Fuel line installation

[nblight]

I havent chosen an engine yet but are building up the fuel tanks and plumbing in the wings of my ch300.

I will be putting in aluminium fuel lines (.375 OD 5052) in the wings, i will have 2 tanks in each wing aux one connected to the mains via transfer pump and vents using standard AN hardware etc. My question is when installing the aluminium fuel lines is there any allowance for movement eg. when running a straight fuel line from connection A to B should i put a bend in it somewhere to cater for any movement similar to what you would do in the firewall to cater for engine vibration?

Thanks

Nick
CH300

 

[proppastie]

I looked up my Mooney Manual, and I believe there is flexible hose between every hard line. Mil-H-6000 3/8 ID.

 

[donjohnston]

I use Adel clamps to support any long runs. For firewall penetration, I just use bulkhead fittings. For anything that needs to accommodate movement, I used braided flex lines.

 

[Toobuilder]

You do not want the line to be a structural element. A good rule of thumb is to have at least 90 degrees of cumulative bend in the line to cripple the column and allow it to move around a bit.

 

[Marc Zeitlin]

My question is when installing the aluminium fuel lines is there any allowance for movement eg. when running a straight fuel line from connection A to B should i put a bend in it somewhere to cater for any movement similar to what you would do in the firewall to cater for engine vibration?

Become familiar with AC43.13-1B, in this case, chapter 8, section 2. Paragraph 8-31(c) says:

...Never install a straight length of tubing between two rigidly-mounted fittings. Always incorporate at least one bend between such fit- tings to absorb strain caused by vibration and temperature changes....​

There are a couple of images that may help. Also see Chapter 9, section 2, for info on installing hoses - it's the hydraulic section, but is certainly applicable to fuel hoses too.

I STRONGLY recommend to my customers that when replacing hydraulic or fuel hoses, they use AE666 teflon/SS hose with firesleeve or AE466 teflon/SS integral silicone firesleeve hose. Rather than the standard 8 year or so life for rubber hoses, these are lifetime hoses, and don't cost much more. Do it once and be done with it - eliminating maintenance downstream will make you (or a future owner) happy.

 

[Victor Bravo]

Yes, definitely have some "stretch" or "compliance" in the line. The last thing you want is for the wing structure to be pushing or pulling on the fuel line. If you use Adel clamps to secure the lines to ribs and bulkheads, I would suggest that you adjust/shim/select the Adel clamps so they prevent the line from "bowing" up and down, but they do not clamp the line so tight as to prevent the line from "sliding" a little back and forth through the clamp.

 

[Winginit]

Each section of fuel line installation will have its own set of circumstances and its own solution. Generally incorporating bends isn't needed other than for purposes of routing around an impediment or simply for changing direction to reach a designated place. Ergo most lines won't be perfectly straight runs and will incorporate some change of direction. Bends should normally be larger rather than smaller and less rather than more. By that I mean if you need to move the line from its current position to a position that is parallel but an inch or two left(right ?), its preferrable to use two smaller angle bends (30-45 ?) rather than two 90 degree bends. Generally I try to place some type of bracket/clamp/support on each run of tubing. Longer runs get multiple supports. Vibration is normally the killer of unsupported tubing and a simple clamp/bracket properly done will usually eliminate that problem.

In the case you mention of running between two tanks located next to each other in the same wing....You should probably make the transfer line between the tanks incorporate some type of bend by welding the tank plugs in slightly offset locations, thereby needing a couple of bends incorporated. Normally I would expect the line to be run from near the bottom of one tank to near the top of the other tank thereby naturally incorporating some type of bend. Thats if the design of the airplane is such that you have that lattitude. You might want to incorporate an inspection cover if possible, and maybe consider using a Stainlees Steel line if its a difficult access area.

 

[Dan Thomas]

A solid line between two closely-spaced tanks is not a good idea. Tanks tend to move a bit and they will place stress on any short line, whether there are bends in it or not. Then there are thermal stresses as the tanks and structure they're in warm up and cool down. And If they're in a wing, that wing will bend a little, which changes the tanks' relative locations, and the line doesn't like that, either. Fill the tanks and their sidewalls will bulge a little, and a short, stiff line will resist that and maybe start a crack in the line or around the welded boss in the tank.

