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Rigid Fuel Lines

efitzgerald

Active Member
I am working on my firewall layout and started on the fuel lines. I am using a SDS fuel injection system with their case mounted fuel block. I had originally planned on using flex hose in fire sleeve from the firewall to the fuel block. I saw another members post with a similar install and he had used hard lines from the back of the engine near to the fuel block and it looked like a great idea.

In my picture the lines are aluminum for the purpose of mock-up only. I was going to have TS Flightlines make the stainless lines. The lines would be flared on each end with aluminum AN nuts. There would be a bracket bolted on top of the fuel pump block off plate to secure the end of the rigid line where the flexible hose connects to it with steel AN fittings. I planned 1 to 2 brackets bolted onto the engine to support the lines.

Is it a mistake to do it this way?

FL6.jpg
 
Hmm. Looks like I'm going to have to reevaluate my routing due to the engine mount. Nw I wish I hadn't just put the holes in my baffle...

Guess we'll see when I go to hang it in a little while.
 
I am using a SDS fuel injection system with their case mounted fuel block. I had originally planned on using flex hose in fire sleeve from the firewall to the fuel block. I saw another members post with a similar install and he had used hard lines from the back of the engine near to the fuel block and it looked like a great idea.

Why does it strike you as a good idea?

Seriously, every possible choice has some advantages, and some compromises.
 
- Dan

The engine has a lot of fluid connections behind the left side. The right side has the backup alternator in the way so it is not an option. I like that the metal tubing is smaller in diameter and can be routed precisely where it is wanted. The engine mount is also very close to the engine in this area, wouldn't chafing / rubbing be a problem with flexible hoses? To me is seems like the rigid tubing is less of a maintenance concern than 3 feet of flexible hose that is covered in fire sleeve. Aesthetically hard tubing looks better than hose to me as well.

This engine uses stainless tubing for the prop oil feed so it seemed like an acceptable option.

I made this post as I really don't know if the potential for failure is worth the advantages that I see. I am simply looking for information.

Thanks
 
The way you have it looks like the hard lines on the rear of the engine would get in the way of maintenance, especially if you need to remove that ignition coil assy. You could use hard line up to the rear baffle and use bulkhead connectors there and hose running down the back of the engine. Or even better is making relatively short hoses from the bulkhead connectors to the firewall and use hard line up the firewall to the same height as the bulkhead connectors. Obviously need enough service loop in the hoses to accommodate the 'shake like a wet dog' that Lycs do on startup/shutdown.

BUT, you have a more serious problem: your lower Lord mounts are installed backwards.
 
- hgerhardt

The upper and lower mounts are installed in opposite directions? The engine is only installed temporarily but thank you for pointing that out. Where are the engine mounts shown in Vans drawings?
 
Thank you, Heinrich.

That is very useful information. It even explains why the Vans engine mount bolt kit includes 2 extra large washers.
 
Make a list!

The key downside is probably vibratory fatigue, so even being stainless, it will need care in fixating it along its length. Take a look at the Adel clamp call out for the prop governor line. Got engine hard points available to mount the Adels?

Half the run will flex lines anyway (firewall to the stainless tube connection), so there is no actual maintenance advantage.

The bracket at the hose/tube junction will need to be pretty tough.

There are a few extra flare connections. Statistically, more connections mean more potential leaks.

You mentioned aluminum hardware on the ends. Why not steel? Stainless tube flares are not soft. Anybody have a torque chart for steel B-nuts on steel lines?

As I recall, certified engines were subject to an AD to replace prop line fittings with steel after some aluminum elbows cracked in service. Given the unknown vibratory behavior of a steel hard line, are you planning to replace the inlet and exit elbows on that fuel block?

Advantages? Small diameter so perhaps easier to squeeze through a tight spot.

Looks nice.

Anything else?
 
