Earthing during refueling

[GRpr]

Apologies if the answer is glaringly obvious, but why do we earth aircraft when refuelling with avgas but happilly fill our cars at the petrol station without doing the same?

If anyone has the answer, rather than a theory, I would be grateful. I was asked this recently and was stumped. I know why we earth aircraft - the theory is straightforward; but why don't we earth cars?

[CaptainSquelch]

Unknowingly we do earth our car. There is a metal mesh in the hose, a bit like the shielding in a coax cable. This in earthed in the pumpunit, and on the other end connected to the dispenser. When refuelling you touch the carwith the metal of the dispenser et voila you are earthed.

Sq

[Unwell_Raptor]

When I was a student I worked in a pie factory for a while, and it was vitally important to earth the flour tanker before it pumped its cargo into the silo. Apparently flour is a greater explosion risk than petrol where static is concerned. At least that's what they told me!

[GRpr]

CaptainSquelch

Many thanks for the explanation of how earthing is accomplished at the petrol station. Do you have any idea why the fuel pipes for avgas refuelling don't employ the same system? Doesn't seem logical somehow!

Unwell_Raptor

Yes! Flour and grain!! I went to a lecture a few years ago on the static induced explosions in grain silos. Most impressive!!!!

[kabz]

Take a biscuit time. Make a hole in the side just big enough to run a (bunsen) tube through. Connect the tube to something in the tin that you can put flour in, and that will get blown out if you blow down the tube. An old tobacco pipe would probably work. Fill the pipe with flour, put a lit candle in the tin, and close the lid (not too tight). Wear safety glasses (just in case)

Blow down the tube... BANG!!

[Tarmach]

What is it in flour that makes it explosive?

Also how can Air to Air refuelling be earthed?

[EDDNR]

Capt Squelch

I doubt very much that your theory is correct, the resultant spark from touching the nozzle to your open fuel pipe would have surely caused an explosion by now.

My guess is that car tyres contain a lot more carbon and conducting material to prevent static when you step out of the car, and it is believed to reduce car sickness as well (hence those black dangly things on the back of Trevors XR3).

An aircraft builds a charge by flying through (friction with) air, hence static wicks, and again, I don't think a car with four tyres permanently attached to the road doing less than 70mph will create such an amount of static.

Rod

[Stroppalot]

Exactly! Sqeuch, you are identifying the exact reason why the earth strap >>IS<< needed on aircraft. If grounding took place when the nozzle touched the filler neck, there would most likely be a spark, which would ignite the fuel vapour... then KABOOM!

The reason cars don't need bonding straps is because the fuel tanks are insulated from the static in the car body. (in fact most fuel tanks now are of plastic construction).

Also remember AVGAS is far more flammable than Petrol.........

And Petrol is far far far far more flammable than Jet A1............

[Luftwaffle]

Tarmach:

What makes flour explode?

oxygen + fuel + ignition = fire

fire + containment + dispersion = explosion

Grain dust or flour suspended in air is a lovely fuel/oxygen mix. Contain that in a grain silo or kabz' biscuit tin, and a spark can lead to an explosion. The fine granularity of the flammable material allows plenty of air around the fire, to fuel the explosion.

In air air to air fuelling, the two aircraft would be "grounded" to each other before the process began, so that they each had the same static potential, and there is no tendency for a spark to jump from one to the other.

[airgeezer]

Static is generated by the flow of fuel, the more fuel that flows the more static is generated. This is the main reason earthing and bonding are more important in aircraft than cars.