Why does a plane need to be grounded while fueling, but we never ground our cars? Does the potential for a spark magically exist for an airborne vehicle but not for an earth bound one?
On cars I think it's because static charges don't build up as much but theoretically it's still possible for a spark to occur when the nozzle touches the tank inlet and provides a ground path.
On airplanes there are two issues:
Static charges built up in the airframe in flight. For this it is essential to ground the aircraft before the fuel nozzle is brought to the tank inlet. The risk is of a spark just before the nozzle touches the filler neck, the nozzle itself being grounded, which is right at the location where the air fuel ratio is favourable to ignition. It's also a good idea to touch the nozzle to the fuel cap before you remove it in case the tank isn't adequately bonded to the rest of the airframe, but I don't think anybody does this.
The other one is static charges built up within the fuel tank by the shearing action of the incoming fuel column when adding fuel. In a large refueling operation, the column of fuel builds huge static charges within the mass of fuel itself, which result in static discharges within the fuel (there are films that show this; a freakin' light show).
Most dangerous is fueling from a plastic can. When I used to fuel my airplane from cans I made a grounding wire arrangement that bonded the fuel in the can, the fuel tank flange and the ground (using a nail on the end of the wire pushed into the dirt) together.
We do ground cars during fueling. The biggest difference is probably just in the way the grounding is done.
Every car I've ever fueled in my life has a spring-loaded metal plate that touches the nozzle as the nozzle first begins to enter the fuel tank, before fueling would start. Any sudden static discharge would happen then. That metal remains in contact with the nozzle throughout the fueling process, keeping the car (or at least the part of it around the fuel) grounded throughout the entire fueling operation.
Fueling Light Airplanes
On the light airplanes I've fueled (Cherokees and the like,) there's a relatively large hole in the top of the wing through which the aircraft is fueled. There's no spring-loaded metal plate or anything like that. Just a hole that is significantly larger than the fuel nozzle with a cap that is unscrewed and completely removed before insertion of the nozzle. It would, thus, be not only possible, but rather likely that contact would neither be made before fueling begins, nor maintained consistently throughout fueling.
Furthermore, even if contact is made before fueling begins, the fuel tank (which hopefully still has some fuel in it) is directly beneath the hole, so any spark that did form at first contact would already be dangerous. With a car, that spark would form outside the fuel tank and would not enter the fuel tank. On the light airplanes I've fueled, it would happen at the top of the fuel tank and fly into it, a much bigger problem.
So, in order to prevent sparks falling into the fuel tanks on a light airplane, we attach a separate cable to the airplane first (away from the fuel tank) that grounds the entire (conveniently conductive) frame before we stick anything into the fuel tank that might have otherwise caused a spark.
- the amount of fuel transferred when refuelling a car is generally too small to generate a dangerous amount of static charge,
you actually do ground your car when you fill it, the action of touching the nozzle to the filler hole grounds it.
EDIT: Also, it's not "magically", but a plane can indeed build up a considerable charge while in the air, from flying through electrically-charged clouds and whatnot - something that is not usually an issue with groundbound vehicles (such as most cars).