L

lr172

Well Known Member
I was going to run a braided fuel line from the firewall to the fuel pump, but planned to run a stainless line from the pump to the carb. Both items are fixed to the engine, so figured there would be no issues with flexing. Also wouldn't need a fire sleeve.

However, I have not seen any lines set up on the pictures I have seen around here. Are there any problems with this approach?

Thanks,

Larry
 
For anyone considering rigid fuel lines on their engine, I would sugest that you read this thread as part of your risk trade analysis:

http://www.vansairforce.com/community/showthread.php?t=57122

This is not a rejection of rigid lines - it is a suggestion to look at all aspects of a "minor" change to a critical engine part before making a decision. Nick shared his story a couple years back to make sure that people knew what happened in his case. I don't know if Lycoming uses rigid lines on any of their standard configurations - they very well might - but thre are a lot of flexible lines flying that work well.

We tend to think of the engine as one big monolithic chunk of metal, but I have seen slow motion films of engines on test stands that show quite a bit of flex between the parts - the cylinders can really wiggle around realtive to the case, for instance, and the case flexes.

Then again, the standard prop oil line is a rigid stainless tube - so that clearly is acceptable in that installation. And it probaby was the result of some engineering work to achieve that.

I'm not saying that rigid fuel lines are bad - I am saying that anyone contemplating a change to the "norm" should think about all of the aspects of the change, and that is true in any critical aircraft system or structure. Vibrational analysys is not simple, and can surprise you. For thsoe who have done the analysis and got the data to back up the design - great! But don't do it just becasue "That Looks About Right".
 
Another consideration. You should be popping the line off the carb every year to check the screen. Would a hard line make that.....harder?
 
Ironflight said:
For anyone considering rigid fuel lines on their engine, I would sugest that you read this thread as part of your risk trade analysis:

http://www.vansairforce.com/community/showthread.php?t=57122

This is not a rejection of rigid lines - it is a suggestion to look at all aspects of a "minor" change to a critical engine part before making a decision. Nick shared his story a couple years back to make sure that people knew what happened in his case. I don't know if Lycoming uses rigid lines on any of their standard configurations - they very well might - but thre are a lot of flexible lines flying that work well.

We tend to think of the engine as one big monolithic chunk of metal, but I have seen slow motion films of engines on test stands that show quite a bit of flex between the parts - the cylinders can really wiggle around realtive to the case, for instance, and the case flexes.

Then again, the standard prop oil line is a rigid stainless tube - so that clearly is acceptable in that installation. And it probaby was the result of some engineering work to achieve that.

I'm not saying that rigid fuel lines are bad - I am saying that anyone contemplating a change to the "norm" should think about all of the aspects of the change, and that is true in any critical aircraft system or structure. Vibrational analysys is not simple, and can surprise you. For those who have done the analysis and got the data to back up the design - great! But don't do it just becasue "That Looks About Right".
Click to expand...

+1 - well stated.
 
Ironflight said:
...
Then again, the standard prop oil line is a rigid stainless tube - so that clearly is acceptable in that installation. And it probaby was the result of some engineering work to achieve that.
.....
Click to expand...

Even that relatively simple line was the subject of a Lycoming AD in 1990 when the end fittings were changed from aluminum to steel.

http://rgl.faa.gov/Regulatory_and_G...D17C07A9D76E6AAA8625684F006E5ADC?OpenDocument

So even Lycoming can get it wrong initially :) even more reason to follow current certified practice.
 
Why Stainless Steel for hard lines? Carbon steel tube has some better mechanical properties, costs less and tends to seal more reliably at the flare? :confused:
Stainless isn't always better in every application, although it does look nice.
 
Yes, rigid lines can fail, especially if done improperly. However, you are not going to tell me that a vibration analysis is required before making a hard line. There are very easily accessible standards for fabrication and application of hardlines and other general hardware like this. It is assumed that these resources are used if you are building an airplane in your garage. If you can't (or won't) follow these standards, THEN DONT BUILD AN AIRPLANE IN YOUR GARAGE!

I've been an aircraft mechanic since I was a teenager. I've maintained airplanes ranging from Cubs to SR-71's and I have seen more failures of hose than hard lines.

