Kitfox Static discharge Fire

[Guy Buchanan]

Well I'm sorry to say you've made me feel a lot better about the silly-ass grounding wire I always get kidded about when I refuel. I've been using it for about 12 years now and occasionally think about giving it up. Not so much, now.

[PaulSS]

I, too, like the idea of the 10' wire with crocodile clips but I was thinking about the order of connection of the wires and real world practice, rather than electrical theory. It's not really something I'd thought about until this thread.

You know how sometimes if you touch a live wire (our statically charged fuel filler lip in this case) and a ground you get a spark, well clearly that would be a bad thing with our open filler cap and fuel vapour. Now I'm certainly guilty of grounding the exhaust pipe, then taking off the filler caps and filling up. Not a problem at the moment because I'm flying an RV, so all the wigglies go straight to ground when I connect to the exhaust pipe. BUT if I was in a Kitfox with my 10' wire, connected ground to the exhaust pipe, crocodile clip to the exhaust pipe, then up to the wing where the filler cap comes off and I then clip the other end of the 10' wire to the filler cap lip, it strikes me that is the time when I'm going to get a spark at the filler cap, as the static electricity at the cap connects to ground through the croc clip, which is a bad thing.

I reckon the 10' wire is a great idea but, if it's not blatantly obvious and I'm the only one who has never thought about it before, I think you need to ensure that you first connect the crocodile clip to the (open) filler cap lip, then to the exhaust pipe and/or direct to ground. This way the static wigglies are already queued up in the wire from the filler cap and will travel to ground when the wire is connected to the exhaust/ground. If we get a spark there then there's no problem, unlike if we do it the other way round.

As I say, probably obvious to everyone else but I'm glad of this thread for making me think about it

[PapuaPilot]

Hats off to you Guy. You aren't being silly. I still need to test my plane with aa ohmmeter between the filler necks and the exhaust stacks (and ground too).

The other day at work I checked a Cessna 206 with a precision tester for bonding to test how the bonding a certified plane is. I checked it from a good airframe ground to the exhaust and the results were not good, it had infinite resistance. Next I checked it from a wheel axle bolt to ground and got .003 ohms (which just meets certified specs for bonding). FYI it is our standard practice (where I work) to hook the ground cable to the axle bolts when refueling.

What this tells me is that using an exhaust system isn't a good idea, even if there is proper bonding to the tank filler neck. I think there are several reasons. Stainless steel is not a good conductor and there are slip joints and gaskets the may be filled with exhaust residue.

Using the landing gear on a Kitfox with the Grove gear would not a good solution because of the plastic mounting blocks and paint. Even if there is bonding from the fuel tanks you would need to find a proper airframe ground to attach the fuel ground cable to.

FYI bonding is making the entire plane one potential. It is needed for several reasons: for static dissipation, to provide a path for lightening strikes and prevention of sparks when refueling. Bonding is done by connecting all of the components together with bonding straps. When you hook up a ground cable before refueling it makes the aircraft the same potential as the fuel pump system, which should be hooked to earth ground too.

In a Kitfox I really doubt there is any bonding obtained through the fiber glass tanks, nor through the fuel lines (I have rubber hoses from the wing tanks to the header tank). I think the only way to get proper bonding would be to connect the filler necks to the airframe or to connect directly to the filler neck as some are doing.

[Esser]

Now maybe this is an oversimplification since the fuel flowing out of the Jerry-can can create static but couldn’t you just touch the Jerry can spout to the top of the fuel cap BEFORE you open the cap and now the two items will have equal charge?

[jiott]

Josh I was thinking the same thing; jerry can or fueling hose nozzle.

[Dave S]

Josh & Jim,

Been studying up on this deal a bit and found some old military information on refueling - a lot had to do with refueling under less than ideal conditions - One of the recommendations was that before a person pulls off the fuel cap - touch the dispenser or funnel to the upwind side of the fuel cap to equalize the charge between tank and the dispenser so if a spark occurs, it will not be in the midst of the fuel vapors- then pull off the cap and attach the funnel or touch the dispenser to the ring and keep it there till the fuel is done flowing - the idea on the second part is to process any charge that might develop once the fuel starts flowing.

[amerkarim]

Hi All,

Thanks for all your comments and the developing discussion on this subject, and for your tips about fixing my wing. Fortunately all the insides were fine so its just fabric and paint, that should be easy to fix. Will post some pics when its done.

For my 2 cents ... I think this is potentially a very serious problem. It certainly has me paying attention to grounding on my airplanes now. I had also emailed John McBean about it with the possibility of him adapting the his kit to allow for better earthing of the tanks. I will get back to him once my plane is fixed with the mod that I am going to do.

