View Full Version : Aircraft refuelling vs. car refuelling
06-28-2002, 11:32 AM
An article on the dangers of static electricity (http://www.snopes2.com/autos/hazards/static.htm) at snopes got me wondering...
Whenever I'm around an aircraft being refuelled, a static (grounding) line is attached to an unpainted metal part of the aircraft before the fuel hose is ever unreeled. This is to ensure that static electricity is discharged before the metal fuel nozzle touches the metal lip of the fuel tank, as a spark in a fuel tank would not be good.
Why don't we do this with cars?
06-28-2002, 11:39 AM
I guess I should have read the entire article before posting. :o
"That risk is avoided when you pump gasoline into your car, because both the gasoline dispenser and the vehicle are grounded.
06-28-2002, 11:52 AM
But the car isn't grounded, it sits on four rubber (insulating) tires. Sometimes I get a hell of a shock when I step out of my car and touch the exterior body.
06-28-2002, 12:02 PM
I have heard this talked about many times. I think it also has a lot do do with the amount of fuel being transfered. You typically put a lot more gas in an airplane.
Still, makes sense to me to ground cars also. Static electricity doesn't know if the vehicle has wings or not.
06-28-2002, 12:04 PM
I've occasionally wondered that myself, Johnny. I think ground wires aren't used at auto pumps these days because of the convenience factor. People just wouldn't bother to use them. But that doesn't address why they aren't at full-service pumps. Decades ago, in the age when station attendants rushed out to your car to fill the tank, check the tires, clean the windows, etc, why couldn't they attach a ground wire, too?
Does anyone know if ground wires were ever used at auto gas stations?
06-28-2002, 12:16 PM
I heard an episode of Car Talk on NPR in which people called in and told how they caused a static spark with their fingers while filling up and it caused a small explosion to come out of the tank. One woman was mildly burned and injured but mainly just shaken up although all of the cars were fine. It can happen but the consequences are not all that bad.
06-28-2002, 12:40 PM
I think jet fuel is more volatile and suspectible to static ignition.
06-28-2002, 12:46 PM
Jet fuel is actually less volatile and suspectible to static ignition. And grounding wires are used when refueling piston powered planes as well.
06-28-2002, 01:18 PM
A big difference is that aircraft can build up a big static charge while flying, because air masses have different potential (the reason why lightning happens). Aircraft can build up so much static that St. Elmo's fire will appear around the plane.
I think this is the biggest worry - an airplane could land and actually have a huge amount of static charge built up in it.
06-28-2002, 01:29 PM
Speaking as a Cessna pilot...
A possible reason why they require airplanes to have the ground wire as a safety measure is that most small airplanes have fuel vent valves in the wings and/or engine compartment, this makes fueling a plane a far potentially more messy endevour than a car.
Take a Cessna for example. The fuel is stored as a "wet wing", which means that the wing is the fuel tank. The air pressure varies greatly during flight, the pressure change in a half-empty wing could crack or even burst the wing if air wasn't freely allowed to escape. Hence, the fuel vent.
The result of this is that, while fueling the airplane on the ground, things get pretty messy. You commonly have gas dripping (or even streaming) out of the vents in your wings, puddling on the ground. When getting back into the plane, I will often get significant drippage on my head, clothes, etc as I walk under the vent. All of this drippy gas means that the air can be very vapor-heavy, this creates an environment of extreme susceptebility to sparks.
If a spark happens while filling your car, at worst you get a flash burn from your fuel opening. If a spark happens while gassing up your Cessna, you could flash all of the vapor and set your wings, the ground, and yourself on fire. Not fun.
06-28-2002, 02:29 PM
There could be a metal conductor, connected to the nozzle and buried in the hose, that grounds the car to the pump as soon as contact is made. Braided stainless steel, which would also help hold the hose round, would work. Just a SWAG.
06-28-2002, 02:33 PM
I must have had some really bad flight instruction. I've never heard nor been warned of fuel spilling out the vents during refueling. I've filled Cessna 150s and Piper Cherokees to the tabs countless times and have never seen spillage. Is this a common thing? Does it only happen when you top the tank off?
06-28-2002, 03:21 PM
FWIW, aircraft have vents for the same reason a car does: To allow the fuel to flow, by preventing a vaccuum in the tanks. The rubber vent on my motorcycle got bent once, and the engine quit. Not only that, but the tank expanded perceptibly when I opened the filler cover.
