A few weeks ago I received an email with a cartoon of a mature lady sitting back with a glass of wine. The caption read, “Another perfect day, and I never had to use algebra once.”
I smiled a little, but then got to thinking about how much we use math and science in our everyday life. And I started to wonder why people look down on their time in school taking these courses as a waste of time, because, in actuality, we use math and science many times every day.
For example, when we fill up our aircraft or automobile, we use physics and chemistry to keep us safe.
One of the laws of physics is that when you transfer a nonconductive fluid — read 100LL, autogas, and Jet-A — a static electrical charge builds up. This charge can cause a spark, and we all know that sparks and fuel do not mix well.
So what keeps us from blowing up every time we refuel our planes or car?
The first thing is that all dispensing hoses are supposed to have a conductive strip in them so that the hose will provide a grounding path to dissipate any static build up. The second safety measure is that a hydrocarbon fuel will not burn in a liquid state. It must be vaporized and mixed with oxygen in the air.
There are limits to the air-to-fuel ratio that will burn. For example, if you fill a beaker half full of 100LL and hold a spark plug 5 feet away and spark it, probably nothing will happen because the air-to-fuel ratio is too lean. Now if you put the spark plug just above the surface down in the beaker, (I strongly recommend that you not try this at home, just take my word for it) and spark it, usually nothing will happen because the air-to-fuel ratio is too rich to burn. So when you refuel your plane or car with 100LL or auto gas, the air-to-fuel ratio at the fill neck is too rich to burn.
Conversely, when refueling with Jet-A or diesel, the air-to-fuel ratio at the fill neck is usually too lean to burn. (This is why you should never mix auto gas and diesel because it may put you in an explosive range, but that is another story.)
When you refuel your car, it is usually not in an open area and the fuel door and fill neck are kind of sheltered. But on an aircraft, the fill neck is usually on top of the wing and you refuel in an open area. This can cause wind turbulence that can “lean out” the air-to-fuel ratio at the neck and put it into the combustible zone.
Now add the fact that the refueling rate for aircraft is usually higher, which increases static generation, and many people hold the nozzle to see how full the tank is, you have more potential for problems. This is why we always recommend using a bonding strap to dissipate the static build-up when refueling an aircraft.
We all agree that an aircraft should have a bonding cable attached to it when refueling, but where do you attach the bonding cable?
I do not know of any GA aircraft that has an identified bonding terminal. Some people use the exhaust and others the landing struts. But the big question is, is there conductivity between the place you are attaching the bonding strap and the fill neck?
If you have some time, you may want to see if there is conductivity between your fill neck and where you usually attach the bonding cable.
Do it carefully by always attaching to the fill neck first so that you do not cause a spark — because sparks and fuel are not a good combination.