Earthing during refuelling

[GRpr]

Nearly a month ago, I posted the a question regarding 'Earthing during refuelling' in the Questions section ref:

http://www.pprune.org/ubb/NonCGI/Forum57/HTML/001208.html

My question was why do we earth aircraft when refuelling, but not motor cars?

On reflection, I think that my original post may have been in the wrong forum.

If anyone can give me an answer, I would appreciate it.

[Don D Cake]

Cars etc are earthed during refuelling.

The fuel pump is bonded to earth.
The fuel line is conductive.
The pump nozzle is electrically bonded to the fuel line.
The car's fuel tank and filler neck and bonded to the car chassis. The car's tyres are loaded with carbon to make them electrically conductive - if they weren't massive static voltages would build up on the car's body during a journey.

Probably not a perfect earthing system but I don't think you could rely on the car owning public to attach an earthing strap to their car every time they refuelled.

Don

[GRpr]

Don

Many thanks

GRpr

[Blacksheep]

But Don,

All of the points that you mention about cars are also true of aircraft.

So why DO we ground aircraft but not cars?

For the larger aircraft at least, the much higher pumping rate generates static that can produce arcs on non-bonded and earthed fuel components. Especially when disconnecting. On cars the flow rates are low enough for static to dissipate as fast as it is generated. Unless the car is a Formula One car on an eight second "splash and dash" of course, which is why there are automatic earthing strips in the pit bays.

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Through difficulties to the cinema

[Hew Jampton]

Earthing is probably just as necessary for cars as it is for aircraft, but isn't done because, as has been pointed out, virtually all drivers wouldn't bother, and there should be some measure of earthing through the tyres. I am not sure about aircraft tyres having as much carbon content. About three years ago there was a massive recall on Vauxhall (GM) Astras to bond the filler neck to the car body, as the lack of a bond caused an explosion/fire at a filling station in Germany with, I think, fatal results. More important for aircraft because the consequences are far more severe: more fuel, more people, aircraft more expensive than cars etc.

[Agaricus bisporus]

Don, my car has those conductive tyres, bonded filler neck etc, I'm conductive, wear rubber soled shoes that should prevent an earth but if I wear an acrylic shirt my car zaps me every time I get out of it.

So who wasn't earthed? And why not?

[Blacksheep]

Yep! My car zaps me every time too.

So drivers wouldn't use earth points eh? They would if the petrol wouldn't come out of the hose unless the earth lead was connected.

Or forecourts could be fitted with those snazzy formula one type earth strips. (The undersealing could be a problem with that idea though.)

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Through difficulties to the cinema

[Don D Cake]

I had been pondering as to why my car occasionally zapped me or my wife as we got out of the car, I had assumed that I/she was discharging the car. I alway let her get out first so she would get the shock. Then the other day it came to me. I got out first and closed my door, no zap. She got out and "oh f*@!", she got zapped. I realised that we were not discharging the static build up on the car through ourselves but discharging static on ourselves through the car.

If you think about it, you are sat in your car with your best lilac nylon shirt on, sitting on a plastic seat, you feet in contact with either a rubber mat or rubber coated foot pedals with your hands holding a plastic steering wheel or gear knob. As you shift around in your seat, turn the steering wheel etc etc you build a great big static charge on yourself (someting like 10kV). You get out of the car, your shoes insulate you from the ground, you touch a metal part of your grounded car and whammo, you utter an expletive and kick the car.

[manuel ortiz]

Don D ,
Just between you and me ... open the door .... touch the metal part of it and THEN put your feet on the floor.
Let me know how it works .

Never tell your wife ....


[This message has been edited by manuel ortiz (edited 18 May 2001).]

[GotTheTshirt]

Its probably a hang over from the"old days" of using avgas and fuelling from cans.

I have seen earth points on various airfields particularly old military one that have been there for donkeys years and never checked !
With pressure fefuelling the mettalic connection is made to the aircraft before fuel flows wheres as with the avgas nozzle it could be pumping fuel before cotact is made.

[alosaurus]

Every part of an aircraft's structure is electrically bonded to every other part such that there is no significant potential diff. across it.This is tested during manufacture (most of the time) down to fractions of an Ohm.It is necessary to bond an a/c structure as carbon impregnated tyres can only dissipate high voltages(watch an a/c arcing a few feet before touchdown at night).
Cars are not electrically bonded throughout their structure (hence no bonding leads and slight difference in price).No point therefore putting a bonding lead between car and pump.
As I think someone said if you put your hand on the metal part of your door frame before your foot touches the ground you will no longer be zapped.
ELECTICITY IS THE ART OF LUCIFER.

