But why do you still see such earthing straps on the underside of tankers if they offer no protection against antistatic charge?
How do small aircraft that are refueled with canisters do this? I can't imagine that a cable / bonding when refueling is always connected? perhaps forgotten?

I always clip the static line on my exhaust. But my fuel tank is polyethylene, which does not conduct well.

Static issues in other industries
 

An interesting discussion but I was surprised that a 10000 ohm resistance mentioned previously was acceptable for the earth rod - I would have expected something no more than 1000 ohms (by way of comparison, earth networks for electrical installations have an earth resistance of a few ohms maximum)

Static electricity is a serious problem in many other industries I have been involved with - exploding grain silos appear in the news every few years & a colleague of mine was working on a bulk powder wagon for BR. When they tested the discharge mechanism the static from the fast moving dry powder caused it to flashover internally, fortunately without anyone being injured.

PS I think car tyres usually include carbon black to ensure that any static can safely discharge to earth & the plastic tanks are very carefully specified to prevent static.

Quote... 'Having a large fan on the front can cause quite a bit of static, which may not have had time to dissipate.'

So does flying though thunder clouds, or brushing your hair.

You are intermixing requirements. Those straps have zero to do with refueling ops or the transfer of any flammable liquid, in general. So even if your fuel truck had those straps there is a separate bonding/grounding system(s) specifically required for the fueling ops. For example, in larger fueling systems a large part of the static charge is created by the fuel being forced through the inline filters. So if the fuel system bonding system fails those straps will not provide a back-up to the bonding system as they are only connected to the chassis.
The long standing guidance is to use metal canisters equipped with their own bonding cable to the aircraft. And while there are plastic canisters and funnels, etc. that can conduct electricity I've never seen references or guidance for their use. However, you also must keep in mind its takes a specific set of conditions to line up to generate a spark to ignite things. Thankfully most get away with fueling ops on any vehicle or aircraft as those conditions don't normally come into play. But when they do boom. Fueling at night in frigid temps from a pumped barrel system can make for quite a blue light show if your bonding clamp gets accidently knocked off the barrel. There are also a few videos out there of statically ignited fuel/chemical fires.

The guidance is to assure that the plastic of the canister is in contact with the airplane fuel filler before pouring, and assuring that it remains in contact the whole time while fuel is being poured. We simply want to prevent an open circuit, which can allow a voltage (static potential) difference between them. Fuel flowing from a plastic canister is going pretty slowly, and not at all under pressure, so it has limited potential to create much static electricity, but good fueling discipline is still a good idea. Technically, if pouring from a plastic fuel canister into the wing of the airplane, bonding the airplane to the ground is of reduced importance, but still a good idea if you can. The point is to eliminate an initial static charge difference between the fuel source and airplane, and then prevent any build up of static which could result from fast moving particles in the fuel as it moves.

The dangerous thing will "only" be that air-fuel mixtures can inflame if the corresponding mixtures are present.
This is the reason for this safety device with the connection of the cable. The crocodile clip falling off alone will not cause a fire. It is solely a matter of preventing fire during refueling or are there any other factors? Have I summarized this correctly?

And an ignition source. But close enough.

True. But it can be the event that leads to providing an ignition source.

Not only during refueling ops. Transfer of flammable fluids, various methods of checking storage containers levels, and even fuel pump filter changes can all lead to a static electricity ignition event. I couldn't find the PDF I was looking for but the one below provides a good overview on static electricity issues and flammable fluids and may answer your questions in a better format.
https://www.chevronwithtechron.com/content/dam/external/chevron/en_us/marketing-support/all-other/Static_Electricity_Hazards_and_Prevention_from_CBT.pdf
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Many thanks!

Additional question - about the PDF
Page 34 - Grounding:

The picture shows that the tank container and the tanker are in contact with the ground. Grounding!

Transferred to the aircraft, this should mean that there is also a certain degree of protection when the aircraft and the tanker are in place. But not the best!

This means that the tanker is also protected to a certain extent by the tyres and the car anti-static earthing strips - even if this is not the best way.

For example, if the earthing cable or clamp falls off or has been installed incorrectly / or has been forgotten.


-> I also believe that antistatic materials are installed in the hoses to protect them and prevent sparks if an earthing cable breaks. Additional protection. True?

I still don't think you fully understand the difference between bonding and grounding. Lets try this way in more general terms.

If you only electrically bond (no grounding) the refuel truck to the aircraft, the bonding system will neutralize the static electricity potential difference between the refuel truck and the aircraft. In other words no static spark possible.

Now, if you only electrically ground (no bonding) the refuel truck to earth and only ground the aircraft to earth, there still could be a static electricity potential difference between the truck and aircraft. So in other words, a static spark is possible if you touch the refuel hose nozzle to the aircraft.

YouTube has a number of videos on static ignited fires where there is no bonding system used. Hence the reason aircraft refueling guidance and rules require bonding systems to be used between the refuel equipment and the aircraft. Make more sense?

