Grounding/bonding when refueling

[engineno9]

Hi all,

Someone recently posed me the question, 'Why do you clip that cable the aircraft when you refuel it?' to which I answered, 'To ground it I presume...', rather hesitantly.

'Why don't you do that with your car then? Isn't it grounded through the tyres?', they asked.

I decided I really should know more about this, so after a bit of very light research, I've discovered that what I'm actually doing is 'bonding' the aircraft to the refueling station, so that any difference in electrical potential doesn't leap between the nozzle and the aircraft. Makes sense I guess...

But surely if we had a fuel truck and an aircraft both grounded via the tyres this shoudn't happen, right? Even if there was a short would the electricity not escape through the ground?

Can any of you ppruners more knowledgeable than me answer the question posed? Why do we do this in aircraft refueling but not at the petrol pumps?

[Above The Clouds]

Certainly in larger aircraft, higher fuel delivery rates producing static, aircraft is also most likely to be live with all electrical systems running powered by either ground power or an APU, and lightning strike risk.

[andytug]

This is a pretty good explanation I think:
https://www.reddit.com/r/askscience/comments/1yqbxw/why_dont_you_have_to_ground_cars_before_fueling/

[Colibri49]

Tyres are rubber, therefore don't conduct electricity. Neither does the ground/earth for that matter, or at least only very slightly.


Grounding or earthing in electrical terms, doesn't necessarily imply connecting to Mother Earth, but rather connecting to the chassis or frame of a radio or television or other electrical apparatus. Here's what Wikipedia says


"In electrical engineering, ground or earth is the reference point in an electrical circuit from which voltages are measured, a common return path for electric current, or a direct physical connection to the Earth."


Fuel flowing in pipes, especially certain types of plastic pipe or funnels, can build up a static electrical charge in that material, which when brought into very near proximity with the aircraft filler neck might allow the static charge to jump the gap i.e. cause a spark.

[Jan Olieslagers]

Tires are not reliable for grounding: rubber is not a good conductor, and you do not know the conductivity of the surface the vehicles are on.

There was a time when one could buy a rubber strip with a conductor integrated, to hang from the car, but I haven't seen those for a long while.

The danger is indeed in static build-up, mainly from the flowing fuel itself; this may provoke sparks which can ignite the fuel fume at the tank, especially when withdrawing the nozzle.

[engineno9]

Thanks for the replies guys. So as I read it, if I were to answer the question as asked, reasons for aircraft to be bonded/grounded but not cars would be:

- higher volumes of flow for aircraft refueling causing potentially greater static charges in the refueling line
- possibility of aircraft systems being still powered on in larger aircraft
- and tyres don't effectively ground anything (I must admit I thought this too)

Are we saying then that for an aircraft fuel pump fixed to the ground (self-service style for example), the supplied bonding cable earths the plane once attached, but for a fuel truck (also with non-conducting rubber tyres), the truck would be both bonded to the aircraft and then in some way grounded itself? Or are we saying that as long as they're bonded together somehow then no sparks should leap across the gap anyway.

And also is there anything in the design of the tanks that makes aircraft refueling more susceptible? E.g. a spark at the fuel filling point on a PA28 or C172 could presumably ignite the vapour over the fuel, whereas on a car any spark wouldn't be immediately above the main body of fuel, which is someway down a pipe and inside the tank?

Apologies if I'm over thinking this - my curiosity has gotten the better of me. And that's what we do here, right? :)

[Crash one]

Most cars now have a flap (metal) that is pushed out of the way by the nozzle, grounding the nozzle in the process. The hose is shielded. Aircraft pick up static whilst airborne, their tyres don't ground it so it is more likely to get a spark. I use cans to fuel from, it's a good idea (essential) to place freshly filled cans on the ground for a while to let the static dissipate before up-ending them into the plastic funnel. Especially if they have been sliding around on the carpet en route. Also make firm contact between yourself and the airframe, don't stand with the nozzle held at arms length gushing fuel into the filler from a distance.
Just my opinion.

