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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?

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    $\begingroup$ There are cars that have conductive rubber like strips hanging down their rear axle to dissipate static and you are discouraged to sit in your car while fueling because of static and there are indeed accidents, though these days systems that don't suck in the gases are a rarity in developed countries $\endgroup$ – PlasmaHH May 19 '18 at 18:47
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    $\begingroup$ They ground the trucks that refills the gas station pumps. $\endgroup$ – pipe May 19 '18 at 20:39
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    $\begingroup$ Race cars are also grounded. So it may be more a matter of the filling nozzle and vapor recovery than of vehicle type $\endgroup$ – Freiheit May 19 '18 at 23:55
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    $\begingroup$ Many maintenance procedures are best done grounded. It's hard to convince the fuel truck to drive that fast! $\endgroup$ – Harper - Reinstate Monica May 20 '18 at 16:54
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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.

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    $\begingroup$ Unfortunately your "nail in the dirt" didn't really do anything grounding-wise, unless the ground was extremely wet. Grounding rods are typically driven 8+ feet into the ground to be an effective "Earth". The nail was also probably galvanized iron which is a relatively poor conductor compared to copper (somewhere between 12-17% whereas copper is 100%). $\endgroup$ – Ron Beyer May 19 '18 at 19:26
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    $\begingroup$ Good thing for me I don't do it any more! :0 $\endgroup$ – John K May 19 '18 at 19:46
  • $\begingroup$ They modified the electrical code to 8 foot, because 6-foot ground rods weren't doing the job. They also require two ground rods some distance apart (unless a complicated test confirms that one ground rod is doing the job), the test costs more than a second ground rod. The best ground is actually an Ufer, which ties into the reinforcing rod in the basement/foundation/pavement, but you must commit to this at pour time. Most people forget. $\endgroup$ – Harper - Reinstate Monica May 20 '18 at 16:45
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    $\begingroup$ @RonBeyer: Galvanizing rods are intended to provide a safety ground in the presence of mains power. That will happily produce a sustained current in excess of 20A. Static electricity is not a sustained power source. in fact, you don't want a too good grounding. Since the charge is limited, a higher resistance will lower the peak discharge current. $\endgroup$ – MSalters May 20 '18 at 20:21
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We do ground cars during fueling. The biggest difference is probably just in the way the grounding is done.

Fueling Cars

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.

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    $\begingroup$ Re "Every car I've ever fueled in my life has a spring-loaded metal plate...": You must have fueled only relatively new cars, then. $\endgroup$ – jamesqf May 20 '18 at 4:48
  • $\begingroup$ @jamesqf If by "relatively new," you mean relative to the airplanes I've fueled, yes, the cars were relatively new. :) Most of the cars I've fueled have been mid-1980s models or later, though with a bit of 70s mixed in a while back. On the other hand, the newest airplane I've fueled was built in 1980 with some dating back to the 60s. $\endgroup$ – reirab May 20 '18 at 6:25
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    $\begingroup$ My 1985 Supra does not benefit from the spring-loaded cover in the fuel filler, just a wide-open hole into the tank, and it does occasionally make me nervous that I'm about to touch two metal objects together in the presence of flammable vapour :) What also likely makes a difference is the filling rate - aircraft generally transfer huge volumes of fuel at a time, typically at a very fast rate while passengers are boarding, much faster than car filling. The fluid velocity in the pipes probably makes static more likely, and you want to take absolutely no chances with passengers onboard. $\endgroup$ – Gargravarr May 20 '18 at 12:32
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    $\begingroup$ @Gargravarr: Re "aircraft generally...", that's one of the (IMHO) problems with this site, that questions about commercial aviation often get mixed up with questions about general aviation. For the planes most of us are likely to own or fuel (unless we are working as ground crew at a commercial airport) the amounts of fuel, filling rates, number of passengers, and so on are not significantly different from cars. $\endgroup$ – jamesqf May 20 '18 at 19:18
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    $\begingroup$ @jamesqf In the US, they've been required since the late 1970s, as part of EPA evaporative emissions fuel tank vapor recovery regulations. Other regions with less sun and smog (Europe) came later. $\endgroup$ – user71659 May 20 '18 at 19:28
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According to reddit,

  • the amount of fuel transferred when refuelling a car is generally too small to generate a dangerous amount of static charge,

and

  • 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).

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  • $\begingroup$ "you actually do ground your car when you fill it, the action of touching the nozzle to the filler hole grounds it" So then the action of touching the nozzle to the filler on the airplane grounds it, too. Still seems inconsistent. $\endgroup$ – birdus May 19 '18 at 17:28
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    $\begingroup$ Aren’t they using tank trucks to refuel aircraft? Those are usually on rubber tires, so no grounding from simply connecting them to the aircraft. $\endgroup$ – Michael May 19 '18 at 17:43
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    $\begingroup$ Not sure whether grounding through the nozzle was a good idea. If there was considerable potential between them, touching them hole could create a spark right at that location where we absolutely do not want it. That's why both the nozzle and the filler hole are grounded before coming close to each other. $\endgroup$ – Bergi May 19 '18 at 17:50
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    $\begingroup$ @Michael I'm really more interested in light planes (e.g., a Cessna 172 or Piper Cub), although I didn't state that in the question. These are typically refueled by the owner at a gas pump not unlike the one at your local Arco. $\endgroup$ – birdus May 19 '18 at 18:06
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    $\begingroup$ @Bergi: Well, theoretically you could have a defined resistance (e.g. 1MΩ) between nozzle and ground to slowly drain the charge in order to avoid a spark. ESD grounding equipment for handling electronics usually works this way. $\endgroup$ – Michael May 19 '18 at 18:31

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