In certified installations like that they use larger nipples on the tanks and a short length of hose clamped to each nipple.

In the fuselage you want to minimize non-metallic lines if possible. Cessna uses aluminum tubing, with short (3") rubber hoses and clamps sometimes used as flexible couplings where necessary. Any line will have bends in it whether necessary (for routing/clearance) or not, and allowance will be made for some means to keep the line from being stressed by the factors I listed. In the older airplanes we usually encounter things like deteriorated cushions in Adel clamps letting the line chafe around or move enough to vibrate against the edges of passage holes in the structure, or they get beat up by the sharp ends of too-long interior upholstery panel screws, or they corrode due to moisture (condensation). Or low spots that don't belong there, accumulating moisture that eats them out from the inside and presenting the risk of ice blockage. We find those connecting hoses old and cracking and crumbling and leaking. An annual inspection means an inspection, not a walkaround and signature.

 

[Winginit]

I think eventually any system you design and install will suffer from deterioration of some sort.... somewhere. Rubber on Adel clamps will eventually deteriorate, almost certainly there will be some chaffing somewhere. Inspections are a good thing and when issues are noted, they need to be addressed. What a builder has to do though is to ask himself , "What is the best and most reliable way to do each tube and each connection". Personally I would prefer to not use rubber hose anywhere that I can use something better. Rubber hose is notorious for deterioration over time, and some types of fuel don't always work well with certain types of rubber.If I did use rubber, I would prefer to use rubber only where its in a noticeable location where the pilot would routinely see it. Inside a wing I would prefer something other than rubber. Now that doesn't mean that rubber won't work for someone, but it would not be my first choice. Also, I don't know how you plan to locate your pump,or the pressure you plan to use, so the use of rubber might not be an option anyway. There are a lot of options available today that were not available when most certified airplanes were built. Some of them make routing much simpler and they often incorporate better materials and outside shielding. They are expensive to buy, but you have to buy more fittings when using solid tubing...so they somewhat offset the cost of one piece flexible hose. They make fittings that can be assembled in your vice. Might be worth looking into something like Earls or Frazzoli. If you look on Utube you can find videos showing the simple assembly methods.

https://www.youtube.com/watch?v=Es8oMQ6noLI

https://www.youtube.com/watch?v=3DXxR-1elak

 

[Dan Thomas]

I have replaced rubber hoses that were in the airplane since it was built 50 years ago. Crumbling but not yet leaking. Some airplanes, if they're in damp environments or really hot places, will need hose replacements long before that. Cessna wants them replaced every ten years and inspected every 100.

And those old hoses weren't made of the better stuff we have now. There is no reason to avoid their use, in short sections, for things like vent connections. If you have to have solid couplings you're going to need threaded bosses on the tank instead of a nipple, and a bunch of fittings and access for their assembly. It can get difficult. Once you start installing systems in an airframe you realize how tight things can be, the difficulty of connecting things in certain places, and the reasons why an OEM used rubber in the first place.

Fuel systems are one of the biggest factors in homebuilt aircraft accidents. A good study of the relevant sections of FAR 23 are instructive. For more detail, the Canadian regs are exhaustive: http://www.tc.gc.ca/eng/civilaviation/regserv/cars/part5-standards-523-sub-e-274.htm#523_951

 

[Marc Zeitlin]

I have replaced rubber hoses that were in the airplane since it was built 50 years ago.

Yup - some rubber hoses are ancient. But I've seen rubber hoses that were cracked (and leaking) after 15 years. So you never know.

Crumbling but not yet leaking. Some airplanes, if they're in damp environments or really hot places, will need hose replacements long before that. Cessna wants them replaced every ten years and inspected every 100.