Without doing any real analysis or calculations, I?m going to SWAG that this is not a good idea. There is relative motion between the engine (on flexible mounts) and the relatively stiff airframe. Yes, there should be plenty of length to not over stress the tubing but the metal will fatigue and subsequently fail at some point. You could ?steam loop? it to spread out the related stresses or install a short run of flexible tubing to isolate the vibs and related cycles. Yes. I?m aware that there are certified aircraft that utilize hard tubing between the engine and fixed connections but they are usually small gage (=less stiff) sense lines and still utilize such stress management loop configurations. As someone has mentioned, designs are full of compromise. Choose yours wisely.
 
- Freemasm

I?m not saying what I proposed is a good idea but you misread or missed something.

The hard stainless tubing would be run between two fixed points on the engine and connected to the firewall with flexible hose.
 
Let me say that I?m wicked guilty of not reading for detail like I should. The plethora of electronic messaging/media has made me scan to try and get through everything. Sounds like you?ve thought it through. My bad. Build safe.
 
Hard lines

I know this is a little bit of apples vs oranges, but the stakes of your decision may be high.

In 1973 I had an engine failure in a Globe Swift when the factory installed aluminum line between the engine-mounted fuel pump and the carburetor failed due to fatigue. I was about 100 feet in the air on takeoff (for what was supposed to be my first trip to OSH) and the engine quit immediately. The fuel spray also caught fire. Although I got it down ok the airplane burned to the ground. Had I been higher when it failed, I would probably not have made it.

Yes you are talking about stainless, but rigid lines are still a potential fatigue issue, and I cannot really see a benefit worth the risk trade.
 
I've said this a million times and probably have a million more in me, but rigid lines are the aerospace STANDARD. Flex lines are the EXCEPTION, used only when there is significant relative motion between components or maintenance access requires it. This is not an opinion of some guy on the internet - it's the engineering standard for aerospace.

Properly fabricated and installed hard lines will last the life of the airframe many times over. There is NO hose assembly that can make that claim.

To the OP - properly fabricate, install and support your fuel lines and you are essentially done forever. Compared to hose, your weight will be down, MTBF higher, cost lower, and maintenance non existent. Other than that, hose is fine.
 
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- Freemasm

I’m not saying what I proposed is a good idea but you misread or missed something.

The hard stainless tubing would be run between two fixed points on the engine and connected to the firewall with flexible hose.

For fixed runs of line that aren't exposed to relative motion - hard line makes sense. As mentioned previously, you should not mix your fitting and line metals. Are you using SS tubing because of either a perceived need for greater strength or fire resistance? if so, why aluminum fittings then? (BTW, I don't consider either drivers for SS pipe on such low pressure systems...different topic)

If it's a cost saving measure, reconsider saving money somewhere else...maybe skip that new lawn troll. ;)
 
- Michael,

I have read many of your posts on hard lines and they were one of the reasons I felt it was an acceptable choice. As well as the prop oil tube being made from stainless with only one support along it entire length.

In the end I'm just going to go with the standard fire sleeved hose. It is less work and probably will be cheaper.

I was planning on steel bulkhead fittings on the firewall. What about the rest of the fittings? Is aluminum acceptable with flexible hose?

Thanks for the input.
 
There is no way hose is going to be less expensive, not by a huge margin. A quality, conductive lined firesleved hose from someone who knows what they are doing (like Tom at TS Flightlines) is going to cost north of $100 bucks (and well worth it, if you need flex hose). That same length of SS Hardline and fittings will cost about $10 bucks at most. And don't even get me started on the difference in weight...

Also, it's perfectly acceptable to mix aluminum nuts and sleeves on SS tube. There are aerospace specifications that describe that exact combination. There are reasons you don't often see aluminum nuts on SS, but it has nothing to do with dissimilar materials. The general reason is because the operating pressure of the system that drives you to use SS tube is so high that it also drives you to steel fittings. Under normal circumstances, our fuel system pressure is so low that aluminum is more than adequate. It's the fire scenario that drives me to use SS tube, not the pressure.
 
