Fuel Tank Bonding and Grounding for Composite Aircraft
And some really good information for the rest of us
By David Gall, , EAA 116499
Static electricity builds up on the surface of fuel - not throughout the bulk of the fuel but rather on the surface. The charge is a result of friction, either due to sloshing within a nonconductive tank or to traveling friction, as when the fuel is 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 by “sparking.”
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 number of refueling fires that occur happen 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 enough of a spark to ignite the fuel-air vapors: boom!
The charge built up from refueling is not the only potential danger. The almost nonconductive plastic that our airplanes are made from makes an excellent charge collector simply when moving through the air. 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, onboard 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. There is a low incidence of fires because there’s normally 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 simply because the spark is too small (not enough heat). The typical autofuel filler neck has too much fuel vapor (too rich a mixture) to ignite from the usually microscopic grounding spark.