[Rob Erdos]

I just noticed that my plans specify aluminum pipe fittings forward of the firewall. Why does Van specify aluminum (e.g. AN822-4D) when steel fittings are not-quite-as-cheap? Wouldn't steel fittings be safer?

Awaiting enlightenment,

Rob Erdos

 

[vlittle]

I have an opinion (not necessarily correct). I try to use steel fittings when:

-the fitting is screwed into the engine or other expensive part such as the oil cooler. This should minimize cold welding.
-the fitting carries fuel (also use steel fitting on the fuel lines). It won't melt during the fire (maybe too late!)

Al to Al fittings work well, as do Al to Steel. Apparenty, steel to steel fittings need more torque because the steel is hard and does not seal as well.

V

 

[frazitl]

I used

and recommend steel, stainless steel, or fire-sleeved flex lines for all fuel and oil lines and fittings FWFD. It's a little heavier and a little more expensive, but the time to burn through aluminum lines and fittings has to be much less than for steel. Sure, the oil cooler is aluminum, and so is the fuel injection servo body and spider. Nonetheless, fuel or oil under pressure spraying around FWFD scares me enough to take every reasonable precaution I can...

Call me chicken

 

[NDrv8r]

steel fittings fwf

another reason to use steel fittings on fluid carrying lines firewall forward is that they are more crack resistant.

 

[Wayne Gillispie]

I am using what is provided, using fuel lube, torqing, and supporting lines properly. If we have a fire hot enough to melt alum ftgs with oil/fuel inside of them then your cowling/bottom skin/oil cooler/fab are in big trouble. All we can hope for is to be in the flare over a lake close to shore.

 

[L.Adamson]

I am using what is provided, using fuel lube, torqing, and supporting lines properly. If we have a fire hot enough to melt alum ftgs with oil/fuel inside of them then your cowling/bottom skin/oil cooler/fab are in big trouble. All we can hope for is to be in the flare over a lake close to shore.

That's been my take for a long time. I figure the cowling and windscreen won't be around too long. My plan is to shut off the fuel. I do have a steel fitting connected to the carb.

L.Adamson --- RV6A

 

[Walt]

I prefer steel fittings FWF for the extra strength they provide, this is especially true on the engine which is subject to significant vibration.

 

[DanH]

I prefer steel fittings FWF for the extra strength they provide, this is especially true on the engine which is subject to significant vibration.

Exactly.

Engines shake. Aluminum has no knee in the SN curve.

 

[Alan Carroll]

I just noticed that my plans specify aluminum pipe fittings forward of the firewall. Why does Van specify aluminum (e.g. AN822-4D) when steel fittings are not-quite-as-cheap? Wouldn't steel fittings be safer?

We can only speculate on Van's motives, but it could have something to do with the fact that aluminum fittings are considerably lighter.

Personally I'm skeptical that steel fittings help much with fire safety. Surely the rubber hose will be gone long before the aluminum fitting to which it is attached? Fatigue strength is another matter however.

 

[jtrusso]

Good timing

I was just going to ask this question.

I just bought into a share of a Glassair and all the fittings FWF on the fuel and oil lines are Al. All the fuel and oil lines are braided stainelss over teflon and fire sleeved, which is good.

I was wondering what the brain trust thinks about leaving it alone vs replacing with steel. I think it's better to leave them alone because the system has been tested and has no issues. If we tear it apart (assuming I could get the partner to go for that) we run the risk of building in a problem. Seems like the risk to reward ratio doesn't pan out. These lines were installed about 7 years ago when a new O-360 was installed.

I know, it's not an RV...but it's close enough for now. I'll still get that RV someday!

 

[Toobuilder]

I have to comment on the "absolutes" of aviation sometimes...

The thing about airplanes is that "stronger" is not "better" once you have met your design requirement... It's just heavier (which is bad for a flying machine). Going steel everywhere firewall forward is a safe (but lazy) approach, and results in a heavier, poorer flying airplane. While there may be some instances where the installation details make aluminum fittings a poor choice from a structural standpoint, I'd hazzard a guess that they are relatively rare. Perhaps when you have added all the weight of the firesleve on the miles of poorly supported hose running through the engine compartment, then the fittings should be steel. But aluminum fittings have been used forever on aircraft and continue to be used. We have scads of them used in our current military aircraft where appropriate.

Remember, a fitting, bolt, part, etc either meets requirements or it does not. Painting all aluminum fittings with the "firewall forward" brush is not design engineering or even adhering to "best practices"... It's just a quick way out. If someone is not qualified to make the determination or can't find someone who is, then go the safe bet (and pay the price in weight).

 

[apkp777]

Where's all the fire coming from that everyone talks about? My fuel tanks are in my wings and I have a shut off valve in arm's reach. Oh yeah, and I tightened all my fittings properly.

For the record, certified aircraft use aluminum fittings all the time. They don't burst into flames all that often. There might me some places where steel is preferred, but on a stock RV, I doubt it.

 

[Toobuilder]

Where's all the fire coming from that everyone talks about...