Do you have a reference for the 10 year lifetime? I've searched high and low for rubber hose lifetimes, and the hose MFG's just say to look at the aircraft MFG's documentation, and I've seen a COUPLE of references to 5 - 8 years. If I could find a definitive reference, I'd really appreciate it.

Thanks.

 

[Winginit]

I guess from my perspective certified airplanes originally came with rubber because that's all that was available at the time, and fuel was basically of one type. Once the rubber was used, it was therefore the only option for future replacement because of FAA regulations. In a home built, absolutely none of the aforementioned things are relevant....so why use rubber? We all know there are better things available today, so why choose a lesser quality product when you don't have to meet an archaic regulation?

 

[Dan Thomas]

I guess from my perspective certified airplanes originally came with rubber because that's all that was available at the time, and fuel was basically of one type. Once the rubber was used, it was therefore the only option for future replacement because of FAA regulations. In a home built, absolutely none of the aforementioned things are relevant....so why use rubber? We all know there are better things available today, so why choose a lesser quality product when you don't have to meet an archaic regulation?

They used rubber not because that's all they had at the time; they used short pieces of it as couplings in areas where they either needed some flex, or they had too little access for aluminum AN fittings. The rubber was MIL-6000 hose (not "rubber") and it's still available today, but some manufacturers are also issuing modern fuel injection hose as a replacement for it. The modern stuff will tolerate ethanol, while some of the older compounds won't. Furthermore, teflon hoses are regularly used in the engine compartment because of their tolerance to heat and their long life. We also find silicone hoses used as vents, drains and so on in the engine compartments of post-'96 airplanes. Vacuum systems are using reinforced polyethylene hose. Lots of modern stuff, see?

Too many people think that certified airplanes are so archaic that they still use old materials. I see modern polymeric compounds in newer airplanes all the time. They still use 5052 aluminum tubing aft of the firewall because it's far safer than using flexible hoses, is much cheaper, maintains its shape to avoid sags and low spots, and doesn't age nearly as quickly as polymers do. Fuels attack the compounds in flexible hoses and it's impossible to see internal deterioration until they fail, which results in the need for scheduled replacement. I use O-rings made of modern compounds, too, where specified.

The wiring used in airplanes is modern Tefzel stuff. A very thin, tough, teflon-based insulation, very fine tinned copper stranding in the wire, far greater quality than I've seen in any automobile. And it has been used for 4 or 5 decades. Cars need to catch up. There are still some areas where airplanes could use more modern technology: wheel bearing seals are an example. Modern seals are found on some new Cleveland wheels, but the design of some installations prevent their use. The whole wheel retention method would need to be changed for them to work. Gets expensive.

 

[Winginit]

Dan Thomas;414106]They used rubber not because that's all they had at the time; they used short pieces of it as couplings in areas where they either needed some flex, or they had too little access for aluminum AN fittings. The rubber was MIL-6000 hose (not "rubber") and it's still available today,

The problem with quoting just Mil-6000 hose is that there are different types of hoses specified. A builder needs to research just which hose might meet his needs with the proper spec. Some are compatible with fuel and some are not. From what I'm aware of, the fuel hose available in older airplanes was always rubber. As I mentioned, there are newer alternatives that are better even if they do/do not happen to meet a Mil-Spec. I would think that all the major manufacturers of hoses do meet and probably exceed some Mil-Spec, and definitely handle different types of fuel. In todays world, a homebuilder should certainly build his airplane with the realization that even if he doesn't plan to use auto fuel, it might be wise to build with that future possibility in mind. Not to distant memories recall the problems that were created in the automotive industry and even in aviation when the petroleum industry started modifying fuels. Rubber was one of the major problems with different fuels. Nowadays its common to use compounds which are impervious to current fuels.

but some manufacturers are also issuing modern fuel injection hose as a replacement for it. The modern stuff will tolerate ethanol, while some of the older compounds won't. Furthermore, teflon hoses are regularly used in the engine compartment because of their tolerance to heat and their long life. We also find silicone hoses used as vents, drains and so on in the engine compartments of post-'96 airplanes. Vacuum systems are using reinforced polyethylene hose. Lots of modern stuff, see?