[Lu Zuckerman]

To: alosaurus

What you stated about the electrical bonding on an aircraft is true with one exception. On the A-310 the slats and flaps are not bonded to the airframe. This causes several problems to include arcing when the flaps are retracted. Prior to retraction the flaps can build up as much as 1400 volts of static electricity. Because of the size of the flaps the arcing can last several seconds and in the process it can electrically etch the rear spar or the wing skin. In the case of the slats not being bonded to the structure this can cause an even bigger problem. According to Airbus there are two points on the A-310 that are the primary attach points for a lightning strike. One is the nose that has diverter strips to carry the charge into the airframe and the other is a partially extended slat. If lightning attaches to the slat edge the charge will arc to the nearest slat jack and into the wing structure. Because of the design of the wing fuel tanks the arcing from the slat jack will go into the fuel tank and blow the wing off.

Regarding the discharge of the airframe static charge it does not go through the tires (tyres) to ground. If this were the case the charge would have to pass through the bearings and the repeated discharge at each landing would eat the bearings due to static discharge erosion. If you look closely at the landing gear of an aircraft there is a flexible cable attached to the landing gear and it has a metal ferule attached to it. This is the first thing that hits the ground and it dissipates the charge.


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The Cat

[alosaurus]

Lu Z.
Check with any Dunlop rep and they will tell you how it costs them a fortune to impregnate their tyres with carbon so that MOST of the static charge can be dissipated through the main wheels.
On my a/c type the lead you describe is only fitted to the nosewheel to dissipate any residual.
With regard to the A310 that is interesting but it still does not invalidate my answer to the question.

[Lu Zuckerman]

To: alosaurus

On the A310 the design called for the use of “Rose Bushes” which were made of plastic and impregnated with graphite to allow the conduction of static electricity. If as I stated the slat were struck by lightening the graphite would cease to be a conductor and due to the extremely high voltages involved would become a resistor and heat up so rapidly as to cause a complete disconnect on the slat system. Since the flap system has the same “Rose Bushes” they too could be melted if a lightening strike were to pass to ground through the driveline.

The same thing would be true for rotor blades made of composite and these blades did not have a substructure of aluminum mesh under the graphite covering. I brought this problem up to the design manager on the Agusta A 129. I left the program so I don’t know if they changed the design of the blades.

Now we get to the Dunlop tires (tyres). It would seem to me that if the tires/tyres were compounded with excessive carbon to allow the bleeding of static electricity then what would happen if the aircraft were struck by lightening while it is on the ground. It would seem to me that it is the Rose Bushes all over again.

My previous statement about the bearings being in the current path would lead to a decrease in the reliability of the bearings due to static discharge erosion would still apply. That is unless they have a way of creating a bonding path that bypasses the bearings.


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The Cat

[alosaurus]

Lu
If an a/c is struck on the ground it is subjected to a special post lx check this would pick up any significant bearing damage.I don't dispute your A310 problem but if you don't believe me about the tyres (Enlish sp)speak to anyone who produces them.

[Lu Zuckerman]

To: alosaurus

What you said about the bearings being inspected after a ground lightening strike is true but it would be obvious when the inspection crew went to remove the wheels and tires (tyres) only to find out that the tires weren’t there. At least, not in the as installed state. My comparison of the tires (tyres) to the rose bushes was this. In each case the carbon or graphite that was incorporated to drain static electricity would turn into a resistor with the very high voltages and currents associated with a lightening strike. This would cause severe overtemp and the tires (Tyres) would explode. IMHO

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The Cat

[GotTheTshirt]

Lu,

The bonding wire dangling from the landing gear may be fine in theory but I can assure you the last time I saw one of those was on a car for travel sickness!

As has been mentioned the tyres (as in Dunlop) are conductive.

As for wheel bearings I have seen them with some indication of burning but not destroyed as in thousands of volts.
As all of the ball bearing provide a contact, and there are fair few on them I would imagine that normal static just passes through without harm.

Worst case senario must be a lightning strike just before touchdown !!

