Virgin Acetal Plastic

[skippydiesel]

While looking for joiners & elbows to suit my 8mm fuel lines, I came across a very light & cheap (compared with brass) irrigation line product by Neta - a whole range of joiners, reducers, elbows & manifolds made out of a material called "Virgin Acetal Plastic".

 

A quick net search suggests this material is unaffected by hydrocarbon fuels such as petrol (& may also be ethanol tolerant). The only small negative appears to a degree of UV sensitivity, hardly an issue in a system which is covered at all times.

 

Does anyone know of any reason for not using these joiners in my fuel delivery system?

 

 

[damkia]

Have they had any vibration integrity testing done? Thermal stability testing?

 

It's one thing to stick them underground in a non-critical fixed environment, another one totally to put them into a hot aircraft subject to a heap of vibration and use them in such a Class 1 critical area as a fuel system.

 

My opinion? There are some things you can skimp on and others you shouldn't skimp on - this is one of the latter.

 

 

[skippydiesel]

I would not consider using them close to any heat source (even though their thermal stability is probably as good as the fuel hose itself).

 

Vibration ?? again unless you have a fuel hose/line up against the engine this should not be an issue. Further I can not imagine a "rat tail" type connector used to connect supported "rubber"fuel lines being subject to destructive vibration. This is the stuff of GA (& higher) hard connect alloy fuel lines.

 

In my situation I must convey fuel from my tank(s) through control valves, a pump, filters and fuel flow sensor - all before passing through the "firewall". It is in this area that I need to use a number of reducer connectors, Y manifolds and elbows - cost & weight are always of interest to aircraft builders/modifiers.

 

I note the increasing use of plastic type fuel connectors and other components in European light aircraft - they seem to be more open to alternative (to metals) modern materials.

 

 

[damkia]

I note the increasing use of plastic type fuel connectors and other components in European light aircraft - they seem to be more open to alternative (to metals) modern materials.

May be better to try to source those.

 

 

[skippydiesel]

I admire those who adopt a risk management approach to life. The rest?? well they had better just stay in bed.

 

I doubt we would be flying at all, if the Wright Bros waited for someone else to combine available technologies into a flying machine.

 

 

[Yenn]

Before you go down the ag pipe road, look at Aircraft Spruce catalogue on line and you will see what is being used for aircraft. I can't see any real savings in price or weight by using plastic, but there are real fears that it may not be safe.

 

 

[skippydiesel]

Have the fears by all means and then apply some science/real world evidence before making an ill informed decisions.

 

Highly durable "plastics" (plastic in this use is a generic term covering all synthetic materials that have the same superficial appearance) are taking over where previously only metals, ceramics, minerals, glass and wood would be considered. In general they are cheaper, lighter, are easier to make and work with (ask your plumber). In addition they have other characteristics, such as impact, fatigue, abrasion resistance, low or nil electrical heat conductivity, chemical resistance, etc etc.

 

I am not "wedded" to plastics, even though much of my aircraft could be considered plastic (glass & carbon fiber) in common with many of the components in the latest military & civilian jets. The reason I asked the question was to get some evidence or scientifically based opinion IS THERE AN INDUSTRIAL CHEMIST / ENGINEER OUT THERE??

 

 

[damkia]

I am not "wedded" to plastics, even though much of my aircraft could be considered plastic (glass & carbon fiber) in common with many of the components in the latest military & civilian jets. The reason I asked the question was to get some evidence or scientifically based opinion IS THERE AN INDUSTRIAL CHEMIST / ENGINEER OUT THERE??

............to save a few grams in weight and a few dollars??????

Total weight of bits you described in plastic would be ~100g. Total weight in brass would be 300-400g.

 

Cost difference in time and consultation costs trying to work out if your cheap plastic couplings are safe to use would negate the cost of "known quantity/known quality" items like brass couplings. Ever heard the expression "throwing good money after bad"? How about "Do it once, do it right"?

 

I'm guessing the QC on any of the plastics used specifically for aviation such as you described will have had all of the testing done in a rigorous way, unlike garden irrigation fittings..........I would even go as far as to say the garden fittings are likely to have a limited life to ensure you come back for replacements - try that with fuel on you lap at 1500'.......

 

 

[skippydiesel]

Great conversing with you Edward - I fly uncertified RAA largely because I was priced out of certified GA - does that give a clue to at least one of the drivers that influence my thoughts & decisions.

 

My little beauty is 13 years old - the main testing it has received is by the time engineer/authority - CAA ,FAA , etc, no where to be seen!!

 

The small/light/ low powered aircraft I now fly is impacted by every gram (adding up to kilograms) - fine if it is necessary, if not, it just reduces performance (fuel burn/cost) for no good reason. It behooves all light aircraft pilots to be obsessive about weight.

 

As far as cost goes - down at my local hardware the Neta fittings are about $0.70 each, compared with $5.00 each for the comparable brass fitting. Aircraft grade brass would probably be very much more expensive (and look just the same).