While Dan has brought up legitimate points concerning the suitability of aluminum from an outright strength/fatigue perspective, fire is the other point often addressed in these debates. And to that point, I am with you. If you have a fire that burns through an aluminum fitting, you have BIG problems - I doubt having a steel fitting is going to make any difference at that point.

 

[terrye]

Firewall Forward: Aluminum vs Steel

The thing about airplanes is that "stronger" is not "better" once you have met your design requirement... It's just heavier (which is bad for a flying machine). Going steel everywhere firewall forward is a safe (but lazy) approach, and results in a heavier, poorer flying airplane.

Looking at the AN standards for fluid adapters (eg. AN815 Union - Flared Tube) the following codes are listed for material options:

No code letter in part number for steel
Add D after dash number for aluminum alloy (except 7075-T73)
Add J after dash number for corrosion resistant steel, type 304
Add K after dash number for corrosion resistant steel, type 316
Add S after dash number for corrosion resistant steel, type 347
Add T after dash number for titanium
Add W after dash number for 7075-T73 aluminum

So there are quite a number of different material alloys available depending on, as you say, your design requirement. The trouble is, the AN standards do not give any guidance on what the design (or installation) requirements for each material are.

For my airplane, the design requirements for firewall forward fluid systems include fire resistance and fatigue/vibration resistance. Several posts assume you already have a fire and that aluminum adapters in the flame zone won't melt and make a bigger fire. My failure mode analysis suggests that if a fitting cracks at the threads (a point of high stress concentration) and sprays the fluid it is supposed to contain, such as fuel or oil, on hot engine parts then this is a fire initiation. In considering the fire resistance and fatigue resistance of the actual fitting, I believe the steel fittings are superior to the aluminum fittings. I plan to use steel fittings firewall forward for my fuel and oil lines.

I do agree with you that once you have met the design requirements, stronger is just heavier, which is not desireable on an airplane. So if you can refer me to or publish some design requirements that define the proper material to be used for various fluids in both the airframe and firewall forward, I would appreciate the education.

 

[apkp777]

While Dan has brought up legitimate points concerning the suitability of aluminum from an outright strength/fatigue perspective, fire is the other point often addressed in these debates. And to that point, I am with you. If you have a fire that burns through an aluminum fitting, you have BIG problems - I doubt having a steel fitting is going to make any difference at that point.

I certainly don't discount the work that Dan has done on this subject. He's brilliant for sure. My point is, a plans built kit is not a powder-keg waiting to burst into flames. I do have at least on fitting that I chose steel. It's the union between my Red Cube and Gascolator. Otherwise, all aluminum. The folks at Van's are pretty smart and the plans are good. I think you're safe to stick with them on this.

 

[TS Flightlines]

alum vs steel-

Aluminum adapters (AN fittings) are fine FWF--as long as they ARE AN , and not a look allike. AN grade fittings are much stronger than the nock-off imports that look like AN. Steel and stainless provide a margin of safety, but are heavier.
As for aluminum hose ends---those made from AN aluminum are also ok--its the rubber lined hose that will give up before the fitting, unless the fitting is damaged. Many companies sell the red & blue anodized hose ends for "701" style hose, but they may or not be AN grade material.
I solved this hose end problem by using stainless, and the rubber hose issue by using stainless braid teflon--AND everything carrying fluid FWF get firesleeved. You can protect against everything, but you can increase the margin of safety. Just my opinion--
Tom

 

[Toobuilder]

... So if you can refer me to or publish some design requirements that define the proper material to be used for various fluids in both the airframe and firewall forward, I would appreciate the education...

The problem is there is no catch all for this ? it?s not as simple as FWF or not. You are looking at the load each fitting sees and have to select the fitting with the appropriate material properties (which are published as the MIL specification). But it?s up to you, the ?designer? to determine the loads. In general, a high vibration environment is not necessarily bad for an aluminum fitting. At least it does not automatically render such a fitting ineligible. It depends how much unsupported weight is hanging off it that makes the difference. A light, well supported hose (or hardline) will generally not cause the aluminum fitting to flex enough to fatigue and fail. However, cobble some AN/NPT adaptors together and cantilever a fuel flow totalizer or flow divider off this same aluminum fitting and you are asking for (and will likely get) trouble. Since aluminum fittings have been used for decades without trouble, it can reasonably be assumed that they work if done properly. I'd suspect that in those rare cases where there was a failure of an aluminum fitting, it was maintenance related (over torqued, damaged, etc) or just poor design.

As the "manufacturer" we are very much on our own here. If you are deviating from standard aircraft practice when installing your fluid systems, then it does not hurt to go conservative and take the weight penalty. But by the same token, the "rules" have not changed for those of us who continue to use aluminum appropriately.

 

[Toobuilder]

...My point is, a plans built kit is not a powder-keg waiting to burst into flames...

We're in complete agreement.

...Disclaimer though - standard aircraft maintenance/fabrication practices need to be followed. No kinks, nicks, gouges, stress risers, twisted hoses, lack of support, chaffing, loose fittings, etc, etc...

...Otherwise it IS a powderkeg!