[Colibri49]

While I'm no physicist, I like to think of such things this way. At the heart of much of physics in the everyday world - not necessarily in more complicated stuff like quantum physics - there is a principle which applies to most phenomena. "Nature is always striving to achieve a balance, or equilibrium". This applies as much to electrical problems as to anything else e.g. water or gas at a higher level or pressure, will tend to flow down to a lower level or pressure.


Therefore you could apply this to the refuelling problem of static build-up and discharge. Just think of wherever static electrical charges might develop for whatever reason and what conductive path they're most likely to follow in order to discharge to a lower potential (pressure). If there is a small gap created between a conductor or body with a higher static electrical potential and another conductor with lower potential, then the static will jump the gap.


It's not for nothing that physical science used to be called "Natural Philosophy". Here endeth the lesson.

[andytug]

Don't confuse bonded and grounded - if you bond truck and plane together then they are at the same electrical potential so no current should flow. This may or may not be the same as "ground" which is just an arbitrary zero. All voltages are relative (hence "potential difference"), so you can have negative voltages as well.
If you called ground 1000 volts then your car in theory would be 1012, and so on.

[Jan Olieslagers]

Thanks, C1, there's some news for me!

[engineno9]

Thanks Colibri - I understand the equalisation of the potentials, it's the apparent difference between refueling cars and aircraft I'm trying to grasp. From the additional replies and further research, it seems most likely that:

- cars have inbuilt mechanisms to equalise any difference between potential, by protecting the fuel tank until the car's metal has touched the fuel nozzle and therefore equalised any difference - thanks also for that Crash One, I didn't know either.
- cars are in constant contact with the ground and therefore can constantly dissipate any static, albeit very slowly, through the tires whereas in aircraft it builds and may not be dissipated by the time you arrive at the fuel pump.
- static buildup in aircraft is greater due to speeds involved
- static buildup in fuel lines is greater due to volumes involved.

Would this be a fair summary?

I'm still not grasping the difference between bonded and grounded though. I always thought grounding was giving electricity a route to earth. And I then assumed that bonding was tying two bodies together to equalise them, which may or may not involve grounding them (so two hypothetical planes connected in the air by a cable and clips would be bonded but not grounded). Am I way off here?

[piperboy84]

I was getting refueled by a truck at a US airport a while back, I took the ground cable from the truck and attached it to the exhaust pipe, the refueller guy took it off and reclamped it to the bolt on the brake caliper, I asked him why, he said he was told by his boss to either use the brake caliper or tie down ring on the wing which ever offered a clean metal to metal connection but not the exhaust. No idea what that was all about.

[ChickenHouse]

I was getting refueled by a truck at a US airport a while back, I took the ground cable from the truck and attached it to the exhaust pipe, the refueller guy took it off and reclamped it to the bolt on the brake caliper, I asked him why, he said he was told by his boss to either use the brake caliper or tie down ring on the wing which ever offered a clean metal to metal connection but not the exhaust. No idea what that was all about.

That is a safety necessity. You should only clamp ground to the exhaust if the aircraft has not flown before and engine is cold. Reason is danger of detonation. If you have flown before and cut the hot engine by mixture, you may still have fuel residuals within the exhaust pipes. If you clamp the ground to the exhaust outlet and get a spark due to voltage difference, it may ignite. So I was told already long time ago at PPL training.

[Colibri49]

Engineno9. You seem to have as good a grasp of the principles as most others. As for the differences between bonded, grounded and earthed, it seems to me that many people use these terms interchangeably and that there really isn't a huge difference between them.


"Bonded" seems the most straightforward to me and suggests that components are electrically joined together e.g. in a metal aircraft the flying control surfaces are bonded to the airframe by flexible conductive straps. This is the Faraday's Cage principle.


"Grounding" to my mind would suggest connecting electrical components to the chassis of an electrical apparatus, while "earthing" might be the electrical connection of something to the earth (or ground).


