I had an aircraft with glass fibre wings and the fuel tanks bonded within the skin. Always earthed to the aircraft but I cannot see what good that was.
My general impression is that the greatest hreat is from static caused by the movement of fuel through the nozzle.

A ground installed refueling point, is more than likely grounded/ connected to an earth stake of some sort. I'm not familiar with them, and havent used one.

The Military (RAAF, USAF, probably RAF) systems I have used involve tankers.

The system taught has always been, that the aircraft when parked is connected to an earthing point (Brass head in the tarmac/concrete). The tanker when it arrives, will have an earth lead run out and connected to the same earth point on the ground. Another lead is run out and connected to the aircraft, usually near the filler point (winch may be a single fuel point connection or filler cap). After the hose is run out and before fueling, a bonding lead attached to the nozzle, is attached to a grounding/bonding point adjacent to the filler point.

Now, the aircraft, tanker and fuel hose are all at the same potential (nominally zero) and the potential for a static spark is pretty much eliminated. For everyone's information, . there is a tinned copper braided wire run down the length of the fuel hose, that clams to the nozzle , and the attachment point on the tanker. Continuity of this lead and the tanker earth leads are regularly checked for good continuity. (or should be).

The grounding/earthing point in the tarmac consists of a brass head attached to a copper coated steel rod about 1 meter long that is driven into the ground. resistance between the earthing points is measured after installation, and regularly (or should be, its a tedious job) and should be below 10000 ohms.

Here endeth the lesson...

Quote Step Turn :


On many aircraft, you will find metal structure bonded to metal parts by small bonding straps. Flight controls to their flying surface being a prime example. This assures no arcing through the hinges.


Quote Wikipedia:


A Faradaycage or Faradayshield is an enclosure used in order toblock electricfields. It is formed by conductivematerial or by a mesh of such materials.


They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.

I have flown in snow in the Aztec, and reaching toward the windshield would produce sparks from my fingertips to the near area of the windshield (like those glass globe which spark under your palm).

For readers wishing to understand the issues with aircraft grounding and bonding from a design and maintenance perspective, AC 43.13-1B, Chapter 11-185 is the best starting point:

http://www.faa.gov/documentlibrary/m...ac43.13-1b.pdf

In particular: 11-187 c. "All isolated conducting parts inside and outside the aircraft, having an area greater than 3 [square inches] and a linear dimension over 3 inchs, that are subjected to appreciable electrostatic charging due to participation, fluid, or air in motion, should have a mechanically secure electrical connection to the aircraft structure of sufficient conductivity to dissipate possible static charges."

In one type of large but unpressurised aircraft, the windshields were made from plastic coated in gold film to be conductive for the purpose of heating to de-mist. There was a thermostatic control mounted very near to the windscreen edge to regulate the heating of the windshield.


On one dark and snowy winter night in northern latitudes, the dry snow was brushing across the windscreen during cruise flight and causing static to build up immensely in the windshield, until it took the shortest path to discharge. (Think of combing your hair with a plastic comb on a dry day)


You've guessed it! The discharge occurred through the thermostat and caused it to fail "On". Of course the windshield just got hotter and hotter until by the time that the aircraft landed, the windscreen was bulging in towards the pilot.


It's worth remembering that whether air is hot or cold, it's dryness that offers the best conditions for static build-up.


Can't tell you the specific aircraft type, otherwise I'd have to shoot you.

Why didn't someone pull the CB?

Dunno! Perhaps the crew couldn't understand why the windscreen was slowly bulging inwards and didn't touch it to feel the heat. They would have been wearing issue leather flying gloves anyway.


But it definitely happened because I knew the people in the organisation whose aircraft it was.

A short, almost relevant story.....

My father bought a new Ford Cortina, (back in those days), and all went well until it was 4 months old, when it would turn over but very slowly on the starter.

I looked under the bonnet, whilst dad tried the starter, and saw that the Choke-Cable was getting hot and smoking. I realised that all the starter current was going though this cable instead of the Engine Earthing Strap.
I took the Earth Strap off, and found that the paint on the chassis was preventing a good contact... I scraped the paint away, re-fitted the Earth Strap, and we had no further problems for many many years.
.

Avtrician,

The RAF refuellers had one bonding reel to attach to the a/c, a bonding plug and a crocodile clip at the hose end (pressure or open line) to plug in/clip on at the fuelling point. We stopped attaching the spiral wound wire within the hose to the hose end filler some years ago, it was said that the fuel within the hose made a bond. The vehicle also had an earth strip, usually at the rear connected to the chassis and trailing on the ground, conductive tyres were also fitted. An electrician was to check all these connections with a safety ohmmeter weekly and sign the vehicle docs as to their integrity.

PM

One can hear many things said, indeed.

And that earth strip and conductive tyres would be zero use on a dusty concrete apron, I understand these are abundant down under.

Hello!

I am very interested in this topic and have other questions:

What happens for example, if the grounding point of the aircraft or refueling vehicle no longer has any electrical conductivity (corrosion / incorrect installation ) or the bonding cable is broken, to what extent is there still a risk of fire or if, for example, the cable is attached to a painted surface?

What happens if the cable comes loose during refueling or does a cable only have to be connected at the beginning of refueling to compensate for the potential difference?

How often do such fires occur? What is more problematic jet or avgas?
You always read that fuels are becoming safer and if I remember correctly you "never" hear of any such incidents that lead to fires?

In short, does this grounding during refueling serve as a safety device to prevent fires - during refueling - or does grounding have any other safety relevance? What about the electrical systems on board? Or is that irrelevant?

