Connecting "bonding" clamp to exhaust pipe during refueling - apparently not a good idea

Lndwarrior

Lndwarrior

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I use to connect to the exhaust until one day I taxied away from a fuel pump with the ground (ie: bonding wire/clamp) connected. I really thought I had damaged something but the ball joint on my exhaust saved me, I think.

Never did that again.

I had been trained to connect here, but that was a million years ago before we got this incredible learning tool called the internet.

It seems like it should work. In my case I have a ball valve and the exhaust gasket between the clamp and the engine. But even then you would think the contact on the ball joint and the exhaust bolts would be enough.

Stainless steel is less conductive that other types of metal so that may be a contributing factor.

It would be interesting if someone with knowledge in this field could fully explain this.

 
With my aircraft being made out of wood, my best options are the exhaust, propeller, or carburetor. The latter is usually a bit oilly, don't like putting an alligator clamp on an aluminum prop, so....

Ron Wanttaja
 
When I started working the line, I was trained to put it on the exhaust or any other unpainted metal part. For some airplanes, we used the gear. Larger airplanes have designated grounding points.
 
He mentions one thing which I think is bad advice. Touching the nozzle to the plane before opening the cap will dissipate any pre-existing charge, but static is also generated by the fuel flowing through the hose, so you want to keep it grounded while fueling.

I doubt the lower conductivity of stainless, even if it's a couple hundred ohms, will matter; it would just take a few nanoseconds longer for the charge to equalize.

If your fuel caps are retained by chains, that's probably the optimum place to ground.

If the tank is secured with padded (non conductive) mounts and plumbed with a rubber line, there may be no conductivity with the rest of the plane.
 
The exhaust does have a gasket, but the studs and nuts connect the exhaust to the engine electrically, and the engine is connected to the airframe electrically also. So I say that's good. It won't help you if you taxi away from the pump with the ground wire still connected though. That mistake is on you.
 
For static purposes you don't need that low of a resistance. Even a few thousand ohms is good enough.

A good walk-around after refueling will catch the grounding wire.
 
frfly172 said:
Have been using the exhaust for years without a problem.
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In fact millions of refuelings have happened as such, with no explosions. Makes one wonder if the bonding is truly necessary in most cases.
 
Let'sgoflying! said:
In fact millions of refuelings have happened as such, with no explosions. Makes one wonder if the bonding is truly necessary in most cases.
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I'd say it is. Ever hear of the Hindenberg? That was caused by static discharge from the hull to the ground ropes.
 
I have seen an aircraft catch fire from a static spark. Although that refueling procedure was pretty negligent, they had drained fuel into a plastic bucket and now they were pouring that fuel back in the tank, using a metal funnel. Big spark jumped from the funnel to the bucket, the bucket caught fire, panicked bucket holder dropped it on the fuselage setting the whole thing on fire.
 
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I have a friend with scars from a fueling fire sparked by static and know two other guys who lost planes to static spark fueling fires.

Most of us use bonded fuel hoses so an additional bonding wire is redundant. Most fuel fires don’t occur while filling with good equipment. They happen when doing simple chores badly, like draining fuel into an unbonded container or hand pumping fuel out of a drum.

If you want to be safe and don’t have a bonding wire? Use a rag around the fuel nozzle at the filler cap to block vapors from mixing with outside air. Combustible fuel vapor concentration has a very small range. Keep vapors in the tank and they can’t burn since the concentration is too rich. That’s why gas station nozzles have those rubber bras.
 
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Dana said:
If your fuel caps are retained by chains, that's probably the optimum place to ground.
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Bad idea. If there's a spark when the clamp gets close to the chain, you might ignite vapor from the tank at that location. Don't make the connection in close proximity to the fuel.

Dana said:
If the tank is secured with padded (non conductive) mounts and plumbed with a rubber line, there may be no conductivity with the rest of the plane.
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I don't really know, but in that case wouldn't the tanks have grounding straps to the airframe? It's also possible that the fuel level senders would create a ground path.
 