It's all a bit confusing! Perhaps a qualified electrical engineer should answer this.

[Jan Olieslagers]

Faraday's cage has nothing to do with this, that one is about blocking High Frequency signals such as radio or transponder transmissions.

But "ground" and "earth" are indeed used interchangeably. For this once I will disregard semantics :) to state the essential: the airplane and the fuel pump must be at the same potential.

BTW the aircraft's 12V= or 24V= have nothing to do with it either: the danger is in static build-up.

[9 lives]

- cars are in constant contact with the ground and therefore can constantly dissipate any static, albeit very slowly, through the tires whereas in aircraft it builds and may not be dissipated by the time you arrive at the fuel pump.

This is the key concept. It's less the risk of static build up during the flow of the fuel in the supply line to the tank, but rather the initial charge difference possible from a plane which has just built up a large static charge during flight. You would rather dissipate that charge with the bonding cable to the airframe ground before the fuel nozzle is anywhere nearby, than between the nozzle and the tank filler neck at the moment you put the nozzle in the tank.

Bond the plane to a good (not painted) airframe ground point. I like the exhaust pipe best, as the engine is well bonded to the airframe (or you have a maintenance defect). Tie down rings are ok, if they are fixed, rather than retractable. Wheel parts are ok, with a caution: I had a fueller bond to the nosewheel of the 182 amphibian. I pointed out to him that the two nosewheel struts on the Aerocet floats are composite, and non conductive. I moved the bonding clip to the exhaust pipe.

[Colibri49]

A brief bit of googling:


"One of the most misunderstood and confused concept is difference between Bonding, Grounding and Earthing. Bonding is more clear word compare to Grounding and Earthing, but there is a micro difference between Grounding and Earhing.

Earthing and Grounding are actually different terms for expressing the same concept.
Ground or earth in a mains electrical wiring system is a conductor that provides a low impedance path to the earth to prevent hazardous voltages from appearing on equipment. Earthing is more commonly used in Britain, European and most of the commonwealth countries standards (IEC, IS), while Grounding is the word used in North American standards"


Reference:


What is the difference between Bonding, Grounding and Earthing? | EEP (http://electrical-engineering-portal.com/what-is-the-difference-between-bonding-grounding-and-earthing)

[andytug]

Bonding and grounding are the same - if the thing you are bonding to is at ground already.

A good example of this is TVs which these days tend to be "floating earth", TV are mostly plastic now so the risk of shock is much less, plus the voltages in Leeds are way below the 25kV+ on the old CRTs. So there is no longer an earth wire in the mains flex. The TV has no idea what voltage "ground" is but it still works because the potential difference between live and neutral is still the same. It can mean that on a cheap set the aerial wire (for example) is at a different potential from ground, so be careful....

[engineno9]

I think we're getting there then!

So bonding seems to be the really the important part as far as refueling is concerned so that no discrepancy in charge causes electricity to leap from pump to aircraft or vice versa.

And if the thing you're bonded to is also ground based, i.e. a self-service pump, you're also grounded (and I imagine said thing is a lot less likely to have any charge of its own built up).

But if you're bonded to say a fuel truck, perhaps you're less 'grounded' due to the poor conductivity of the tires.

Then again maybe fuel trucks also have the rubber conductor strip that was mentioned as being present on some older cars.

[andytug]

Bl@@dy autocorrect, should be LCDs not Leeds!

[glum]

To put it in aircraft terms, the systems on board need a certain voltage to work.

This is merely a potential difference between the power input line and power return line of (normally) +28Vdc, or 115Vac.

This difference can even be +14V on the input, and -14V on the return: the difference is still 28V and the + and - are purely with respect to a nominated reference, such as the batteries.

Imagine you have two 14V batteries and need to make 28V: You'd connect them in series (i.e. +ve on battery 1 wired to -ve on battery 2) such that a multimeter with the black lead on the negative terminal of one battery and the red lead on the positive of the other would show a 28V difference in potential between the two points. You ALWAYS need a reference point. In this instance, the black lead is the reference point, and the red shows the difference to that point.

