Tips on Self-Fueling

[NC Bill]

AvWeb ran this article on self fueling an airplane I found interesting.

 

http://www.avweb.com...st=email#221179

[Al Downs]

Great article. So how does all of this apply to our composite airplanes? Is grounding the nozzle effective? How about keeping the nozzle in contact with the filler/ Does that do anything?

[Ed Cesnalis]

Don't confuse bonding with grounding. The fuel trucks used to do both now they just bond.

 

The reel from my fuel trailer to my exhaust is my bond and the wire to the hangar is my ground.

 

I also contact my nozzle to the fill before I begin pumping and leave it in contact as I pump.

[N89WD]

Good article, they definitely went in depth on info.

[Doug G.]

Seems to me we have been here before. does bonding to the exhaust pipe have any affect on the wing or filler. Has anyone checked the conductivity for that path? Or are we doing things that are created for metal planes?

[Anticept]

Carbon fiber is relatively conductive. 178CT had to have a cowling repair because the battery trickle charger cable slipped off of the positive wire and came to rest on the cowling, it smoldered before the epoxy burned off and the cable fell to the ground. FDUSA had that happen on a plane they were working on once too.

 

The exhaust pipe is the only place we can safely clamp to. I suspect that static can discharge through the fuel lines (fuel is a little conductive, but not much) and carbon fiber just fine.

 

Also, tires have carbon in them to encourage discharging the static to the ground.

[Doug G.]

Yes, carbon fiber is conductive, but that doesn't tell me anything about the conductivity path from the exhaust pipe to the fuel filler. If the carbon fiber were enough there would be no reason for pass through grounding for the instruments and everything could be grounded to the airframe.

[Doug G.]

Still doesn't say what the conductivity path is to the filler. Is the filler in direct contact with the carbon fiber in the wing? is the putty that holds it there conductive? Is the wing solidly grounded to the body of the plane?

[Anticept]

Yes, carbon fiber is conductive, but that doesn't tell me anything about the conductivity path from the exhaust pipe to the fuel filler. If the carbon fiber were enough there would be no reason for pass through grounding for the instruments and everything could be grounded to the airframe.

 

I said relatively conductive. Static electricity has a very high potential and only needs a tiny amount of conductivity to discharge. The resistance of carbon fiber is high enough to interfere with return electrical paths in any medium and low voltage applications though, and will heat up if you push any more than a few hundred mA, which is why it is not used as a ground path.

 

Carbon fiber is a decent conductor, but it is not an ideal conductor.

[Doug G.]

Evidently no one has ever checked to see what the actual situation is. I don't buy the, " It has always been done that way" argument. What is the grounding path to the wing and filler? The wing root and fuse are painted, the fuel tank is isolated by rubber tubing and Hylomar, the lights are directly grounded, the file is held with putty against paint. If there is a ground to the wing it work be through the flap and flaperon controls which have plastic inserts in the control linkages.

So I'll ask the question again. Is there a path from the exhaust to the filler?

This might be a job for those fancy Fluke meters some of you own. So far I don't think anyone knows.

[Anticept]

Evidently no one has ever checked to see what the actual situation is. I don't buy the, " It has always been done that way" argument. What is the grounding path to the wing and filler? The wing root and fuse are painted, the fuel tank is isolated by rubber tubing and Hylomar, the lights are directly grounded, the file is held with putty against paint. If there is a ground to the wing it work be through the flap and flaperon controls which have plastic inserts in the control linkages.

So I'll ask the question again. Is there a path from the exhaust to the filler?

This might be a job for those fancy Fluke meters some of you own. So far I don't think anyone knows.

 

No, it is not in the design to make it directly grounded (like bonded connections). This is why the CT is not allowed to be used in IMC. Composite aircraft used in IMC have to have a conductive copper mesh built into the skin to keep static from arcing around the body of the airframe (metallic aircraft even use bonding straps, there is that much static buildup when flying through the clouds), and provide a lightning path. It's not a serious enough issue for VFR only aircraft to worry about because the carbon fiber is conductive enough to transfer static buildup, and there isn't much buildup in VFR.

 

EDIT: The fuel will conduct any fuel tank static buildup through the fuel lines to the fuel valve and engine. However, this isn't really a factor at all, because the tanks aren't where static builds. The risk of static discharge is from the nozzle to the fuel port. That's semi-grounded through the frame.

[FlyingMonkey]

I fuel from five gallon plastic cans certified for gasoline. I make sure when pouring that the nozzle touches the metal tank neck ring, and that I have skin contact with the gas can, the body of the airplane, and the tank neck ring so everything is at the same electrical potential. So far I'm still here.

[Doug G.]

Anticept, the CTs have a " rubber" portion of line between the tank and engine, so that theory does not apply. (as per my last post).

I use Andy's fuel system and I am also still here. That is simply anecdotal evidence. I would like to know the reality rather than a bunch of guesses and how it works in other planes. (I don't don't believe the CTs are made with the copper mesh.)

[Anticept]

Anticept, the CTs have a " rubber" portion of line between the tank and engine, so that theory does not apply. (as per my last post).

I use Andy's fuel system and I am also still here. That is simply anecdotal evidence. I would like to know the reality rather than a bunch of guesses and how it works in other planes. (I don't don't believe the CTs are made with the copper mesh.)