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- Michael

I bent tubing out of aluminum and was going to have Tom copy them in SS. I never got pricing from him but he made it sound expensive, or at least time consuming. It is my understanding that flaring and bending tools for aluminum are not appropriate for SS tubing.

I am aware that the firesleeved tubing is not cheap.
 
There is no way hose is going to be less expensive, not by a huge margin. A quality, conductive lined firesleved hose from someone who knows what they are doing (like Tom at TS Flightlines) is going to cost north of $100 bucks (and well worth it, if you need flex hose). That same length of SS Hardline and fittings will cost about $10 bucks at most. And don't even get me started on the difference in weight...

Also, it's perfectly acceptable to mix aluminum nuts and sleeves on SS tube. There are aerospace specifications that describe that exact combination. There are reasons you don't often see aluminum nuts on SS, but it has nothing to do with dissimilar materials. The general reason is because the operating pressure of the system that drives you to use SS tube is so high that it also drives you to steel fittings. Under normal circumstances, our fuel system pressure is so low that aluminum is more than adequate. It's the fire scenario that drives me to use SS tube, not the pressure.

I agree with everything you said except:

Also, it's perfectly acceptable to mix aluminum nuts and sleeves on SS tube. There are aerospace specifications that describe that exact combination. There are reasons you don't often see aluminum nuts on SS, but it has nothing to do with dissimilar materials.

Aluminum fittings on stainless lines and hardware WILL corrode over time. In theory the anodizing should protect against it but it doesn't seem to be enough. I've seen Apollo program hardware that used aluminum AN nuts and sleeves on CRES lines and there was enough corrosion growth build up between the sleeves and the lines to split the sleeves. This hardware was plumbed inside nitrogen purged explosion proof enclosures but it was 30 years old at the time. This was on either a LOx system or Isopropyl Alcohol system but there was no evidence that there were any fluid leaks (the IPA was dyed) thus the fitting sleeves were likely not exposed to the propellants. I've seen many instances of aluminum B-Nuts corroding on the threads when they were used on CRES or carbon steel fittings. Generally when other types of steel and aluminum hardware are interfaced in the aerospace industry, a corrosion inhibiting primer is required on at least one of the interface surfaces.

Occasionally you might see aluminum fittings used on stainless hardware on short life-span hardware where weight is extremely critical but the stainless lines are needed for strength or fatigue life such as on rocket boosters or prototypes. For aircraft that expect to be in service for many years, this is not a good idea unless it is a regularly replaced item.

As I recall, Space-X had a failure of one of their early Falcon-1 rockets due to fitting corrosion.

Skylor
 
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- Michael

I bent tubing out of aluminum and was going to have Tom copy them in SS. I never got pricing from him but he made it sound expensive, or at least time consuming. It is my understanding that flaring and bending tools for aluminum are not appropriate for SS tubing.

I am aware that the firesleeved tubing is not cheap.

The SS tube I use (Spruce) takes more effort to bend, but unless the tools are really sketchy, they should work fine. A bit of lubrication on the sliding shoe goes a long way to reducing the effort. All my bending tools are common stuff most of us are already using. Flaring is also no problem with reasonable quality tools.

Spend a few bucks on some stainless tube and give it a shot - besides, you already have the patterns made!
 
Skylor - I'm not an materials engineer so I'm not going to offer an opinion concerning the "cause" of the corrosion you speak of. All I can stand on is my experience in big aerospace and the process specifications I've seen that allow certain mixing and matching of materials. As an aside, one company is MILITANT about keeping cadmium and Titanium apart at all costs, while others are much less concerned. I HAVE seen stainless Bnuts literally welded to stainless fittings due to severe galling, however... Anyway, I have made many SS/aluminum tube assemblies and have yet to see any corrosion first hand. The lines I made for the RV-8 back in 2009 still look like the day I installed them.

