Grounding/bonding when refueling

[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. It is formed by conductivematerial or by a mesh of such materials.


They are also used to protect people and equipment against actual electric currents such as lightning strikes and electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.

[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/m...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-bal...efuelling.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]

Originally Posted by chewing4gum

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 and US DoI.

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.

Originally Posted by Pilot DAR

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.

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 connection

Wonder 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]

Earth Strip

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]

Originally Posted by 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.

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.