Grounding/bonding when refueling
 

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?

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.

This is a pretty good explanation I think:
https://www.reddit.com/r/askscience/...efore_fueling/

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.

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.

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? :)

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.

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.

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.

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

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?

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.

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.

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.

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.

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

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....

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.

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

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?

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.

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.

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.

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!

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".

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!

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.

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.

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.

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.

Interesting! 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?

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.

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.

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!

+1

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

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.

@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.

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

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.

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.