But as with a car, if you top off the tanks and leave the aircraft sitting on the ramp on a hot day, the fuel may expand and find its way out of the vent.
06-28-2002, 03:37 PM
How about this – Typically, when you get out of a car, your feet touch the ground and you are touching the car at the same time (shut the door, whatever). This grounds the car and dissipates the static build up. I think that on some low wing planes, you basically jump from the trailing edge of the wing to the ground. You are not in contact with the plane and the ground at the same time. No static gets discharged.
Wow, I’m really reaching.
06-28-2002, 05:51 PM
I think I gave you the Straight Dope, guys. Airplanes are usually fueled right after landing. An airplane can build up a hellacious charge while flying through different air masses. You ground the plane to dissipate the built-up charge.
I don't buy the fuel vapor from the overflow argument, because typically you ground the airplane at a point closer to the fuel vents than the actual nozzle hole is. But overflow vapor is certainly a really good reason not to smoke around airplanes. On a hot day, you can literally see the fuel vapor rising from a fully-fueled airplane as the gas expands.
The big worry is that you'll open the tanks, and gasoline vapor will rise out of fuel filler neck. Then you touch the metal nozzle to it, a spark jumps across, and ignites the vapor.
ElvisL1ves is wrong to suggest grounding the nozzle, because the last thing you want is to complete an electrical connection right at the fuel nozzle. That's why the grounding strap is far away from the filler neck - if a spark jumps, you don't want it jumping anywhere near the fuel nozzle.
For the record, I used to be a 'duty pilot' on weekends, running our flying club. I've fueled hundreds of airplanes. I'm also a ground school instructor. Static potential buildup while flying is always given as the reason why the grounding strap is so important.
Cars don't build up a huge static charge because the tires are generally good enough conductors to dissipate the charge, especially if they are dirty, oily, or wet (and they almost always are, to some degree). Airplane tires will eventually dissipate the charge as well, but if you fuel immediately after landing, the charge may still exist. But while flying, air friction against the body of the aircraft acts just like rubbing a balloon in your hair - ions are released and trapped by the airplane. And because an airplane in flight is not touching anything, the built-up charge has no place to go. Eventually, if the charge builds up to a great enough potential to break down the dielectric constant of air, it will dissipate back into the atmosphere in the form of sparks or ball lightning.
If you pay close attention to some airplanes, you'll sometimes see little metal braided straps coming off a part of the trailing edge - these are static dissipators, to help the airplane shed its charge gracefully. Otherwise, you'd be hearing pops and clicks on your radio all the time. Some airplanes need them, some don't.
Once you land, the remaining charge will slowly dissipate through the tires, but it may not be gone by the time you start fueling.
The risk of static discharge is even higher on non-metal aircraft. The newer composite planes are basically a big freaking capacitor, and can build up a large charge.
06-28-2002, 05:54 PM
Here's an article about plastic bed liners in pickup trucks causing explosions and fires when fuel cans are filled. The plastic bed acts as an insulator, allowing a big static charge to build up. This would confirm my assertion that in a metal vehicle, the charge slowly dissipates away through the tires.
06-28-2002, 06:29 PM
Originally posted by Sam Stone
If you pay close attention to some airplanes, you’ll sometimes see little metal braided straps coming off a part of the trailing edge - these are static dissipators, to help the airplane shed its charge gracefully. Otherwise, you’d be hearing pops and clicks on your radio all the time. Some airplanes need them, some don’t.
All of what Sam said makes good sense to me. Sailboats can also use static disipators on top of the mast. Looks like a little chimney broom. Sail boats also generate a LOT of static electricity. I was hooking up my brothers boat (already on a trailer after we got it out of the water on) to his truck and literally got knocked on my ass. I forgot about that. I was leaning over, putting on the safety chains and got knocked on my ass. Completely. Sort of a huh, what, why am I sitting on the ground? Very strange.
I have taken 110v a number of times, but this was something else.
06-28-2002, 07:42 PM
One thing that is different between cars and airplanes is that cars have a door-valve-thingie that the nozzle of the fueling hose must be pushed through before fueling begins. This probably tends to keep any vapors from the tank down farther inside, away from the point of first contact between the nozzle and the receptical, where a spark is likely to occur. Airplanes, generally, have caps that open directly into the tank. A three to five inch diameter opening above a pool of gasoline. When the nozzle touches the side of the wing, there are plenty of vapors there.