But the overarching fact remains that there is little to be worried about WRT properly constructed and installed hardline. I know that in my 30 years in aerospace, I've replaced a lot more hose due to age, leaks and abrasion damage than hardline. And considering that a typical military aircraft uses so little hose compared to hardline (maybe 50:1), that's saying a lot.
 
no alum hard lines where you have engine vibration period. use steel or inconel tubing. engine rpm =2700 equals 45 Hz frequency. you should probably be 25% above a 4 per rev for a 4 cylinder, that would be 1.25(4 x 45) = 225 Hz. tube clamps can help you achieve that. I modeled what you have with no tube clamps, just fixed at the ends so you can see the response. this model is AN6 alum tube filled with fuel, .035 wall. the high stress points are at the fixed connections at the ends. obviously this doesn't work as is because mode 1 and 2 are below 225 Hz. also, make sure you tubing is not stressed by thermal movement of the engine. choose your clamp locations wisely.

another option could be flex tubing that is designed to move. use tubing that can take the heat. clamp it several places. replace it periodically as a normal maintenance item.

Frequency response:
mode 1 = 72 Hz
mode 2 = 203 Hz
mode 3 = 232 Hz

mode 1


**
mode 2


**
mode 3

 
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There is no way hose is going to be less expensive, not by a huge margin.

Gotta buy hoses anyway, for the run from the firewall bulkhead fittings to the ends of the stainless hard line.

Spend a few bucks on some stainless tube and give it a shot

Mike, where and how would you fixate the tubing to the engine case? I'll bet the aerospace applications specify mounting points.

I modeled what you have with no tube clamps, just fixed at the ends so you can see the response. this model is AN6 alum tube filled with fuel, .035 wall. the high stress points are at the fixed connections at the ends. obviously this doesn't work as is because mode 1 and 2 are below 225 Hz

Where's my like button?
 
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Gotta buy hoses anyway, for the run from the firewall bulkhead fittings to the ends of the stainless hard line.



Mike, where and how would you fixate the tubing to the engine case? I'll bet the aerospace applications specify mounting points.

Yes, you need hose to get from the ship to the engine, no way around that, but when paying by the inch, shorter is better. My solution was to call Tom and order a pair of 11 inch hoses which went from the firewall to a stout combination fuel pump block off plate/mount bracket. The hardline ran up the back of the accessory case, grabbing a bolt here and there for Adel clamps, then through the baffles. I didn't use the fuel block like the OP (I built a rail), but there are opportunities to add brackets to case bolts at the splitline or Adel butterflies on the pushrod tubes. There's many opportunities for effective support for a hardline. And of course there is plenty of engineering documentation out there to decide what "effective support" means.
 
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OK---basically what I told Eric was, NORMALLY we'd take a mockup, in his case his aluminum tubes, use the CMM scanner and get the bend dimensions, angles, and spring backs for stainless. Bend a set, and do a comparison. YES----tube itself isnt expensive, but doing things so its DUPLICATEBLE can sometimes be, and alittle time consuming as well. Its like using a Auto parts hand bender verses a good Imperial Mil-Spec one, or the CNC dies we have ( which BTW are mil-spec).

YES---you can take a bent tube and duplicate it "pretty close enough to fit" but that not what I was proposing. And stainless nuts and sleeves. So know I didnt have enough data to give Eric a price. And this project is still in its infancy, and I dont know if he decided what HE wanted to do. BUT, I was always planning on stainless, all thickness TBD, but probably .035. We did discuss the need for suppor in at least 2 places above the cylinders. That also was TBD, but NOT around the pushrod tubes.

Granted from my perspective, a good firesleeved teflon hose would be just fine if the bends could be relaxed just a little bit. Hoses definitely from the firewall to the engine area anyway.

Tom
 
Lycoming use to use aluminum hard lines for prop Gov. They cracked and they went to stainless or flexible lines.

Metal tubes do fatigue as the engine shakes. However if the tube is well supported and does not flex the fatigue life will go up. With the lack of test and analysis, I'd consider these life limited and replace them at engine overhaul.

This being the fuel line over your HOT engine, I would go conservative. My opinion only, flexible (which is also life limited).
 