Specified/certified flammable fluid hoses do have an integral bonding layer that can be used as the pump, filter, hose, and nozzle refuel bonding system to meet various guidance. So instead of a separate bonding cable to connect the truck to the aircraft, there usually is a shorter bonding cable connected at the nozzle which is connected to the aircraft before refuel ops.

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Exactly this.

The earth is a poor conductor, it's just better than nothing. Bonding the airplane to the fuel source is the important thing, grounding to the earth, each or both, is less effective.

To add one more theme to this, as firefighters, we were trained that if we ever had to work around downed, possibly energized, electrical wires at a car accident scene (or direct car occupants to self rescue), all walking steps were to be very short shuffling, definitely not strides. The reason being that the poor conductivity of the ground (in the earth sense) meant that over the distance of a stride, there could be a sufficient voltage difference to be harmful - lower voltage up one leg suddenly getting higher voltage up the other when the step was completed. This could be injurious. Short steps or shuffling would minimize that as much as possible. 'Never a good situation, just make it the least bad it can be.

Happy New Year!

Is it still possible for an aircraft to become statically charged during refueling?

When does this charge dissipate? On the Ground?

The fuel pistol is itself grounded via a ground cable/shield/conductive rubber in the fuel tube and the fuel pistol is in contact with the airframe.
Once the pistol has been put in the aricraft is effectively grounded. However that is the riskiest moment as a spark may be generated right in the most dangerous place where fuel vapor and air oxygen are nicely mixed.

The airframe may get charged if the pistol is not in contact with the airframe and the fuel is poured in the tank due to some triboelectric effect.

The charge and potential is equalized between the airframe and the ground.

There are millions of cars filled with gasoline every day around the World yet you don't hear of a single incident of fire attributed to static discharge. I suspect that with the similarly small quantities of AVGAS we put in our 'planes that we have a similarly small risk. I think if you're filling up a DC6 or similar then the quantities concerned might generate sufficient static.
However, we still stick rigorously to the routine of attaching the static line to our PA28 and C172s when filling up at the pump.
Now, our friends who fly microlights upend their Jerry cans of gasoline and merrily glug the contents into their aircraft without even a thought about static bonding!
TOO

It's not just about fire prevention when refuelling.

But also about the electrostatic charging of an aircraft and the danger to electronics…

Electronics are not affected by the charge accumulated on the airframe. Certification provides for direct discharge to exposed parts. There are ESD protections in place just for that.

Radio devices may be affected by radio noise created by continuous discharge during flight as static may build up due to friction with ice or water droplets at high speed, that's why high speed aircrafts have static dischargers.

Fuel inlets in cars have flame breakers, have a metal spring-held vane that allow for grounding before fuel starts flowing.

The biggest danger is people entering into the car, charging, exiting and then touching (while charged) the pistol and igniting the fumes. There are some YT videos that catch that same scenario:


Aircraft tanks are more susceptible to sparks igniting fumes because they don't have such safeguards: the fuel cap, once removed, gives direct access to the tank. As the pistol closes in a spark may jump in the very wrong place, with plenty of fumes well mixed with air.

Ok. That's news for me. I didn't know that.

Under what conditions can a static charge occur on the surface of an aircraft or refueling vehicle on the ground?

An electrostatic charge on the ground will probably not be standard, but several conditions will be necessary. I would like to know which ones?

An electrostatic charge cannot build up on the ground, as the ground is, well, grounded.

Electrostatic charges may exists as a difference in electrical potential between two things, or one thing and ground. If you effectively ground the one thing to ground, it is grounded, and the charge will dissipate instantly, and not reoccur as long as the ground remains effective. If you electrically connect two un-grounded things to each other, you are bonding them to each other, but either are grounded. Again, an electrostatic charge will not for between these things, as long as the bond remains effective.

An electrostatic charge can build up and be held by anything, and is commonly associated with it moving past poor conducting particles. Airplanes build these charges up easily (large surface area, move quickly through lots of particles in the air, cannot be grounded on flight. Cars build up charges also, and you can get a spark if you exit a car in your bare feet, but, much less common or severe. In any case, all purpose made fuel delivery hoses have a bonding cable in the hose, to provide the best possibility of bonding, and grounding as the fuel nozzle is brought into contact with the vehicle. A very big no no, and I have warned people at gas stations - when filling plastic containers, they must be on the ground (not in the truck of a car, or bed of a truck).

When I flew and fueled Aztecs in the winter, after connecting the grounding cable before fueling, I got into the habit of momentarily contacting the back top of the fueling nozzle to a bare wingtip screw as I moved in along the wing toward the fuel filler - with the intention that if any small charge remained to be dissipated, it could do so from the back of the nozzle chassis to the airframe, well away from the filler neck. I don't know if doing so was effective, but I'm still here!

Yesterday, I refilled my empty airplane from the four plastic fuel containers into which I had drained the Avgas prior to maintenance. To assure bonding, I placed each plastic fuel container on the wing itself (to allow any possible equalization of the container to the airplane), then assured contact of the container spout to the funnel in the fuel filler before and as I poured. So far, so good, and had a great flight!