Having your plane burn to the ground, or worse... having someone or yourself get burnt by a fuel fire is no fun, and I have had the misfortune of seeing 2 pilots in hospital with severe burns fighting for their lives after catching fire in a plane crash.

If there is a simple solution that requires a bit of planning or forethought, or a simple modification that can be done afterwards to our planes that will reduce the risk of this happening, then that is a worthwhile investment of our time. I promise you, even if the chance is 1 in a billion, it is worth it. You don't ever want to be that guy in the hospital bed.

I was thinking of going with the modification of wrapping some wire around the filler neck, but the way my fabric is done, I would have to strip the other wing as well, and I am not keen on that. I am going to see how well earthed the flapperon mounts are. I may just end up drilling a hole in the gas caps and then riveting an earth cable from them to the flapperon mounts (they are in contact with the tank lips from the inside), and then hide the cable under a strip of fabric. By having a simple small connector and a few cm of slack, one could disconnect the gas caps if needed. An earthing strap from the engine block or airframe, to one of the exhaust pipes should then be sufficient to bypass any gaskets that act as resistors. A multimeter should confirm the earthing before I start. and then a short earth rod and a crocodile clip to the exhaust should discharge all the static before I start fuelling in the future.

That tip about wiping plastic jerrycans with a wet cloth is also one worth remembering for next time.

Thanks and regards to you all

[Rodney]

This has been a really valuable discussion. I've learned a lot from all the posts.

I'm not much good with electricity so tell me if you think this would work, ok?

I checked continuity between a bolt that holds the "N" strut on and the exhaust pipe. Good continuity there. I checked between the filler neck to the bolt and then the filler neck to the exhaust pipe and no continuity, as expected.

So I bought a static discharge wrist strap like computer tech's use when handling mother boards which I guess can be ruined by a static electrical discharge.

I clip the alligator clip to the "N" strut bolt and as I take the fuel cap off I make sure to touch the exposed metal on the fuel filler neck with my hand that has the wrist strap on. Then, when I pick up the fuel nozzle in the other hand I make sure I touch the filler neck again with the hand that has the wrist strap on it. I hope that makes sense.

Am I right in thinking I have now equalized the potential between the fuel filler neck, the rest of the airplane and also the fueling nozzle??

Thanks
Rodney

[rv9ralph]

I have been following this thread, and re-read the initial post about the sequence of the incident. Here are some thoughts. The static discharge happens when there is a difference of electrical potential, that has been agreed upon. In this instance the discharge happened when the fueler touched the aluminum funnel to the fuel tank causing the discharge (ignition spark). If he had discharged his potential to the airframe away from the fuel tank, there would not have been the spark to ignite the fuel fumes.

I was talking to the owner of the FBO at KLVK and discussed this event. We agreed that the solution is to make a connection of the fueling apparatus (funnel, fueler, nozzle) to the airframe away from the combustable fumes.
The grounding clip at the fueling stations does not ground the aircraft, but connects the aircraft to the fueling nozzle, eliminating the potential for a spark at the fuel tank. (The fuel hose has a conductor from its base to the nozzle).

I hope this make sense.
Ralph

[amerkarim]

I agree with the principal that static electricity builds up over the wing and concentrates at the filler necks, or can be contained within the fuelling can.

The problem is the discharge of the static from the filler neck of the fuel tank to the can or to the person who is acting as the earth. This discharge can be large enough to produce a spark if the tanks are not grounded in some way and the static has not been discharged beforehand.

In principal, you can just tough all around the wing with your hand and touch the filler neck and gas caps.. by doing so you are the earth and you are discharging the static through your body to the ground, but even this action can create a spark between your hand and the filler neck. As you are most likely to have fuel vapour around the gas caps / filler neck ... in the right conditions of dry air, high temp, this can start a fire.

The spark will only happen at the point of contact of the earth to the statically charged airframe. As the airplane is sitting on rubber tyres, it is insulated from the earth anyway. and so will tend to collect static electricity all over the airframe. This static can be discharged by making a contact from the airframe to the ground either by touching the airframe or using a grounding wire and a crocodile clip. The key is to ground away from the fuel source... so even if a large spark is generated on contact... it is away from the fuel vapour source.

By connecting the fuel tank filler necks to the airframe.. when the static is discharged by touching the exhaust pipe or any earthed part of the airframe away from the fuel vapour source.. as long as their is continuity between the wing and the airframe.. all the static will be discharged, even that around the wing and fuel filler caps.. and any spark generated will be away from the fuel vapour.