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We hear these kinds of stories often. Here is a video of a rubber lined hose failure. This hose had been in service for many years and developed a leak in the rubber liner. The owner was noticing fuel smells in the cabin. As soon as a little pressure was applied on the hydrostatic pressure test bench, it began to leak like a sieve. At the extremely low pressures it was normally under, it was only weeping slightly versus spewing fuel out. https://www.youtube.com/watch?v=uu__...ature=emb_logo |
I’ve fabricated fuel hoses in the past but would definitely not do it again. You really need to know what you’re doing when you fabricate your own fuel, oil or brake hoses. There are a lot of potential pitfalls and the consequences of a hose failure can certainly be fatal in a very ugly way. The trouble for many amateur builders who attempt to fabricate their own hoses is that they don’t know what they don’t know. This is definitely not the area of the build where you want to try and save a few dollars.
There is no greater area of ignorance in the fabrication of home built aircraft hoses than in the requirement for pressure testing of the hose. Some home fabricators don’t test at all because they have no equipment to do so...that’s really putting your life on the line. Others will connect the hose up to their compressor and pump a couple of hundred psi into it....that’s entirely inadequate as a test and super dangerous as well...you do not want fittings flying off like ballistic missiles. The pressure testing must be done hydraulically. If the builder doesn’t have an hydraulic pressure tester then he needs to have the pressure testing done by someone who does. And that tester needs to be using the right type of hydraulic fluid so that the hoses are not contaminated. This is not necessarily straightforward. Perhaps the biggest ignorance in pressure testing is the pressure itself. I keep hearing builders say how they tested home made fuel hoses to say 200 psi because that’s “way beyond operating pressure”. The correct pressure to test a hose to is the proof pressure of the hose. That is the pressure that ensures that the hose has no defects and that the end fittings are properly fitted. Typically the fittings will be checked after proof pressure has been applied to ensure that they are not slipping. The proof pressure on a fuel hose varies depending on the manufacturer, the type of hose, and the hose size...but for a -6 hose it might typically be around 1500 psi. Testing to say 200 psi is virtually useless. It’s typically not the operating pressure of the hose that causes the end fitting to fail....it’s the flexing of the hose in the end fitting caused by differential movements. And finally, there is a broad misconception on Teflon hoses that they last forever and never need to be replaced. That is not necessarily true. Teflon hoses are “on condition”. That means that they have to be inspected at predetermined intervals to ensure they are still functional. If they pass pressure testing they can be returned to service. The big problem with Teflon hoses is that they form a cast (a shape, a curvature) and when the hose is pressure tested that cast needs to be maintained to prevent the hose from straightening out under the pressure. That is not always easy to do. |
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These recurring threads about DIY vs buying fabricated hoses should definitely have their own section in the “Never Ending Debates”.
I don’t want to take away from what I’m sure are very professionally made and tested hoses advertised here, but if you are interested doing it yourself, the fact is that you can make quality hoses on your own, easily and with normal shop tools. No Fear, Uncertainty and Doubt required. As for the high test pressures, we test utility gas lines, steam lines, hydraulic lines and many other high pressure systems in the utility industry to ASME standards, which generally require hydro testing at 150% of the operating pressure. You can certainly test your lines at higher pressures, but you won’t be able to convince me that an oil system that is fully relieved by 100 psi needs to have hoses good for 10,000 psi. That’s a safety factor of 100. And then consider that nuclear power plant pressure systems are designed to a minimum safety factor of 2, with some as high as 5. While you can have a confident feeling that your oil, gas and brake lines are tested to a certified standard, created back when fluid lines were all made of natural rubber - just know that it’s unnecessary. And it still won’t guarantee they won’t leak. I test my lines using 150 psi compressed air, the earl’s test kit and a 5 gallon bucket. Works great. BTW, if you buy the fittings and hoses from ANPlumbing, they’ll assemble them with crimps and test for you at essentially the price you pay for the materials. It’s a nice low cost option if you don’t want to do it yourself. |
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We didnt make up the pressure ratings on the hoses, nor the testing pressures.
We took the Certified hose working pressures of hoses that were used for the applications and adapted it. For example, a -6 303/111 hose (666/124 teflon) used for fuel is rated at 1500 psi. Now in most of your injected applications, youre looking at 29-31 PSI with the boost pumps. (SDS and EFii run +- 49 psi.) But the hoses used to plumb them in a certified application were rated at 1500, so thats the spec we used. So you can see that the pressure test margin is not excessive, but over the working pressure. Since oil systems can run 80-90 PSI on occasions, the same basic hose (303/111 for certified applications) was used. Again---it's what was designed for the application. So similar hose, similar HOSE working pressures, similar test pressures. Even carbureted fuel systems at 5 psi use a 303/111 hose rated at 1500 psi in certified aircraft. YES YOU CAN use a hose with a much lower WP and have even seen Tygon used, believe it or not. WE chose to use teflon not only for the pressure ratings, but for the fluid compatibility. Fuel is a great example---we never know what additives are in fuels that may react with the liner materials in a negative way. Like the 701 hose in the video that Steve posted. That hose was sent to me to test. The owner though he had a hose end leaking at the screw in collar. As you can see, it wasnt the case. Fuel had deteriorated the liner of the hose to the point where it was leaking THROUGH the liner out out of the reinforcement braid. LITTLE PRESSURE--I think it was 1 psi---just enough to get fluid flow. When I got started in this, I wanted to give experimental builders a resonable, reliable alternative to expensive certified hose assemblies, and a faster response time. 10 years later, I think we've accomplished that. Tom |
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This isn't a never ending debate like "Primer Wars." I think everyone can agree that the truth is that an aircraft should utilize high quality hoses. High quality hoses can be built yourself,or they can be sourced from a company that fabricates that professionally. Poor quality hose components/assembly can result in a less than desirable outcome. We completely agree with your statement that it is possible to build very high quality DIY hoses. We have some customers/fellow builders that want to do so for the experience of building. We have no problem at all recommending brands for high quality do it yourself lines. It's not about saving money, but rather about the fabrication experience. AN plumbing sells about 15 different lines of fittings and hoses. To the untrained eye, it can be challenging to know the difference between what is suitable for an aircraft and what isn't. The ones that are not suitable for aircraft use may work fine at first, and cause problems down the road. We decided long ago that we would not compromise on quality. We could fabricate hoses for half the cost of what we do, but the components would not be ones that we would utilize on our own aircraft. We refuse to go down that road...ever. Regarding pressure testing.....a 150 PSI pressure test will give peace of mind, but will not serve as a proof test for the fittings. We could crimp a hose that would hold at 150 psi that would fail over time under normal operating pressures and vibration. Putting 150PSI of air pressure into a hose is no problem....UNLESS A FITTING FAILS. In that case, your day will get very interesting. There is a reason that hydrostatic tests are utilized when testing hoses under pressure. Ultimately, there is no right or wrong answer because there are various paths to reach the end goal. For builders who want to use professionally fabricated hoses, we can help. For builders that want to build their own lines, we are happy to point you in the right direction too to help make sure you are utilizing high quality components. If anyone has any additional questions, please feel free to contact Tom or I. |
the ''untrained eye''. ok, so it was just a phone call to ANPlumbing to get specific recommendations for assemblies to be used in airplanes.
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