Personally I would use high pressure flex lines, but if you do choose stainless hard lines please don't use aluminum fittings as you are asking for a crack down the road. The proper fittings are suffix "J" as in AN818-4J, AN822-4J etc. the "J" suffix represents corrosion resistant steel fittings. Fittings without a suffix are plain steel and could work. I wouldn't use suffix "D" aluminum fittings in that application. In addition all fuel injected Lycoming's have an AD regarding the proper support, with Adel clamps, of the ridged injector lines..... due to cracking. So you need to properly support the hard lines. I would use the MS21919 WCJ series of Adel clamps for that application. I also would recommend Loctite 567 for any pipe threads used. Good luck, Russ
 
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Was it aluminum, stainless, a copper primer line or a brazed fitting on the end of a fuel injection line?

This was an aluminum line coming out of the firewall that twisted around to the gascolator which was also mounted to the firewall but on a bracket that allowed just the slightest amount of flex. Slightest.

So the story goes that I'd read a comment by Van that RV pilots never really explored stalls or low speed handling but rather were content to just go blast through the sky. Unrelatedly, I also realized that he I'd never really thought about emergency procedures in my RV-4, so one day I spent a few minutes in the cockpit, thinking about same.

One evening, I went out to do a few stalls and no big deal, power off. I added power to climb back up and the engine quit, prop still windmilling. Emergency procedures to the fore! Hit the boost pump, engine runs, happy boy.

Oh, what the hey, let's go back home and check it out. On that flight, I was also looking to burn off some fuel so I could remove the left fuel tank for maintenance, so I *knew* from the dipstick that I had four gallons in the left and 13 in the right when I took off, and I was burning from the right tank.

Went home, nice landing, taxied to the hangar, engine running, wondering what test I could do before shutdown when the engine shut down all by itself. Was going to restart it but, no, let's take a look first.

All seemed good but the right gear leg was black and blue. There are no black and blue fluids in the plane...

On further examination, the aluminum fuel line had cracked at the firewall. Just a little crack. When this happened, the engine-driven pump was sucking air and the engine quit. When I hit the boost pump, the fuel line was pressurized on the other side of the crack and the engine ran.

But here's the scary part -- when the boost pump was running, I was pumping a gallon a minute overboard inside the engine compartment, exhaust stacks ten inches to each side. At the hangar, the engine quit due to fuel starvation.

The black and blue was blue avgas and black was grime cleaned out of the engine compartment.

One other poster to this thread pontificated about hard lines being the standard in other parts of aviation, but you had to read the fine print, several paragraphs down, that his comments were for properly supported and properly installed hard lines. There's more...

Aluminum hard lines have no place when one end is rigid and the other can move. In my case, it was firewall to gascolator. In all RVs, it's firewall (or gascolator) to engine. And don't forget that a prototype F-14 was lost when hydraulic lines cracked and leaked -- and I think those were stainless steel.

"When things work right, it's recreation. When they don't, it's education."

Be safe, be well, wash your hands...
 
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Bug in the posting software.

Curious! I submitted my post, the first on page 4... but it didn't appear, even though it was listed as the latest post on this thread. Then when I created a post about its disappearance, like this post, and it reappeared. So I'll leave this post in...
 
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All---just for the record----IF we were to make and use rigid tubes FWF they WOULD BE STAINLESS. That been my personal policy for years, and its not going to chnage. And yes, stainless nuts and sleeves. Only difference is the wall thickness of the tube. Not that .035 isnt sufficient for most all of our applications, we dont have some things where we crimp a teflon hose to the tube, making an integral tube/hose assembly. That being the case, the hose end of the tube needs to be machined to accept the ID of the hose--so a thicker wall is necessary. Our Andair valve/wing hoses are dont that way for the RV7s, only we use 6061T6 for those.

Aluminum tube has its place, but my opinion (ONLY) I dont feel FWF is one of them. Eric asked if we could duplicate his tubes, and we responded.