This alone would make an airplane more suspectible to an explosion, apart from the fact that a plane that has just landed has a much greater chance of producing a spark due to static electricity.
06-28-2002, 08:07 PM
Not all cars have that little metal flapper valve over the gas tank opening. The recesses of my brain tell me that they came along when leaded fuel went the way of the dinosaur. Unleaded gas, if I recall, is more volatile, so the flapper valve probably serves a dual function of preventing the gas inside from evaporating, and also keep crap out of the gas tank.
Once again: The reason it's not as important to ground cars, is because cars A) don't build up as much static charge in the first place, since they go slower and are always on the ground, and B) because car tires are mildly conductive, and the static charge dissipates through the tires and into the pavement. Asphalt, btw, is a better conductor than concrete. So a car on an Asphalt road will dissipate the small amount of charge it may be carrying rather quickly.
Occasionally you'll see a little conductive strap hanging from the back of a car. This is a grounding strap that dissipates static buildup into the road when the car is moving. I've seen them advertised as preventing car-sickness, but this is probably nonsense. The real reason for them is probably to make the AM radio easier to listen to, or else the static buildup would cause static and pops and clicks.
Airplanes, on the other hand, build up big charges, and the tires are in contact with the ground for a shorter period of time, and the fueling areas are often concrete and not asphalt. in the case of my Grumman, the gear legs were fiberglass, so there isn't even an electrical connection between the tires and the plane, other than what there might be through the brake lines. Lots of airplanes have fiberglass gear legs.
06-29-2002, 12:55 AM
Searched the web and didn't find any info on how the pumps and vehicles ground at the retail level. Best guess is that the nozzle and hose carry the bond while the pump itself goes to a ground. When exiting a car or truck, most people will touch a part of the vehicle body (filler door, maybe?) and ground the vehicle through themselves. With the nozzle in the filler neck, the vehicle has a bond to the pump and then is grounded the same way.
This is not a perfect system, though. The API seems to say that if you go back and sit in your car during refueling you can pick up a charge which you will carry and discharge as a spark at the nozzle when it is grabbed. This causes a flash fire. So maybe my whole hypothesis is wrong and nothing is grounded, just bonded. Anyway:American Petroleum Institute (http://www.api.org/consumer/refuel.html)
Static electricity-related incidents at retail gasoline outlets are extremely unusual, but the potential for them to happen appears to be the highest during cool or cold and dry climate conditions. In rare circumstances, these static related incidents have resulted in a brief flash fire occurring at the fill point. Consumers can take steps to minimize these and other potential fueling hazards by following safe refueling procedures all year long.
Most important, they should not get back into their vehicles during refueling -- even when using the nozzle's hold-open latch. This will greatly reduce and minimize the likelihood of any build-up of static electricity.
A build-up of static electricity can be caused by re-entering a vehicle during fueling, particularly in cool or cold and dry climate conditions. If the motorist then returns to the vehicle fill pipe when refueling is complete, the static may discharge at the fill point, causing a brief flash fire with gasoline refueling vapors.
Motorists who cannot avoid getting back into the vehicle should always first touch a metal part of the vehicle, such as the door, or some other metal surface, away from the fill point upon exiting the vehicle.
Bonding and grounding are two big safety issues that were pounded into our heads when I was an aircraft refueler. We would bond vehicle to aircraft, bond both to ground points, and even had a clip or plug to bond the refuel nozzle to the aircraft. During over-wing or open-port refueling you were to keep the nozzle pressed against the filler point so that a bond was maintained there as well. Don't forget, the refueler was to touch a metal part of the truck or aircraft before all of this started so that he, too, would equalize his charge.
Remember, not only do vehicles pick up a static charge, so too will fuel as it moves through a hose or falls through the air. This is a big problem when reloading, say, a tanker truck through one of its top hatches. The fuel is falling a long way and the fumes that are replaced are forced out of the top where the nozzle is inserted. If the nozzle is not bonded to the truck properly the whole thing can go KABOOM and make a rather nasty mess.
Hueys and Chinooks had similar issues when you had to use a hand nozzle on them. The fuel tanks were large enough that under the right conditions a fairly substantial charge could be built up by the fuel falling into their tanks. Though rare, improper fueling technique had resulted in the loss of aircraft.
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