I think thats exactly what I was saying, why use rubber when there are newer things available.

Too many people think that certified airplanes are so archaic that they still use old materials. I see modern polymeric compounds in newer airplanes all the time. They still use 5052 aluminum tubing aft of the firewall because it's far safer than using flexible hoses, is much cheaper, maintains its shape to avoid sags and low spots, and doesn't age nearly as quickly as polymers do. Fuels attack the compounds in flexible hoses and it's impossible to see internal deterioration until they fail, which results in the need for scheduled replacement. I use O-rings made of modern compounds, too, where specified.

Again, I think thats what I was originally saying....especially rubber.

The wiring used in airplanes is modern Tefzel stuff. A very thin, tough, teflon-based insulation, very fine tinned copper stranding in the wire, far greater quality than I've seen in any automobile. And it has been used for 4 or 5 decades. Cars need to catch up. There are still some areas where airplanes could use more modern technology: wheel bearing seals are an example. Modern seals are found on some new Cleveland wheels, but the design of some installations prevent their use. The whole wheel retention method would need to be changed for them to work. Gets expensive.

It may be a better type of wiring, but I think the wiring used in todays cars, and even much of the aftermarket wiring is available in very good quality,and is easily sufficient for aviation use. I'd venture to say that 99% of all homebuilt airplanes are not wired to Mil-Spec standards. For the most part, common connectors terminate most wires, so what good does it do to use Mil-Spec wire if you aren't using Mil-Spec connectors on the ends of them ? If a decent quality wire is utilized, properly assembled to a connector, and properly supported,there is no reason for it to fail. Most wires fail because of vibration, chafing,overloading, or improper assembly....not because the wire itself is somehow inferior to Mil-Spec wire. I think its great to find that something someone wants to use meets some Mil-Spec, but that doesn't mean it won't fail if its put in the wrong situation. When I was a buyer for Military components, the common understanding was that a Mil-Spec identifies the minimum acceptable standard the government will accept. There are many products which exceed Military specifications, but we could not use them as a substitute for the specified item. Kinda like the fact that you can't upgrade many things in older certified airplanes with something newer and obviously better. Now, that being said, Military Specs are usually quite rigorous and you get a modicum of quality assurance with their use, and that gives everyone a feeling of confidence.....but you also need to use a little common sense as to whether there is something that will serve your needs better.


We had some electrical components which initially required 100% testing before installing in a weapons system. One particualr test was called "shake and bake". The components were put thru a torture test of heat and vibration before use. Then they were tested to see if they still worked and installed in the system. Kinda like buying a car that had been run thru a Baja Race and expecting it to be "good as new". Eventually they began to test a percentage of the components instead of 100% of them. A few random units were pulled from each batch and tested. Finally that evolved to just accepting units from a company because the units they supplied had a track record of doing well back when we actually tested them. Any new vendor had to get tests done and then we could use them as well. So, the question is, What is the best way to be sure something is of good quality ? The answer is, by how it does when you use it. Apparently the lower cost method worked fine because they still do it that way.


Mil-H-6000A 7 Jan 1960 (Original Standard)

Mil-H-6000B Supercedes Mil-H-6000A Effective 22 March 1982
3.3.1 Inner tube. The tube of the hose shall be o seamless construction of suitable synthetic rubber compounded with the necessary ingredients co meet the requirements of this specification.
3.3.3 Outer coating. The outer concing shall be predominantly polychloroprene rubber compounded co meet che requirements of this specification.

Mil-DTL-6000 Rev D Supercedes Mil-H-6000B effective 20 Sept 2007
3.3.1 Inner tube. The inner tube of the hose shall be manufactured from synthetic rubber compounded with the necessary ingredients to meet the requirements of this specification
3.3.3 Outer coating. The outer coating shall consist of polychloroprene rubber reinforced and compounded to meet the requirements of this specification.