I would be interested

Thanks for answering.

It is common, if not locally mandatory that fueling hoses have an internal conductor, to bond the nozzle to the pump. And, in is intended that the fuel nozzle be in contact with the aircraft filler neck during the entire fueling. Therefore, the most critical time to bond the aircraft to the fuel supply (truck or ground mounted pump) is before the nozzle is brought to contact the aircraft filler neck. Obviously, the action of making that contact at the aircraft has to potential (see what I did there!?!) to create a brief spark. That is best away from the fuel filler neck itself. So, before getting the nozzle out, bond the aircraft, and in doing so, bring the aircraft and fuel supply to the same electrical potential. Thereafter, when the nozzle contacts the filler neck, and while it remains in contact during fueling, the potential will remain the same, and the grounding wire is redundant.

If there is a difference in potential sufficient to create a spark, it's a pretty high voltage, so pretty good at finding it's way through small amounts of corrosion, but, yes, you could have so much corrosion so as to have an incomplete circuit. I always give the ground to aircraft clip a little scratchy wiggle after attaching it for this reason.

Irrelevant to aircraft electrical systems (and they should be turned off anyway, other than helicopter hot fueling).

I'm not aware of an aircraft fire resulting from eclectically discharge during fueling. But I know that one of my clients bonds all airplanes in the hangar to hangar ground while they remain in the hangar. He'd had an airplane catch fire in the hangar (before he was my client) and they pushed it out while on fire, to save everything else. I think it was memorable for him, so now, prevention! There is a very scary security video "out there" of a fuel tanker being filled at the depot, and the contact of the filler nozzle and the filler port (being used on top of the tank) contacting. You don't see the spark, but you sure see the resulting fire!

Well explained. Thank you very much.

At smaller airports, if refueling is done from canisters, or if the grounding / bonding cable is forgotten in a hurry, isn't that a major hazard?

Do I understand correctly that there is double and triple protection if the bonding cable is out of order for any reason?

Firstly through anti-static earthing strips on the underside of the refueling vehicle (earth belt) and secondly through the earthing in the refueling hose itself?

You'll find aircraft refueling is very regulated so the issues you mention should not happen if those regulations, rules, and laws are followed. However, as does happen, people do not follow that guidance or have an equipment failure resulting in a fire. For reference here are 2 examples of that guidance: IATA and US DoI.

But to answer your questions:
What happens for example, if the grounding point of the aircraft or refueling vehicle no longer has any electrical conductivity (corrosion / incorrect installation ) or the bonding cable is broken, to what extent is there still a risk of fire or if, for example, the cable is attached to a painted surface?
>>The possibility of a fire increases if the refueling system bonding cables are compromised. As to what extent depends on the conditions and circumstances.

What happens if the cable comes loose during refueling or does a cable only have to be connected at the beginning of refueling to compensate for the potential difference?
>>Bonding cable(s) must remain attached before, during, and after the actual refueling ops.

How often do such fires occur?
>>Not often but they do occur. However, the reason they dont is because most people follow the appropriate guidance and maintain their refueling equipment per that guidance.

What is more problematic jet or avgas?
>>AVGAS.

You always read that fuels are becoming safer and if I remember correctly you "never" hear of any such incidents that lead to fires?
>>You never hear of them as they're more an "industry" issue than a general public headline. As to fuels being "safer", I guess that is subjective to the person. If a fuel can burn in an engine it can burn outside an engine.

In short, does this grounding during refueling serve as a safety device to prevent fires - during refueling - or does grounding have any other safety relevance?
>>Prevent fires during refueling process. But bonding aircraft is also used during a number of maintenance operations as well.

What about the electrical systems on board? Or is that irrelevant?
>>Depends on the situation. If there is an empty fuel tank/cell and a fuel pump electrically shorts and produces a spark then boom. Same with using a cordless drill around or in fuel cells.

There have been a few fires due to static discharge. There are a number of reports and references on them. Heres one.
FYI: there are a number of aircraft OEM procedures that include bonding aircraft to earth prior to performing maintenance even in a hangar. We had ground points in the hangar floor for this at the old day job.

The static charge that can build up on an airframe in flight can be quite substantial. Helicopters doing sling loading some times have the hook up person earth the helicopter before hooking up the load, have heard of cases where the hook up individual has been given a very large jolt, charges of 250,000 volts are mentioned in CAP 426 (Helicopter external sling load operations).

The CAP says,Wonder if you FW folk are the same? One reference says while the tires will discharge static electricity, because of the presence of carbon black within the rubber matrix, they are not relied on to earth the airframe; therefore, when buildup of static electricity is of concern, it is always important to use mechanical means to ground the aircraft.

Earth Strip
 

However, it must also be said that the earth strip on the underside of the refuelling cars also provides double protection if other systems fail.

If I'm wrong, let me know.

Technically, the bonding between the aircraft and fueling system structure is the primary requirement. The goal is to eliminate the static potential between the fuel system and aircraft and not grounding it to earth. Several older references still incorporate earth grounding but the current go to guidance does not. So those "earth strips" don't really provide a backup to the primary bonding.

Has anyone mentioned our cars don’t have a massive fan on the front.

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

Back in the Dark Ages when UK SAR was done with Whirlwind 10s, I participated in dinghy drills off Bridlington. I distinctly remember a crewman on the launch, throwing a wet rope at my legs, as the winchman and I were lowered onto the deck. I didn't feel a shock but maybe it was masked by the impact of the rope.

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
​​​​​​​

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.

.
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!