Bottom line: If you're going to do something on your airplane, learn the requirements, and execute the fabrication properly. This is a learning exercise, after all.
 
I want to clarify my post about using stainless rigid tube. Obviously a stainless braid teflon FIRESLEEVED hose is the best answer here. Larry's question was whether there was a problem with this approach. Technically, probably not, but I dont recall seeing any rigid tubing for this application. I'm guess the he wanted to use rigid tubing, and my comment about stainless was to keep him away from aluminum.
Lycoming did use rigid tube for governor lines in some applications, and they were of a similar distance, with I would guess, similar motion. So--I guess you could do it, and yep stainless tube looks good, but flex hose is the best bet. But---FIRESLEEVE the hose.
Tom
 
Tom, define "best" from an aircraft design/ engineering standpoint.

I can tell you with absolute certainty that my engineers use flex hose as a last resort. Flex hose is a " specialty" application- used only when there is relative movement or maintenance access is required. Compared to hard line, hose is heavy, expensive, fragile, and bulky. Hose is the EXCEPTION - hard lines are the standard.
 
SHIPCHIEF said:
Why Stainless Steel for hard lines? Carbon steel tube has some better mechanical properties, costs less and tends to seal more reliably at the flare? :confused:
Stainless isn't always better in every application, although it does look nice.
Click to expand...

Corrosion resistance. You don't want something like a fuel or hydraulic line corroding and blowing out at an inopportune time. In a hot, potentially humid area like under the cowling, you don't want something that corrodes because that's a nearly ideal place for it to happen.
 
well...
Stainless steel is unfortunately named. It does stain & corrode, and in some insidious ways. I've had a fair bit of bad experience with it at work.
Stainless steel suffers from Crevice Corrosion, which can occur in areas that are difficult to inspect, like wet enclosed areas with poor circulation, or areas of mechanical stress. For example, around the flared end of a tube, inside the flare nut. Also under an Adel Clamp or Zip Tie.
Stainless Steel has a lower tensile strength than Carbon Steel. Also it's surface is harder, making leaks at flare joints more likely. Finally, it tends to micro weld when in compression, leading to galling and stripped or seized threaded parts.
Carbon steel, on the other hand, tends to get uniform surface corrosion which is readily controlled with coatings like paint (engine mounts) or oil (always available as an unwanted mist in the engine compartment).
I have had to replace many stainless steel JIC fittings at work with carbon steel because of hydraulic fluid leaks into salt water. Also, Stainless Steel for fire mains didn't work out. Standing water in the lines corroded pits and worm tracks into it in a matter of weeks. It was a major engineering miscalculation upstairs.
Lastly, even the finest cars use carbon steel tubing thruout, and the under hood environment must be worse than our engine compartment?
I'm just suggesting that carbon steel automotive brake line, available in straight sections at your local auto-aerospace parts store might be more appropriate? More available, cheaper, stronger, and proven safe on millions of old rusty cars daily?
 
SHIPCHIEF said:
well...
Stainless steel is unfortunately named. It does stain & corrode, and in some insidious ways. I've had a fair bit of bad experience with it at work.
Stainless steel suffers from Crevice Corrosion, which can occur in areas that are difficult to inspect, like wet enclosed areas with poor circulation, or areas of mechanical stress. For example, around the flared end of a tube, inside the flare nut. Also under an Adel Clamp or Zip Tie.
Stainless Steel has a lower tensile strength than Carbon Steel. Also it's surface is harder, making leaks at flare joints more likely. Finally, it tends to micro weld when in compression, leading to galling and stripped or seized threaded parts.
Carbon steel, on the other hand, tends to get uniform surface corrosion which is readily controlled with coatings like paint (engine mounts) or oil (always available as an unwanted mist in the engine compartment).
I have had to replace many stainless steel JIC fittings at work with carbon steel because of hydraulic fluid leaks into salt water. Also, Stainless Steel for fire mains didn't work out. Standing water in the lines corroded pits and worm tracks into it in a matter of weeks. It was a major engineering miscalculation upstairs.
Lastly, even the finest cars use carbon steel tubing thruout, and the under hood environment must be worse than our engine compartment?
I'm just suggesting that carbon steel automotive brake line, available in straight sections at your local auto-aerospace parts store might be more appropriate? More available, cheaper, stronger, and proven safe on millions of old rusty cars daily?
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Yep, I had some stainless bosses corrode sitting on a diesel engine raw water aftercooler at the dock in 60 hours. I had to stop production. One, yes , 1 inch hole corroded through a sea water after cooler. the hole was a little worm hole less than .040" in diameter. It was 400 series though, and I learned a lesson. 300 was much better but still has it's limitations. Not to condemn the material, just know its application parameters. Without seawater RV's are probably OK.:D
 