[Lu Zuckerman]

To: alosaurus and Got the T shirt

Here is the definitive answer regarding conductive tyres (tires) made by Dunlop and I assume other manufacturers of aircraft tyres (tires).

Dear Mr. Zuckerman

Tyre conductivity has been a contentious issue for some years now.
During the late 1930's Dunlop started advertising it's tyres as helping to
dissipate static electricity.
The US military included a statement requiring a level of conductivity for
all tyres in MIL-T-5041, which has been a governing spec for most tyres for
many years. It has always been difficult to measure due to the weather and
other environmental factors. (Two measurements taken thirty minutes apart
or even two hundred yards apart can be significantly different.)
Due to the introduction of new materials in tyres and wheels during the last
fifteen years, the conductivity of the tyres has been reduced considerably.
Most airframe manufacturers now require an aircraft to be earthed using a
cable connected to the ground during re-fuelling etc.
This matter was discussed in length at an industry meeting a couple of years
ago and it was decided to remove the conductivity from the latest
specification MIL-PERF-5041-J due to the difficulty in measuring it and the
introduction of hard tethering of aircraft.

When retreading, it is left to the approved retreader to use the compound
that best suits his equipment and provides the best performance. At Dunlop
we normally retread using the same compound as is used on the new tyres.

I hope this answers your questions, if not please don't hesitate to contact
me.

Best Regards

Ray Fletcher
Programme Manager
Dunlop Aircraft Tyres Limited

Phone: 44 (0)121 627 7227
Fax: 44 (0)121 377 7150
Website: www.dunlopaircrafttyres.com (http://www.dunlopaircrafttyres.com)

Regarding the damage to the bearings it is not the surge of static build-up that causes the most damage it is when the wheels are rolling and the balls in the bearings are moving in relation to the inner and outer races. As the ball rotates a new point of contact is made and as the ball moves away from the original point of contact a small spark is generated and this etches both the races and the balls. I once saw a demonstration where this condition was illustrated using a flashlight battery as the DC power source. Helicopter transmissions are also susceptible to this spark erosion especially in the gears. As the gears rotate a spark is generated causing etching of the mating surfaces.




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The Cat

[Bally Heck]

The danger when refueling arises if a potential difference develops between the bowser and the aircraft. The action of making fuel flow through a pipe creates a static charge. If this charge exists in the bowser and not the aircraft then nature will try to equalise the potential difference by way of a spark jumping between adjacent surfaces. A miniature lightning bolt if you will. The hose is earthed to the aircraft through metal to metal contact. Belt and braces requirements make an additional earth wire desirable. As aprons are in general poor conductors, the conductivity of the tyres is irrelevant from this viewpoint. Also the earthing is required between the aircraft and bowser and not the earth although this is also achieved during hydrant refuelling.

In the seventies a number of supertankers exploded due to static charges supplying an ignition source during tank cleaning operations.

[ShyTorque]

Someone once told me that the static charge generated by fuel flowing through a refuel hose was greater for kero based (jet engine) fuels than for gasoline based (piston engine) types.

I can't back this up by hard evidence because I have forgotten who it was; however if true it could explain why cars don't need a bonding lead at the filling station.

BTW, every wheeled helicopter I have flown has been fitted with u/c static grounding leads that touch the earth first.

ShyT

[FL390]

Thi is interesting..........I had a question in a practise GCSE Physics paper about why the aircraft was earthed! :)

[Mariner9]

Just to further confuse the issue, anti-static additive is added to Jet kero, but not to gasoline.

[jetset pete]

Sorry to go slightly back here, but in the 'car static' situation whats the physical difference between putting your hand on the metal before stepping out of the car or stepping out of the car first?
Assuming i've built up a static charge in the car why wouldn't I get a shock by earthing myself on the door before stepping out, but would by standing on the ground and doing it?
Admittedly I haven't tried this yet but I await an answer with interest.

Cheers...

[Checkboard]

In the first case (stepping out of the car) you are just earthing yourself. as the car seat is a pretty good insulator. In the second case you are earthing the entire car.

[Dan Winterland]

The USAF lost a KC10 (Military DC10) some years ago due to a lack of bonding while refuelling. The boom operator died in the incident, he had scratched the glass on the window at the refuelling station with his fingernails trying to get out.

[manuel ortiz]

An A320 was also lost at Minatitlan Mexico last year due to the same reason.
Right wing cought fire , total loss.