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!