Move the red lead to the middle point (i.e. the +ve of battery 1 or the -ve of battery 2) you'd see +14V with respect to the black lead (your reference point).

Put the black lead in the mid point and your reference point changes. Now if you put the red lead on the +ve of the second battery you'd see +14V with respect to the black lead . Move the red lead to the -ve of the first battery and you'd see -14V because the reference is higher than the point you're measuring.

You could connect 10 batteries in series, and measure the voltage difference between any of the points, such as -14V to +140V, depending on how many batteries lay between your black and red leads, and which way round you had them. If you do try this, beware; 10 batteries can flow a LOT of current!!

When the aircraft is flying, the airframe could be at -1000Vdc when compared to planet earth, but as long as the on-board system has a difference of 28V (in the right sense) across the power lines it will work quite happily.

Until aircraft came along, ground / chassis / earth were pretty much the same thing, as everything was attached to planet earth. Once we could escape gravity, voltages were no longer 'tied' to the planet and it's global earth reference and were free to wander about without any real effect to the onboard systems.

Does this help or have I made the waters even muddier?

[andytug]

Talking of waters... You can imagine electric current as water current, where the height of water (head) is the voltage, the width of the pipe is the resistance, and the current is... well the current! Higher head of water > more pressure > more current.

Re sparks, dry air takes ~3000 volts per cm to break down and spark iirc.... so to get lightning takes..... a lot.

[Above The Clouds]

engineno9
Thanks for the replies guys. So as I read it, if I were to answer the question as asked, reasons for aircraft to be bonded/grounded but not cars would be:

- higher volumes of flow for aircraft refueling causing potentially greater static charges in the refueling line
- possibility of aircraft systems being still powered on in larger aircraft
- and tyres don't effectively ground anything (I must admit I thought this too)

Are we saying then that for an aircraft fuel pump fixed to the ground (self-service style for example), the supplied bonding cable earths the plane once attached, but for a fuel truck (also with non-conducting rubber tyres), the truck would be both bonded to the aircraft and then in some way grounded itself? Or are we saying that as long as they're bonded together somehow then no sparks should leap across the gap anyway.

All correct except I think the bit you missing is, a fixed fuelling installation will already be grounded as part of its construction then be bonded with the aircraft via the bonding cable you attach to the airframe.

Are we saying then that for an aircraft fuel pump fixed to the ground (self-service style for example), the supplied bonding cable earths the plane once attached, but for a fuel truck (also with non-conducting rubber tyres), the truck would be both bonded to the aircraft and then in some way grounded itself? Or are we saying that as long as they're bonded together somehow then no sparks should leap across the gap anyway.


You may not have noticed but the fuel truck will normally or should connect to a grounding point via a bonding cable in addition to the bonding cable attached to the aircraft.

[xrayalpha]

Interesting experiment on Myth Busters regarding mobile phones and fires at petrol stations.

Turns out that, in the States, you very often pay up front for your petrol. Then you click the nozzle so you don't have to keep holding it, unlike in the UK.

People then go and sit in their (nylon fabric) car seats in their (artificial fabric) clothes and use their phones while the fuel is pumping.

If the tank has not enough space for the fuel pre-ordered, and the nozzle fails to shut off, then the excess fuel pours out onto the forecourt.

As it turns into vapour, the driver leaps out of their seat, and causes a spark because of the plastic fabrics and the ensuing static.

Since they are holding a mobile, the mobile phone gets the blame!

But the lesson for pilots is: natural clothing, stay alert and bonding!

[Crash one]

Many moons ago, circa 1960 ish, the Navy issued ground crews with a lightweight jacket made from nylon, bad move. It was withdrawn after about six months and a few "incidents".