 

CTs do not have a mesh. And, I was referring to static being conducted via the fuel itself. It's such a low conductivity though, and honestly, I wouldn't worry about it. All these parts shielding the tank with non-conductive material also means it is very very hard for static to build inside the tank in the first place.

 

Roger also already said that if you check the fuel port and the engine exhaust, there is continuity.

 

Also, I was referring to the fuel itself being used to conduct the static to the engine. Fuel has a high resistance, but it can conduct! Still, I'm not worried about static inside the tank. It's the skin of the airplanes that builds up.

 

Finally, I would argue that it is better to follow the "it's always been this way" until you actually do find empirical evidence that you should do the opposite for something that has had a history of injuries and fires

[Doug G.]

How many ohms is it from the exhaust pipe to the filter neck, Roger? (Sorry, I missed your reply above.) This it's what I have been trying to find out. I appreciate someone having the capability to do this.

[Tom Baker]

You're right about the rubber piece of hose in the fuel line at the header tank. The carbon fiber wing spars are touching each other and the fuselage. The carbon fiber makes contact through out the plane.

 

Roger, I am pretty sure that the spars do not touch the fuselage, and are just tied to each other. The pins on the ends of the wiing do touch the fuselage, and the forward pin mounts in the end of the fuel tank. I think any conductivity would be carried through these pins, the flap linkage, and the aileron linkage.

[Tom Baker]

It's just our definitions of touching that are different.

 

They do touch as they pass through the tunnel and then again when you put the spar pins in place. Plus the two alignment pins. Conductivity is carried through out the entire plane. I used a meter and put a charge down on the cowling and went around the plane. There is conductivity. It doesn't make any real difference where we think things touch. Every thing is all interconnected somewhere in one or multiple points and you only need one point of contact to complete the circuit.

 

Roger, there is no difference in the definition. When the wings are installed on the airplane the spars do not touch the fuselage at any point. They sit inside the hollow tunnel with clearance on all sides, including the main pins which do not touch either. If the spars were touching it would create a wear point which could lead to a failure.

[Anticept]

There are 4 stainless steel pins, two on each wing, that go into the fuselage. The carry through spar is for the wings to keep them straight, while the plane itself hangs on the 4 pins. See here:

 

http://documents.flightdesignusa.com/CTLS%20MM%20rev.4.pdf Page 4-3, Figure 3, for an example. The pins are a couple inches long,

[FredG]

Tom and Anticept, thanks for the explanation regarding the spars and the rectangular section tunnel that "houses" the spars. Does that mean that the tunnel is not structural? It seems to have very thin sides (at least front, back and bottom), leading me to wonder if it has no structural role in the fuselage (and is only there for cosmetic purposes). Thanks.

[Anticept]

Tom and Anticept, thanks for the explanation regarding the spars and the rectangular section tunnel that "houses" the spars. Does that mean that the tunnel is not structural? It seems to have very thin sides (at least front, back and bottom), leading me to wonder if it has no structural role in the fuselage (and is only there for cosmetic purposes). Thanks.

 

The housing is not structural at all. It is strictly cosmetic.

[Doug G.]

Am I right in assuming then that the putty holding the filler on the wing is also conductive?

I realize that if there is conductivity through to the tires static will drain from my plane as it sits. I always place my conductive plastic containers on the floor of the hangar before fueling. And make physical contact with the plane in the process of refueling. ( In case anyone gets uptight, they containers draw air from inside the wing tank to displace the fuel that comes from the container so fumes aren't vented externally.)

[Anticept]

Hi Tom,

 

I disagree. The spars do touch especially when you apply 200 in/lbs of torque on the pin retainers. The metal rings inside the spars where the pins go through touch when tightened down.

 

Like I said, it really won't make a difference since the whole aircraft is linked in some way shape or form. Put a power supply on the right wing fuel insert then take a reading on the other wing insert or put the power supply on the exhaust pipe and take a reading on the fuel insert. There is conductivity.

 

I think he meant they don't touch the fuselage in any place, that's the pins doing that.

[Tom Baker]

Roger, the spars do touch each other. The main pins go in the metal bushings in the spars, but none of this touches the fuselage. You could pin the wings together on a pair of saw horses and it would be the same. There are no metal inserts in the fuselage for the main pins. The alignment pins do touch the sides of the fuselage, this is where the conductive path is.

[Ed Cesnalis]

I guess I am wondering why question if Roger has tested the plane and says he shows a full circuit. Does it matter what all the glue, bolt, washer, rubber parts, hoses, links, lines, cables, tubes, carbon fiber shell, leather in the seats, glass in the panel, rubber in the tires do if there is conductivity end to end?

 

No.

 

What does matter is that you can indeed put a static ground on the tailpipe and fuel the plane without concern for static discharge and fire, UNLESS, you somehow managed to goof up putting the clamp on the tailpipe.

 

We have continuity but at what level of resistance? Is it a reliable path that exists in all CTs including mine?

[Ed Cesnalis]

I used my multi-tester and found that I do have continuity from my gas fill to my gas nozzle when my bonding cable is attached from my fuel trailer to the exhaust. I guess I have no need to contact the fill with my nozzle.

 

I have continuity from my gas fill to my hangar frame which is my ground.

 

The airframe does does have continuity from metal part to metal part like exhaust to wheel pant fastener but there is none to painted surfaces. If you think you are discharging just by touching the painted wing when filling from cans I don't think so.