Half Fast said:
Bad idea. If there's a spark when the clamp gets close to the chain, you might ignite vapor from the tank at that location. Don't make the connection in close proximity to the fuel.
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Good point. I keep thinking on the static generated by flow while forgetting about the potential difference before you start.

Half Fast said:
I don't really know, but in that case wouldn't the tanks have grounding straps to the airframe? It's also possible that the fuel level senders would create a ground path.
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Come to think of it, even rubber fuel hoses are made with carbon black to make them conductive. There will be a fair amount of resistance, but as pointed out above it doesn't need to be that low.
 
When I was flying a PA-46, I was pleasantly surprised that they have a built-in, designated grounding clamp attachment point on each main gear leg. Unpainted, and a good size for the clamp. I don't know why more light airplanes don't have a similar designated grounding point - instead we often have to hunt for a good, conductive, grounded, unpainted surface that we don't mind possibly damaging a little each time.
 
Dana said:
Come to think of it, even rubber fuel hoses are made with carbon black to make them conductive
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Fun facts.

You can replace spark plug wires on an engine with black rubber vacuum hose (5/32" fits nicely on automotive spark plugs) and the engine will run. (Yes, I did that.) Don't touch the hose... (No, I didn't do that.)

At one time, it was common for cars to have white tires. Now, they are all black. The reason: Radios. The carbon black in the tires conducts away static electricity and improves AM radio reception.
 
Dana said:
I doubt the lower conductivity of stainless, even if it's a couple hundred ohms, will matter; it would just take a few nanoseconds longer for the charge to equalize.
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With very low amperage, as with static, you'd need a lot of resistance to make any difference. E = I x R.
Dana said:
If the tank is secured with padded (non conductive) mounts and plumbed with a rubber line, there may be no conductivity with the rest of the plane.
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FAR 23 required that tanks be bonded to the rest of the airframe.

SECTION: Sec. 23.867
Amendment Number: 23-49, Effective Date: 03/11/1996

TITLE: [Electrical bonding and protection against lightning and static electricity.]

SECTION RULE: (a) The airplane must be protected against catastrophic effects from lightning.
(b) For metallic components, compliance with paragraph (a) of this section may be shown by--
(1) Bonding the components properly to the airframe; or
(2) Designing the components so that a strike will not endanger the airplane.
(c) For non-metallic components, compliance with paragraph (a) of this section may be shown by--
(1) Designing the components to minimize the effect of a strike; or
(2) Incorporating acceptable means of diverting the resulting electrical current so as not to endanger the airplane.
 
Part 23 only governs Part 23 certificated airplanes. N/A to the vast majority of small aircraft.

Bonding fuel tanks to the airframe is common practice because it’s common sense.
 
RussR said:
I don't know why more light airplanes don't have a similar designated grounding point - instead we often have to hunt for a good, conductive, grounded, unpainted surface that we don't mind possibly damaging a little each time.
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They do. They are more commonly called 'tie-down rings'.
 
Always best to check section 8 of your POH (Handling, Servicing, and Maintenance) if your plane is new enough to have that.

It really is on a per-type basis where the best grounding/bonding spot is. Exhaust/nose gear is common, but won't always work. DA40 for example has a specific grounding point under the wing.
 
Lndwarrior said:
I use to connect to the exhaust until one day I taxied away from a fuel pump with the ground (ie: bonding wire/clamp) connected. I really thought I had damaged something but the ball joint on my exhaust saved me, I think.
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I have an idea: Disconnect the ground wire before you taxi away.
 
camorton said:
They do. They are more commonly called 'tie-down rings'.
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In my experience, many tie down rings are painted, or are the retractable, swing-out type. Or if they're steel, the paint was scratched off long ago but replaced by a layer of rust.

In any event, the tie-down rings are often convenient, but certainly aren't specifically designed to be grounding points. And of course, they usually work (when they're there) - but that doesn't mean the grounding point couldn't be better.