[9 lives]

It's important to remember that the need to bond the aircraft to the source of the fuel (which might involve "grounding" too, but not necessarily) is because of the possibility of a latent static charge residing in the aircraft which has the "potential" to jump to the fuel source in the form of a spark. This has nothing whatever to do with the aircraft electrical system, nor household "mains". After all, we also bond aircraft which do not have an electrical system during fueling too!

[Maoraigh1]

I took the ground cable from the truck and attached it to the exhaust pipe, the refueller guy took it off and reclamped it to the bolt on the brake caliper
With our wood-and-fabric Jodel, I insist on clamping to the exhaust, as I'm uncertain if wheels have a bond to fuel system, after two rebuilds in her 52/56 year life.(Rebuilt from two crashed Models 30 years ago.)
After night flying on a frosty evening, I bonded cans to aircraft before pouring. No problems.
Sitting in my car, I unzipped my leather jacket. Static electricity sparks flashed.

[Crash one]

In a wood and fabric aircraft is the aluminium tank bonded to the engine/engine frame? Is this why a fueller once clipped the cable to the filler cap, which I thought was a dangerous move in itself?
Point two: does a wooden aircraft build up much of a static charge? Guessing possibly it does if nylon causes so much problems. I'm not the sharpest tool in the box when it comes to fizzix.

[john ball]

I have first hand knowledge of static fires. In August 1988 on a very hot dry afternoon after flying our Taylor Monoplane, we decided to refuel. We were based on a farm strip and always bought 4 star from the local petrol station filled our big plastic can ( actually a water can bought at a camping shop ).
We used a plastic funnel which sat into the tank on top of the cowling.
I had filled about 20 litres and was just coming to the end and having held the heavy can at shoulder height was just swilling the last bit out when there was a large click followed by flames that stated to melt the full funnel. lots of fuel poured down the fuselage and wing, which was alight, this caught the fabric on fire. I was holding a flaming can which Ii threw as far from the aircraft as possible, but it caught the grass on fire. The whole episode lasted several minutes as we did not have a fire extinguisher and had to beat out the fire with rags. We were stupid and complacent. After this we grounded the METAL funnel and METAL Fuel container to the METAL hangar door. An expensive lesson learnt.

[oldpax]

On one occasion refueling a Varsity I forgot to put the earth to the filler cap and was surprised to see sparks leaping the gap!!Soon had the refueling stopped and said earth lead put in place.As I recall the nosewheel was special rubber that earthed any static buildup in the airframe and was usually tested on minor inspections.

[Jan Olieslagers]

the nosewheel was special rubber that earthed any static buildup in the airframeInteresting! Very correct use of the term "earthed" in this case. But it will help naught when refuelling from a bowser, or that bowser should be similarly equipped. And even then, the surface must be conductive - a dusty concrete apron on a dry summer day would still leave the danger in place. What matters is that the fuel tank, and/or the filler cap, are at the potential as the source of fuel, nozzle or jerrycan or whatever.

@john ball: thanks very much, it isn't always easy to tell of one's less bright moments. Switching to metal jerrycans and funnel is a very good idea; otoh I see little advantage to earthing to the hangar frame. Though of course it can never hurt - and after an experience like yours one would naturally be doubly careful.

@C1: I do not think a wooden airframe would build up a lot of static, there is always a little bit of conductivity unless the wood goes so dry that in-flight breakage becomes a far more serious issue. But large amounts of synthetic stuff like the modern covering tissues are worrying, and reinforced fibres (glass especially) even worse. Perhaps it is another reason to prefer carbon fibre, notwithstanding the cost?

[scifi]

It's not just the airplane that needs earthing; If you are wearing a woollen jumper over a nylon shirt, you might just be at 10,000v relative to earth with the static build-up.
You can earth yourself when you grab hold of the nozzle, as I think the hose contains conductive material.


The size of the spark is largely dependant upon the size of the object which is charged. So you would have less Coulombs than the airplane, and the airplane would have less than the hangar doors (if they are insulated from ground.)


