[RV8R999]

Lots of discussion on this topic.

Couldn't find anything in Advisory Circulars or FARs on this subject.

Lyc installs Alum lines on the engine (which is FWF)


I have my FF sensor mounted to the FW on a 1/4" thick alum angle with a 4" run of alum tube from firewall fitting to the FF sensor with a slight Z bend in the middle and wrapped in firesleave. From the FF sensor to the engine is flexible hose with the first three inches of flex hose supported by 2 adel clamps on the Fire Wall to provide smooth outlet flow for the sensor and relieve any stress on the FF sensor outlet fitting.

I don't see how a 4" run of alum tubing is in any jeopardy of fatigue failure any more than the alum tubes running just behind the FW would be. Properly supported alum lines mounted to the firewall should not be a problem from a fatigue life point of view. Fire, fire..yes I know, we've covered this.

Does anyone have a technical reference discussing this topic?

 

[Alan Carroll]

Lots of discussion on this topic.

Couldn't find anything in Advisory Circulars or FARs on this subject...

I don't see how a 4" run of alum tubing is in any jeopardy of fatigue failure any more than the alum tubes running just behind the FW would be. Properly supported alum lines mounted to the firewall should not be a problem from a fatigue life point of view. Fire, fire..yes I know, we've covered this.

Does anyone have a technical reference discussing this topic?

Good question - although I don't know the answer. I'm particularly curious about installations like the one pictured below (borrowed from Randy Lervold's excellent web site). I would guess that most RVs with firewall gascolators have used some variant of this approach (note- not trying to restart the debate over gascolators).

 

[JAT]

Ken and Alan,

My tech counselor had me change out the aluminum line from the boost pump to the gascolator to a stainless steel line. He had fifty years of experience in the aircraft manufacturing/maintenance business, so I did the swap. There is a saying in Norway that makes sense: "When an old dog barks, we should listen." I took his advice and the swap only took twenty minutes. Randy's naked 4" aluminum line would nag at me, but to this day I haven't seen any technical info on the subject; just the "old dog's" advice.

Jim

 

[DanH]

The aluminum oil return line application under a Lycoming cylinder is not at all like the average aluminum fuel line install. In this app (1) the maximum possible relative movement between connected parts is well defined (individual cylinder vibration.....yes, the cylinders move) and (2) one end of the aluminum tube is clamped in a low modulus rubber hose connector. Stress is obviously very, very low.

The problem with our fuel line and fitting applications is that they tend to be unique to each builder. There are endless variations of angle, length, fitting selection, hose type, suspended mass, bracket strength, etc. A well-considered application mixes engineering, physics, materials science and all the good craftsmanship and best practice rules.

Done correctly, there is no reason why you can't safely incorporate some aluminum lines and fittings. The trouble starts because it is very hard for most builders to think their way through a correct application. OK, so the smart/educated guys can do it with impunity? No. One of life's basic rules is "The more you know the more you know you don't know." Ever notice how professionals apply all their powers to design, then build a prototype to test and measure so they can see where they were wrong?

Given we don't have the tools to engineer to the bleeding edge of adequate, our best bet is to build in plenty of margin. Steel lines and fittings have at least twice the fatigue limit. Quality flex hoses installed with a little strain relief don't have a fatigue factor within their expected calendar life.

Look at the example photo above. So far the install seems to have worked well, and thus we can quite reasonably argue there's nothing wrong with it. However, the exact same install can be done in a more conservative fashion with steel fittings and a firesleeved flex hose between firewall and gascolator. Using the best hose you can buy it will cost about $50 more, the only downside.

Your choice; you're the builder.

 

[Alan Carroll]

Ken and Alan,

My tech counselor had me change out the aluminum line from the boost pump to the gascolator to a stainless steel line. He had fifty years of experience in the aircraft manufacturing/maintenance business, so I did the swap. There is a saying in Norway that makes sense: "When an old dog barks, we should listen." I took his advice and the swap only took twenty minutes. Randy's naked 4" aluminum line would nag at me, but to this day I haven't seen any technical info on the subject; just the "old dog's" advice.

Jim

Jim - the Tony Bingelis articles and books served the "old dog" function for a lot of us, and were very helpful. They're becoming a bit dated now however. I just re-read one of his old articles in which he found it necessary to advise builders not to use plastic tubing for fuel forward of the firewall! I don't think he ever wrote about anything as fancy as a fuel flow sensor. Unfortunately it doesn't seem that anyone of Tony's unique talents has come along since, or maybe they're just not getting published.

By the way I'd note that Randy's photo appeared to be of the firewall "in progress"; he may have added firesleeve to that tube.