Tom
 
Not sure why this subject always goes down this same path. Hard lines are the aviation standard, and anyone who has been inside the guts of an airplane and/or employed by a major aerospace company knows it. It's fact, get over it and move on. That said, proper design, fabrication and installation is required. That's not "fine print" - that's "airplane building 101" and should be assumed. The exact same caution applies to hose assemblies and EVERY OTHER aspect of the airplanes you are screwing together in your garages. Yep, hard lines have failed. So has hose. So has the fuel system when RTV made it to the flow transducer, or the rudder fluttered because of too much Bondo and paint, or a trim tab had too much slop. The list of transgressions that have killed people in aviation is so extensive that an argument could easily be made that aviation is too risky to bear. Using the same "debate tactics" used in this thread, it would be very easy to conclude that flying single, piston engined airplanes built in a garage is the height of irresponsibility - and there is a plethora of data to back it up.

So if you want to stick with hose because you are not willing to learn how to do hardline correctly - fine. But don't think for a moment that you are off the hook in your responsibility to use the same level of care in your hose installation, because you're not.

Whether you have 30 years in big aerospace like I do or you are just building something for the first time, we are building airplanes here - there's no shortcut in the responsibility to build a safe product. Even blindly following the plans does not absolve you of the responsibility to build with standard aviation engineering principles. This forum can be a great resource at times, but there is no way it's going to be a step by step "how to" for every build scenario. Bringing it back full circle, the OP's scheme for hardline is valid, and I'd do something very similar, but that's with the caveat that he's looked at the readily available engineering texts (or at least 43.13), AND has a basic grasp of mechanical engineering. I make that assumption for EVERY post I make on this and other forums.

This is serious stuff people - even if you follow the plans to the letter - remember that.
 
back to the OP topic...

I am working on my firewall layout and started on the fuel lines. I am using a SDS fuel injection system with their case mounted fuel block. I had originally planned on using flex hose in fire sleeve from the firewall to the fuel block. I saw another members post with a similar install and he had used hard lines from the back of the engine near to the fuel block and it looked like a great idea.

Eric- as a suggestion, I would hold off on fuel line routing till later in the FWF process. After you get the baffling fitted, and oil cooler and lines in, you will have a better idea of obstacles and clearances (oil cooler & lines, etc) before mapping out your fuel lines (the most easily routable items, actually).
Also a question - why are you positioning the fuel manifold farthest forward on the engine case? I have mounted these mid and aft top positions in the past and had good results, as well potentially saving of between 2 to 3 foot less fuel circuit run length.

& actually, I'd stick with flex lines throughout and save the complexity...

Good luck with your build!
 
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On this engine the fuel block can only go in two spots. The middle mounting point leaves it sitting at an angle due to the bolt holes being at different heights. I don't believe that was the situation on my Superior 382, but I could be wrong. I could have SDS make a custom mount.

I am going to wait until later for the fuel lines. I am also thinking that if I go with the hard lines at the manifold I will do it a bit differently. I will keep the lines as close to the center of the engine as possible so a sturdy mount can be attached to the center bolts. Then use a heavy support at the engine baffle with bulkhead fitting where flexible lines would be used to run to the firewall. Originally I was thinking that I needed to stay below the engine mount but that doesn't seem to be the case.

And so we are totally clear: There was never any plan to use aluminum tubing other than for mock-up. The aluminum nuts and sleeves were a mistake. I have seen them used on stainless tubing / hose assemblies but that was probably because they were for aluminum fittings.

I appreciate the comments and information.
 
Thought I'd follow up on the running comments regarding the "incompatibility" of aluminum B nuts and tube sleeves on SS tube. Since the cowl is off the RV -8 for an ignition upgrade and the SS line from the fuel pump to the servo has been untouched for 11 years, I figured I'd crack the nuts and slide the sleeves back to take a peek. Surely, with 11 years of these two incompatible materials being in touch with each other's there would be some evidence of the corrosion that grows so forcefully to split the sleeves as described earlier in the thread? Nope. Not in this case. Not a speck of any corrosion. The sleeve slides and rotates on the tube freely, and a visual inspection shows it looks like the day I installed it.