When I worked in Telecoms, we used to test the underground cable from Chester to Manchester, with a 500v Mega. It took an age to charge the cable up, and several minutes for the charge to decay.. Indicating that there was no fault or breakdown of its insulation.

[tinmug]

The exhaust is attached to the engine manifold, the engine is pretty much electrically isolated from the airframe apart from some control cables, by the rubber engine mountings.

Thanks colibri. Research done. Don't believe everything your instructor or anyone who has been flying for years tells you.

[Colibri49]

Tinmug. Where'd you get that novel idea? "the engine is pretty much electrically isolated from the airframe" In a metal aircraft the engine is very deliberately electrically connected to the airframe by substantial flexible metal straps or cables to create a path for current to flow between the battery (also thus connected to the airframe) and the engine starter motor.


Furthermore the alternator or generator needs a similar connection for the rectifier/regulator wherever that is located and some other electrical components like the pressure and temperature sensors also need the engine to be well bonded to the fuselage.


Even wooden and composite aircraft need their engines electrically connected, usually to the battery negative and perhaps to provide connection to some bus-bar for the negative sides of some electrical bits.


Think again!

[9 lives]

In a metal aircraft the engine is very deliberately electrically connected to the airframe by substantial flexible metal straps or cables to create a path for current to flow between the battery (also thus connected to the airframe) and the engine starter motor.

+1

After all, the engine is a part of the fuel system!

[Crash one]

We are talking about wooden airframes, you can't get an electrical connection to wood. But the question I asked was is the metal engine bonded to the metal fuel tank ? Other than by dubious, sometimes plastic pipe work.
I do know from personal experience (because I removed it) that the rear tank on my Emeraude fitted behind the seat was not bonded to the engine/firewall or anything metal attached to them. To remove the tank I only had to remove the plywood shelf, disconnect the rubber feed pipe, remove the filler cap, disconnect the gauge wiring and lift it out.
The only bond that may have been possible would be the sender unit wiring, but the gauge sender is fitted with a rubber seal. All a bit dubious I thought.

[Jan Olieslagers]

@Crash one: there cannot be a general rule, on non-certified planes. The only way to be sure is to check on the given airframe. I fully agree the bond ought to be present. If it isn't there, it shouldn't be hard to add.

[Crash one]

Jan.
Agreed though as the aircraft is 56 years old and hasn't burnt to a crisp yet I must admit I never bothered.

[pulse1]

Back in the 50's I was lucky enough to get a flight in a C119 Packet. I noticed that there was a flexible rod behind the main wheels which would touch the ground at the point in touch down, thus grounding the aircraft. I was told that this was to prevent disembarking passengers from getting a shock as they stepped of the aircraft.

[9 lives]

Yes, the DC-3's I work on have grounding cables at the tailwheel, which rub the ground. We still bond them to the fuel truck.

In a certified airframe, metallic or otherwise, every fuel and powerplant component will be bonded to each other, and the electrical system ground. If an aircraft (certified or otherwise) is not so configured, it is not representative of a compliant or ideally safe arrangement. 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.

One of my client's maintenance shop will have every aircraft in the hangar bonded to the steel structure of the hangar by a clip on cable. This is based upon their very unhappy experience of having a customer's Cessna Cardinal catch fire while they worked on it. They were able to open the door, and roll it out on fire, so only it burned, not the hangar and contents - but they could not extinguish the plane in time.

[funfly]

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.

[Avtrician]

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...

[Colibri49]

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 (https://en.wikipedia.org/wiki/Electric_field). It is formed by conductivematerial (https://en.wikipedia.org/wiki/Electrical_conductor) or by a mesh of such materials.


They are also used to protect people and equipment against actual electric currents such as lightning (https://en.wikipedia.org/wiki/Lightning) strikes and electrostatic discharges (https://en.wikipedia.org/wiki/Electrostatic_discharge), since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.




Automobile and airplane passenger compartments are essentially Faraday cages, protecting passengers from electric charges, such as lightning.

[9 lives]

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/media/advisory_circular/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."