 

[Toobuilder]

...However, the exact same install can be done in a more conservative fashion with steel fittings and a firesleeved flex hose between firewall and gascolator. Using the best hose you can buy it will cost about $50 more, the only downside.

Higher cost is not the "only" downside... Compared to stainless hard lines (which in this application could last as long as the airframe), the hose/firesleve combination is several times the weight.

 

[RV8R999]

The set-up in the photo is nearly exactly how I built my first plane, a BD4, and flew for nearly 800 trouble free hours. This in and of itself doesn't prove much as 800 hours is still pretty low time in A/C hours.

To me this another one of those, "better is the enemy of good enough" discussions. Is SS the most fatigue resistant material or would CrMo be even better still (corrosion resistance notwithstanding)? What is better than CrMo?

I agree Dan - we have to build in margins, but design is driven by more than just function. Cost, weight, supply availability, maintenance, fabrication complexity, life cycles, etc are also significant factors, many of which compete with one another. Although we are not building in capacity I think most of us still have to factor some of the same variables in determining our own personal optimums. If I had $1M is disposable cash my RV-8 would probably be a different plane than it is going to be. Is an extra $50 a big deal, no, but since i'm already 45% over budget (despite having done this before) cost is a factor. If something can be made functional and safe while saving $50 I consider this to be a wise engineering choice.

My point was simply that CORRECTLY done, there isn't anything WRONG with using aluminum FWF... but this statement is based entirely upon my personal compromises of all those factors as is any statement from those who say you "CANNOT" use alum FWF.

I do try to always clarify my comments as my opinion so as not to be misconstrued as restating some statutory requirement which may or may not exist.

This forum is always such a great place to safely and informatively HASH things out and I always appreciate counter points of view. thanks!

 

[L.Adamson]

My point was simply that CORRECTLY done, there isn't anything WRONG with using aluminum FWF... but this statement is based entirely upon my personal compromises of all those factors as is any statement from those who say you "CANNOT" use alum FWF.

I too, don't have a problem with the short aluminum tube..........in the firewall pic. Mine is much like it, except it's an "S" bend which also allows for a bit of flex. I base my thoughts on nearly 40 years in the heating/cooling industry, where copper and aluminum tubing are regularly subject to vibration, such as you would encounter on the firewall. In normal circumstances, unless the tubing connections has faulty welds, or has welded branch tubing in high vibration areas without dampening..............these products have worked well in the HVAC industry for a long time.

L.Adamson --- RV6A

 

[n8zg]

My point was simply that CORRECTLY done, there isn't anything WRONG with using aluminum FWF...

Well... maybe. Depends on which aluminum and where it's going. If you're talking about the soft aluminum tube that came in the kit, it's great for initial fit-up, but I wouldn't use it forward of the firewall for anything other than vent lines - specifically the fuel pump overflow, sniffle drain and the crankcase breather tube (in appropriate sizes, of course).

If rigid tube is indicated, tinker with the cheap soft stuff till you're happy with the routing, length and support isues. Then use it as a template to make the flight-worthy tube out of AT LEAST 5052. Or Stainless. Or Teflon lined braided flex hose with stainless fittings.

 

[RV8R999]

Thanks for your "opinion".

 

[Toobuilder]

...To me this another one of those, "better is the enemy of good enough" discussions. Is SS the most fatigue resistant material or would CrMo be even better still (corrosion resistance notwithstanding)? What is better than CrMo? ...

Here is what the T.O.1-1A-1 says about stainless tubing:

3.9.1.2 Stainless Steel Tubing

?CRES tubing does not have to be annealed for flaring or forming. In fact, the flared section is somewhat strengthened by the cold working and consequent strain hardening. The high tensile strength of stainless steel tubing permits the use of a thinner wall than does aluminum alloy tubing. Therefore, the weight is about the same as thicker-walled aluminum alloy tubing.

...My point was simply that CORRECTLY done, there isn't anything WRONG with using aluminum FWF... but this statement is based entirely upon my personal compromises of all those factors as is any statement from those who say you "CANNOT" use alum FWF...

I agree in principle. The "normal" pressures and temperatures ahead of the firewall will allow the aluminum to survive in many cases. The problem is if there is a fire, exhaust leak, poor design (no support, no strain relief, poor workmanship, etc) or external damage like chaffing due to a broken zip tie, etc. The aluminum has FAR less margin than stainles tube (or hose) in these cases, so this is why I try to substiture stainless hard line wherever possible. I only run aluminum where it can be readily inspected, does not have any possibility of chaffing structure, is not subject to high heat, and/or does not carry vital fluids. This is my personal "comfort zone".