest). This may require more than one wire.  The ONLY solution that satisfies all the foregoing requirements is one which is permanently installed in the fuel tanks and has a grounding/bonding-cable attachment point well away from any potential source

of fuel vapor. Ideally, this grounding/bonding point is accessible before the fuel tanks are opened, in order to minimize the release of vapors prior to grounding, and the grounding/bonding cable connection is completed prior to opening the fuel filler cap. The Central States Association newsletter and others have published several ideas detailing both new construction and retrofits for existing composite airplanes. The drawing to the left is a rendering of one of these,  and it should be noted that the connection should be made prior to removing the cap. Remember, there WILL be a spark....

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