[Colibri49]

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.

[glum]

Why didn't someone pull the CB?

[Colibri49]

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.

[NutLoose]

http://www.pprune.org/spectators-balcony-spotters-corner/375387-earthing-aircraft-when-refuelling.html

[scifi]

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.
.

[kaitakbowler]

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

[Jan Olieslagers]

it was said that the fuel within the hose made a bond

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.

[chewing4gum]

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.

[Pilot DAR]

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!

[chewing4gum]

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?

[wrench1]

I am very interested in this topic and have other questions:.
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 (https://www.iata.org/contentassets/828efe1a6a2a487aacaa8fe7642f0c72/iftp-standard-fueling-procedures_service-levels-and-safety-v1.00.pdf) and US DoI (https://www.doi.gov/sites/doi.opengov.ibmcloud.com/files/uploads/Aviation_Fuel_Handling_Handbook_1994.pdf).

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.

I'm not aware of an aircraft fire resulting from eclectically discharge during fueling.
There have been a few fires due to static discharge. There are a number of reports and references on them. Heres one (https://flightsafety.org/ao/ao_mar-jun01.pdf).
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.
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.

[megan]

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,Helicopter pneumatic tyres are made from electrically conducting rubber to ensure that helicopters with this type of landing gear make a good earth connectionWonder 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.

[chewing4gum]

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.

[wrench1]

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.
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.

[kghjfg]

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.

[India Four Two]

Helicopters doing sling loading some times have the hook up person earth the helicopter before hooking up the load, ...

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.

[chewing4gum]

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.


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?

[IFMU]

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

[HowardB]

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.

[scifi]

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.

[wrench1]

But why do you still see such earthing straps on the underside of tankers if they offer no protection against antistatic charge?
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.
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?
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.

[Pilot DAR]

And while there are plastic canisters and funnels, etc. that can conduct electricity I've never seen references or guidance for their use.

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.

[chewing4gum]

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?

[wrench1]

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.
And an ignition source. But close enough.

The crocodile clip falling off alone will not cause a fire.
True. But it can be the event that leads to providing an ignition source.

It is solely a matter of preventing fire during refueling or are there any other factors? Have I summarized this correctly?
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
​​​​​​​ (https://www.chevronwithtechron.com/content/dam/external/chevron/en_us/marketing-support/all-other/Static_Electricity_Hazards_and_Prevention_from_CBT.pdf)

[chewing4gum]

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?

[wrench1]

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.
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?

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?
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.

[Pilot DAR]

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.
.
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.

[chewing4gum]

Happy New Year!

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

When does this charge dissipate? On the Ground?

[vihai]

Happy New Year!

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


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.


When does this charge dissipate? On the Ground?

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

[TheOddOne]

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

[chewing4gum]

It's not just about fire prevention when refuelling.

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

[vihai]

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.

[vihai]

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.


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:

https://www.youtube.com/watch?v=FPKen4QwY7I
https://www.youtube.com/watch?v=jKu2G0Ex23Y

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.

[chewing4gum]

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.


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

[chewing4gum]

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?

[Pilot DAR]

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!

[chewing4gum]

The regulations on refueling state:

"Vehicles and containers used to refuel and defuel aircraft must be earthed and also conductively connected to the aircraft."

Apart from sparking during refueling, what other dangers exist if the earthing or connection fails?
I read on another website that one only grounds his aircraft but does not connect. Apparently everything went well?

The danger is inflammation when refueling?

[Pilot DAR]

I read on another website that one only grounds his aircraft but does not connect. Apparently everything went well?

So an anonymous person who may not be doing things correctly has not had something go wrong yet. The internet.

Apart from sparking during refueling, what other dangers exist if the earthing or connection fails?

I think that the topic has been well covered in this discussion.

"Vehicles and containers used to refuel and defuel aircraft must be earthed and also conductively connected to the aircraft."

That sounds like good regulation wording, do it that way.....

[chewing4gum]

Thanks :)