Grounding/bonding when refueling

[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 airframe

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?

[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.