A different kind of grounded

When refueling your airplane, you know it’s important to use a grounding cable. But do you know why and the proper way to ground your aircraft?

First, a quick science lesson: Hydrocarbon fuels like avgas and Jet A do not conduct electrical current. Therefore, when they are transferred from one container to another — say a fuel truck into an airplane — they can build up a static electrical charge. This charge can then cause a spark, which is not a good thing in a flammable liquid, especially if enough air is present.

To solve this safety problem the aviation community uses a grounding strap from the aircraft to the dispensing unit, whether it’s a fuel pump or a fuel truck.

At large airports that serve airlines, they used to connect a ground cable from the pumping unit to the aircraft and another cable was connected to a grounding point. But there was a problem. Many airliners are refueled in inclement weather. If lightning should strike the jet during refueling, the path to ground could increase the damage to the airliner, especially to the electronics.

That’s why a number of years ago the airline industry changed its fueling procedure, eliminating the second ground cable that connected the refueling unit to a ground terminal. Now they just connect the aircraft to the refueling unit in a process called bonding.

In general aviation, most refueling is done at a fixed base pump with a grounding cable. (I know that a lot of refueling is done with a truck where bonding is applicable).

So is grounding safe at a pump? Generally, yes because most pilots do not refuel in lightning conditions. Additionally, GA aircraft are much smaller targets for lightning than airliners. Another factor in our favor: In most GA aircraft the electrical system is turned off during refueling.

One of the questions I get often is why a ground cable isn’t needed when we put fuel in our car or truck.

It brings to mind a demonstration I saw when I was at Shell Oil during a regular safety meeting. There was a beaker half full of gasoline. They dropped a connected spark plug into the fuel and sparked it. We could all see the spark jump across the electrode, but nothing happened because liquid fuel does not burn. They then raised the plug to right at the surface and again the plug did not ignite the fuel when charged because the mixture of air to fuel was too rich to burn. They then raised the plug to about a foot above the fuel surface and arced the plug. Again nothing happened because the mixture was too lean to burn.

It was only when the spark was a few inches above the surface that the mixture ignited. (This demonstration was done under carefully controlled conditions, so please don’t try this at home).

What keeps cars safe when refueling is that the hose has a ground cable in it and the mixture at the fuel neck is too rich to burn if a spark should happen. In a diesel truck, the air to fuel mixture is too lean to support combustion so, again, no problem.

When refueling an avgas-powered aircraft, a grounded hose is also used. But avgas has a lower vapor pressure than mogas and refueling is usually in a higher air current area. This means that the nozzle to the tank neck has a greater possibility of being in the right air to fuel ratio to support combustion. For safety’s sake a ground or bonding connection is required to dissipate the charge.

One of the biggest concerns for general aviation aircraft is where to connect the grounding or bonding cable.

On some certified aircraft, there is a designated grounding terminal. Usually, it is on the landing gear, but sometimes it is on the exhaust.

But what about homebuilt aircraft, especially composite aircraft where the frame and body are made from nonconductive material?

Here it is very important that the pilot do some homework. Take a volt ohm meter out to your aircraft and check to ensure that the chosen location for the grounding terminal on the plane has an electrical conductivity to the fuel filler neck. (Do not check conductivity with an open filler cap.)

Also, if you use autogas in your aircraft and haul the fuel in cans, you should put a jumper wire from the can to the filler neck.

If you use plastic containers, they recommend putting a metal strip into the fuel with a jumper cable to the neck.