 

[Dan Thomas]

We have to define "rubber" here. True rubber is a product made from a tropical tree and has little resistance to fuels. It hasn't been used in cars or airplanes for a long, long time except for the rubber seals in DOT-3 or -4 brake systems, where a lot of synthetics don't work. Everything else, including MIL-6000, has been made from Buna-N or other synthetics at least since WWII.

If autos are using a teflon-based wire insulation now, all well and good. It's used in aircraft because it doesn't burn easily like the typical PVC, which generates toxic fumes. And it's more resistant to chafing. In vehicles that cannot simply pull over and let you jump out when something catches fire, we need better stuff. The copper strands in aircraft wire are tinned, which minimizes the oxidation problem that leads to resistance and heating at crimp terminals when the wiring gets old. Bare copper oxidizes very quickly. And anyone building an airplane should be using a good ratchet crimper as well; the cheap squeezers often don't compress it to the MIL-spec dimension to get adequate contact and mechanical pull-out strength. A good crimper, crimping MIL-spec terminals, will crimp the wire as well as the insulation clamp that cheaper automotive terminals don't have.

 

[Winginit]

Dan Thomas;414148]We have to define "rubber" here. True rubber is a product made from a tropical tree and has little resistance to fuels. It hasn't been used in cars or airplanes for a long, long time except for the rubber seals in DOT-3 or -4 brake systems, where a lot of synthetics don't work. Everything else, including MIL-6000, has been made from Buna-N or other synthetics at least since WWII.

I think the idea is that the black flexible tubing that is used in older airplanes is universally refered to as "rubber" in a generic sense. That particular item, whatever it is, and what ever its called , can and will deteriorate over time. For that reason it must be inspected at least yearly and replaced periodically. Inspections are basically going to be visual with no real way to verify the true condition of the hose. Since many of them are located in obscure and inconvenient places, they do not get checked regularly.Every gearhead out there has probably seen situations where a hose looked perfectly OK except for the place where it cracked and was now the source of a leak. While some hoses may show signs of deterioration before leaking, many don't....so it becomes a matter of replacement on a regular basis if someone wants to be sure the hose is OK. I'm simply suggesting that either newer types of hose be considered for their longer lifespan, or that a builder plan his connections in a manner that eliminates the need for replaceable hoses.

Dan Thomas ...If autos are using a teflon-based wire insulation now, all well and good. It's used in aircraft because it doesn't burn easily like the typical PVC, which generates toxic fumes. And it's more resistant to chafing. In vehicles that cannot simply pull over and let you jump out when something catches fire, we need better stuff. The copper strands in aircraft wire are tinned, which minimizes the oxidation problem that leads to resistance and heating at crimp terminals when the wiring gets old. Bare copper oxidizes very quickly. And anyone building an airplane should be using a good ratchet crimper as well; the cheap squeezers often don't compress it to the MIL-spec dimension to get adequate contact and mechanical pull-out strength. A good crimper, crimping MIL-spec terminals, will crimp the wire as well as the insulation clamp that cheaper automotive terminals don't have.

Yes, using quality ratchet crimpers is the best way to assemble wire harnesses. The wire and connectors being used are something a builder should put some thought in to. While Mil-Spec wiring is better than a lot of the wire used in todays automobiles, it really does not help a builder that much. Most issues are going to arise where the wire and the connector meet. The length of wire from connector seldom gives anyone a problem unless its unsupported or being rubbed and chafed by something. The wire itself is usually pretty dogone reliable and trouble free. Probably 99% of all electrical wiring problems are simply corrosion at an exposed terminal connection or a bad ground connection. The connectors almost never just fail, and neither does the wire. How many millions of cars have you seen or passed in the last 20 years of driving ? How many (%) have you seen "burst into flame" because of faulty wiring ? Older cars prior to say 1980 didn't have nearly as well designed connectors or wiring, but todays connectors are very troublefree. They have to be that way in order for the computers to get accurate information. I think this is another myth similar to "airplane engines are more reliable than automobile engines", and its time to rethink that perception now that technology has moved on. If someone properly sizes their wiring, properly routes and supports it, makes and uses quality connectors, they will have a reliable wiring system without using Mil-Spec components. IMHO