Here is a PA-46 main landing gear. Note the grounding attachment point, it's mounted on the forward part of each upper main gear leg (so it's definitely electrically connected to the rest of the fuselage), and is a good size for normal ground clamps and even has a little ridge to give the clamp something more to grab.

1734102449152.png

Another view:

1734102481604.png
 
Lndwarrior said:
I use to connect to the exhaust until one day I taxied away from a fuel pump with the ground (ie: bonding wire/clamp) connected. I really thought I had damaged something but the ball joint on my exhaust saved me, I think.

Never did that again.
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Will you never forget to disconnecting the bonding wire or do you never bond your aircraft after that incident?

Correct me if I'm wrong (I'm not an electrician nor an electrical engineer), but I didn't think the purpose of the bonding wire is to ground or remove static electricity, but rather to bond the systems (the aircraft and the fuel pump) so they are at the same potential so the static electricity that builds up from the fuel moving through the host, etc. doesn't discharge in the form of a spark. If there is no difference in electrical potential, there is no discharge.
 
My mechanic isn't overfly fond of using the stack as the ground point because he finds a lot of the clips scrape up and cause eventual damage to the end of the stack. I just use the part of the nose gear that you'd connect the towbar to.

Amusingly, nearly every line guy who goes to ground my plane puts the clamp on the nosegear centering tab. I have to point out to them that that is a piece of (non-conductive) bakelite.
 
Our club Cessna's had an active life, and tie down rings had no paint or rust on them.

Most of us used the pilots side ring, well away from the propeller, and readily seen when returning to the pilots seat.

Our preferred alternant was the top casting of the nose gear, and as in Russ's picture, a solid connection to the structure of the plane.

Taxi straight up to the pump or truck, necessitating a push back before starting, and removal becomes obvious.

As others have pointed out, exhaust pipes have been used for at least half a century with good results, as any "insulation" is still a fairly good conductor at these voltages and slight required current for effective grounding.
 
Most grounding clamps I have seen have copper or brass (bronze?) teeth, which are softer than SS. I suppose it's possible to damage the exhaust pipe but it would take a thousand years.

As for driving off with the cable connected, IMO that is an argument in favor of the exhaust or nose gear. I personally would be more likely to see a wire off my nose than a wire off to the side on a tie down loop.

I always wind up tripping over the wire with the ladder or tangling it up with the hose, so forgetting to disconnect it is not usually a concern for me!
 
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I use my own fueling equipment. My fuel hoses are internally bonded and all my fixtures and fittings are steel. My tank is grounded and the pump is bonded to the wall-mounted power switch. I don’t use a separate bonding cable to the airplane. I had one for my truck-mounted tank but not my home tank.
 
I keep the nozzle in hand as I climb up on the tires and cowl step. I presume all that must equalize it pretty well as I’ve never seen or been able to create a static spark in 30 years of doing it. I’m much more careful when wearing static-producing clothes like nylon winter jackets and gloves. With those I can feel the static discharge when I touch the plane with bare skin.
 
AV8R_87 said:
Sounds like you actually did that.
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Lol! No. Sometimes the dang clip hangs so loosely that it seems like it would just fall off. Add engine start, vibration, and prop wash and if it’s still hanging on for dear life then taxi will disconnect it.

I think doing the continuity test like in the video is a good idea though to make sure the exhaust is ok to ground to. I’ll check that next time at the the airport. Thanks to @Lndwarrior for posting.
 
flyingron said:
Amusingly, nearly every line guy who goes to ground my plane puts the clamp on the nosegear centering tab. I have to point out to them that that is a piece of (non-conductive) bakelite.
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I've taken my Fly Baby to full-service fuel maybe three times in the nearly 30 years I've owned it. Each time, the line person has clipped the ground to the non-rusty, non-painted, shiny steel tie down ring in the wing.

Had to break the news to them that the steel ring was bolted to non-conductive spruce.....

Ron Wanttaja
 
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