I'll report back in another 11 years.

But for those so concerned, please inspect all the stainless steel screws holding your aluminum airplane together at once, and for goodness sake inspect the SS firewall in case it touches the aluminum fuselage anywhere.
 
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Regarding the above observation. I’m not surprised, assuming your aircraft has spent most of all of it’s life in the high desert. Dissimilar metals (per the galvanic series) can exist just fine without a catalyst like moisture. Here in Florida, stainless screws will corrode the adjacent aluminum in fairly short order, even with perfect paint. Combined with heat, this speeds the reaction even more. Said moisture will make 7000 series aluminum corrode from the inside and effectively “total” an aircraft. There’s more factors in play when considering a variety of conditions.
 
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7000 series aluminum is a special corrosion case on its own merit. Fortunately, it's use is rare in our world.

That said, I wholeheartedly agree that the environmental factors are an important element of the corrosion puzzle. But there is no denying stainless and aluminum are used together in primary structure without incident generally. The SS firewall riveted directly to the aluminum fuselage is perhaps the best example as related to the RV fleet. And of course the very popular "stainless screw kits" used to hold panels on all our airplanes and the spam can fleet. Or the popular stamped stainless rocker cover screw kits threaded into the aluminum heads. Seems if stainless screws can "total" an aircraft we would have heard about it by now? Does Van still provide aluminum fittings for the SS firewall penetrations?

Yep, metal can corrode. I have some nasty, 60 year old AN fittings from my old Mooney that I would not re use. It happens. I've also had brand new flex hose collapse the inner sleeve and starve an engine of oil. But that's not reason to throw the "it might happen" boogie man at a situation just to prove a point.

It's STANDARD practice in aerospace to mix SS and aluminum. The trick as an airplane builder is to be smart enough to know when it's appropriate and when it's not. If someone isn't willing to invest the time to learn, then stick to the plans.
 
A nice controlled response. I stated intergranular corrosion can total an aircraft, not an uncommon event in Mooney world. Stainless and aluminum has a galvanic risk and should be avoided in structures. Look at a metal galvanic chart. There?s a big difference in design compromise when boundary conditions dictate such e.g. your stainless steel firewall is a fine example. BTW there?s large metallurgical differences between the plethora of stainless alloys.

That said, making a broad statement that mixing metals on tubed systems is fine because it was for you has risks for others. Having a system failure and possible related consequences, because of such stated factors is derelict; especially when there are alternatives that eliminate such risks.

I love this forum. It does lead to ?standardization of deviation? in some cases; an attitude/human factor that greatly contributed to the deaths of seven astronauts.

Build safe/fly safe. I wish you nothing but the best.
 
Slight thread drift here, I hope it's ok.
In the case of a hard line from the fuel pump to servo; is there a way to increase protection of the line from heat? If one were to use hose, the necessary fire sleeve would help. But in the case of a SS hard line; how would one insulate it? I ask because I have a slightly less than normal situation running this line in a very tight route close to engine case, cylinders, and exhaust.
 
Slight thread drift here, I hope it's ok.
In the case of a hard line from the fuel pump to servo; is there a way to increase protection of the line from heat? If one were to use hose, the necessary fire sleeve would help. But in the case of a SS hard line; how would one insulate it? I ask because I have a slightly less than normal situation running this line in a very tight route close to engine case, cylinders, and exhaust.

Sure, you can use silicone-impregnated fiberglass sleeving from McMaster. I used this on my home-swaged steel-braided teflon hoses, but could just well be used on rigid tubing.

To help installation, I made a bullet-shaped guide from a wood dowel turned in a lathe and stuck in the end of the B-nut. It doesn't say so in the description, but this stuff does stretch a bit to go over the nut. Then I used heat shrink tubing on each end to keep the fibers contained and finally a band-type hose clamp on each end to hold the sleeve in place like you see on commercially-made firesleeved hose.
 
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