Is this a reasonable fuel line routing method?

[Tom Martin]

I like saving dollars and weight as much as the next guy but I also know that these engines are not solid blocks; they do move a bit in service. I know that some governor oil lines are rigid lines,but are there hard fuel lines used in the certified world on with these engines?
After 800 hours I have some cracks in my baffles and aluminum plenum and these hard lines make me a bit nervous.

[Toobuilder]

I agree. Nice job!

You now have an appreciation for how much stiffer this material is than aluminum. This is a clue for their general fabrication. Though the servo and fuel pump are not moving relative to one another, there will be slight dimensional change as the engine heats and cools. So in this case, the general rule concerning bends is, the more, the better. In your case, you have several nice 90 degree bends which allow a little bit of flex without over stressing. Same concept applies to hose- you never want it installed "tight".

[Toobuilder]

Mr. Martin -

There is some precedent for rigid fuel lines in the certified world, though admittedly rare. I believe this has to do with the fact that engines are supplied to the OEM's as "generic" power plants. It would be difficult to fabricate a hard fuel line that took into consideration every exhaust system, engine mount, nose landing gear, etc. Better to supply a hose and let the airframe manufacturer deal with it.

However, you are spot on in your caution that the engine does move around a bit when heating and cooling. That movement needs to be addressed in the design of the tube. It is up to the builder to decide just how to allow for that movement. Hopefully people realize that a perfectly straight, 2 foot long hardline would break very quickly if connected between the servo and fuel pump. But the meandering path taken over and around the engine mount, oil pan rails, etc. as shown above gives a measure of strain relief. The key thing is to just make sure the line does not become a structural member - either in tension or compression.

Bit of trivia- There is at least one retractable gear aircraft that uses a single piece of hardline from the wheel mounted brakes to the fuselage. the 90+ degrees of retraction is accomplished with no flex hose or swivel fitting - it's all done with flexing the hard line. On the SR-71, the brake line travels from the wheel, up the gear leg, then coils around the gear trunion about a dozen times and attaches solidly to a fitting up in the wing. As the gear retracts the hardline bends, but it is only a fraction of a degree per inch through the coil. It's an extreme solution to an extreme mission, but does show that properly designed, even solid hard line can be pretty flexible.

[grayforge]

The other difference between Tom's baffles/plenum and these hard lines is that his are made of aluminum and these are made of stainless steel. I also used stainless steel end nuts and steel collars.

[Tom Martin]

I am interested in this thread as I am always looking for alternate ways of doing things. There is not just thermal movement in these engines, there is actual movement between the parts. It has been well documented that cylinders move relative to each other and that even in the block there are some movements. There is most certainly a lot of vibration. That we have used hoses as the predominate method of moving all liquids north of the firewall does not mean that other methods will not work. It does however mean that we have a well established failure mode with hoses.
I would feel a lot better about this discussion if one of the expert mechanics would step forth and say that a rigid line from the fuel pump to the fuel servo has been used successfully on a Cessna XXX for 30 years with no issue.
The fact that the tubing is stainless does not mean that it is not susceptible to vibration, just that it could take longer to show that wear. Sometimes a more rigid part may be more susceptible to fracture.
A good friend of mine pointed out that the small stainless lines from our flow divider on injected systems are stainless and properly supported, give good service.
Again I am not just being negative here, fuel system issues are the predominate cause of most engine related problems and I would hate for this to be perceived as an industry accepted practice unless in fact it is. If it is an accepted practice with thousands of hours of trouble free hours I will withdraw my concerns.

[BillL]

Originally Posted by grayforge

Michael, I like your hard fuel line install between the pump and servo... so I decided to give it a try. You're right, besides requiring more muscle, bending hard lines isn't much more difficult than soft aluminum. One difference is that it's not as easy to tweak the tubing one way or another to make small adjustments. After about 90 minutes of work, I ended up with a pretty decent first attempt. I'll be adding an Adel clamp and maybe some fire sleeve later. Photos below...

Thanks for the idea!

Russ

Where do you plan to put the adel clamps to terminate vibration? It looks like two good spots long the sump. ( like the governor line)

[Toobuilder]

Originally Posted by Tom Martin

...I would feel a lot better about this discussion if one of the expert mechanics would step forth and say that a rigid line from the fuel pump to the fuel servo has been used successfully on a Cessna XXX for 30 years with no issue...

Not claiming to be an expert, but here's the "certified/TC'd" setup for the -B1E:




Also keep in mind that short= stiff, so the B1E example is a pretty severe case. A longer run with a few bends in it is actually (relatively) flexible (as found in the SR-71 trivia above).

[billdianne]

[quote=
I would feel a lot better about this discussion if one of the expert mechanics would step forth and say that a rigid line from the fuel pump to the fuel servo has been used successfully on a Cessna XXX for 30 years with no issue.
.[/QUOTE]

Actually there is some history to look at relating to the s/s governor line, excluding the AD for replacing aluminum fittings with steel.

[Ironflight]

That's a good picture Mike - thanks for posting it. To me, it is clear that trying to fabricate a hose with that tight of a bend would have been tough - the hard line looks to me to be the better of the two options because of the closeness of the two ends. So there was a clear engineering trade that had to be made.

I would be careful about the SR-71 analogy though - as we well know, and aircraft like the Blackbird received meticulous attention from mechanics between every flight - lots of eyes on the machinery in case there was something wrong. Most people don't look under their cowlings on a GA aircraft between oil changes - so the comparison isn't necessarily valid. A good example for what it is, but have to remember the limitations/assumptions that go into every aircraft and the way it is operated.

[grayforge]

Originally Posted by BillL

Where do you plan to put the adel clamps to terminate vibration? It looks like two good spots long the sump. ( like the governor line)

Yep, one of the sump studs along the top of the sump. In fact, if you use a magnifying glass, you can see a sump stud under the straight stretch of line above the sump that already has its nut removed, ready for installing the adel clamp.