TM-584C Rev C

TM-584C Rev C
Corrosion Control And Treatment Manual
http://corrosion.ksc.nasa.gov/pubs/tm584c.pdf‎
is a NASA publication, easily downloaded.
It's easy to access the info.
I looked it up to reference Crevice Corrosion, but I'm glad I did, because it's a great guide to extending the service life of your plane, and understand what's going on.
 
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When I saw the title of this thread in "New Posts," I thought
"What? For your engine's pleasure?"
 
Michael---my 'best' was from a installation and fabrication for experimentals point of view. Certainly there are many applications where rigid tubing is best. We see it in industrial hydraulics all the time.
No offense was intended!
Tom
 
No offense taken Tom. Just want to ensure that we don't spread false or misleading information if we can help it. There seems to be a bit of that going on in this thread. It is as if flex hose use has become so rampant, that it has not only become the "new normal" but people are beginning to question the validity of a rarely seen, yet well accepted standard like hardlines. Zip ties are becoming commonplace FWF... Are we going to get to the point where we start questioning the use of Adel clamps?

Let's lay some truth down for a minute:

Stainless steel hard lines are a common, well established method for fuel, oil, hydraulic and pneumatic aircraft systems, both high and low pressure.

Stainless hardlines can be fabricated properly using the tools and processes readily available to the home builder.

Aluminum nuts and tube sleeves on stainless, while not a "best practice" from a metallurgical perspective, ARE accepted and used in aircraft systems by OEMs.

Flexible hose assemblies have their place. But like hardlines, their use should be appropriate to the job at hand, properly fabricated, installed, and maintained.
 
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Take a look at all the Grumman AA5-series airplanes. They ALL have a stainless hard line between engine-driven fuel pump and carb. While being "certificated" doesn't make it "right", a fleet with many thousands of hours of in-service time and few to no problems with this part is a strong indicator that a stainless hard line in this application is not necessarily wrong.
 
Michael---, yes all of that is true for sure. Teflon hoses are generally more expensive that stainless rigid tube, mainly because of the hose ends. I will say that some builder have problems fabricating rigid tubes, and substitute teflon flex hoses. As the 'manufacturer' of thier plane, they can do that.
You are correct in saying that the system should be analyzed and the proper acceptable standards used. Yes, I built ALOT of teflon hose assemblies, but also some rigid tube assemblies.
Tom
 
Thanks Tom. You provide a required, high quality product and great service. I just want to make sure that this popularity does not overshadow other "accepted" alternatives. Hose definitely has its place - let's just make sure ease and convenience does not displace design elegance.
 
Anyone Else Using Rigid Line To A Carb?

My O-320 will have a line between the carb and the fuel pump. What is the general practice for this? Flex line or rigid line?

Tom, and Michael, please, you've both made excellent points here. But I'd like to see what other people are doing.

Thanks!

Dave
 
My SuperCub with a C-90 had a carbon steel fuel line between the fuel pump and the carb. The fuel pump were mounted on a pad right front. Just in case you are wondering why this Cub had a fuel pump: This engine probably came out of a low wing aircraft and they usually have the fuel pump installed.
 
David Paule said:
My O-320 will have a line between the carb and the fuel pump. What is the general practice for this? Flex line or rigid line?

Tom, and Michael, please, you've both made excellent points here. But I'd like to see what other people are doing.

Thanks!

Dave
Click to expand...

Flex lines are much more common FWF (even in the certified world) than hard lines.
 
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