 

The Neta fittings look very well manufactured being very clean sharp moldings, compared with the rather course finish on the brass. Appearance is only a clue to function/permanence, however the nice prominent "sharp" "barbs" on the plastic "tails" suggests a nice secure leak proof fit. The lighter fitting may also put less weight/strain on the union especially during turbulence or hard landing.

 

One of the great things about modern high volume molding production is the high standard and consistency that can be achieved without additional cost - try that with brass.

 

I have no intention of risking life & limb for a few $$$$$ however after 25 years mucking about with aircraft I have noticed that much of our attitude to the flying world is slavishly accepted from "Uncle Sam" and the risk avers bureaucrats that bedevil the western world.

 

Someone somewhere noticed that polly fibre cloth shrinks when heat is applied - only museums striving for "authenticity" till use cotton, linen & dope to cover aircraft.

 

If a "virgin acetal plastic" fitting is equal to or better (function & cost) than a brass fitting, why not give it a go - that's why I asked the question. Anyone got the answer?

 

 

[Gnarly Gnu]

Hey Skippy seems like you're not impressed with the answers + suggestions provided & are keen to use these plastic garden fittings regardless so why not just let us know back how it works out for you, others may be interested.

 

 

[Old Koreelah]

Hey Skippy seems like you're not impressed with the answers + suggestions provided & are keen to use these plastic garden fittings regardless so why not just let us know back how it works out for you, others may be interested.

Plenty of medical breakthroughs have resulted from scientists taking risks and even experimenting on themselves. As long as Skippy is aware of the possible hazards, why not try it? We may all benefit from his experiments. The future of our sport is in constantly reducing weight.

 

 

[Bernie]

Just a tad off topic, but I have those elbows and joiners in my motor home, not the one in the avata a differant one they hold pressure without any leaks.

 

But I don't use them for fuel, so can't comment on such.

 

Bernie.

 

 

[Head in the clouds]

I am not an industrial chemist but I have used many types of plastics in industrial applications, and have developed a number of products which are manufactured by injection molding, I used to have my own injection molding machine which I used for prototyping. So I'm not an expert but I do know a few things about the reason irrigation fittings manufacturers now use acetal, and why the barbs on them are made sharp.

 

Bear in mind that these types of fittings were originally molded in polypropylene, then they changed to PVC and more recently started to use acetal. 'Virgin' acetal, BTW, is a nonsense, presumably meaning it is not recycled, but recycled acetal is never used for injection molding anyway.

 

The best product for the irrigation fittings would be UV stabilised PVC but it is very expensive and although relatively easy to mold it is slow to cool so the cycle rate of the molding machine is relatively slow, allowing sufficient time for each shot to set before de-molding. Which is why the PVC plumbing and electrical fittings are costly. Acetal is cheap, molds well i.e. it flows well through thin cavity sections and it reproduces the mold well, getting into sharp corners. Polyproylene is not so kindly in the molding department and is slower to cool, when compared with acetal.

 

Acetal is reasonably chemical resistant where hydrocarbons are concerned but it ages poorly. The main problem with acetal is that it is one of the more brittle plastics, and ageing and UV damage exacerbate those conditions.

 

Keeping acetal in the dark does reduce its rate of decay due to UV but it doesn't stop it, the rot begins and continues from the time of first exposure to UV.

 

Although some plastics are more tolerant of liquid hydrocarbon exposure than others it is generally accepted that all plastics are degraded by them to one degree and rate or another. Polyethylene is the most tolerant of the common plastics but is soft and slippery and slow and difficult to injection mold. It is used for rotational molding of fuel tanks, jerry cans etc.

 

The reason that garden fitting tails are made sharp is to help to stop the water pressure from blowing the connections apart, some of the smaller sizes do not use hose clamps. However, have you ever had a good look at old irrigation piping? Over time the piping adopts the shape of the tail and the fit is no longer tight, especially at the sharp edges of the tail. In fact as far as leak prevention is concerned the sharp edges don't do any favours, preventing leakage is dependent on the tightness of the hose stretched over the enlarged part of the tail and rounded barbs provide better sealing area than sharp ones. Anti-blowout is provided by the hose clamp being smaller than the enlarged part of the fitting, so the sharp barbs serve no useful purpose at all.

 

Have you ever seen those fuel sight gauges made from vinyl tubing that runs up the outside of the fuel tank? Vinyl is very fuel resistant but give those tubes about two years and they're brittle, five years and they're dark coloured and very hard and usually starting to crack. They are ten cents worth of tubing, literally, and people don't bother to change it annually even though they can no longer easily see their fuel level. Will you change all your plastic fittings every two years without fail?

 

What it all comes down to is that the plastic fittings will require replacement at some point in time and would anyone know when that time is? Some of the fittings may last longer in the dark recesses than others in UV exposed areas, so the tendency might be to replace the exposed ones regularly and forget about the ones that are out of sight and hard to get at. At least with brass fittings you can more or less set and forget them. I use 6mm annealed copper tubing in all inaccessible areas and silver-solder the barbs on. BTW I only paid about $2 each for the barbs by buying double ended barb joiners and cut them in half and drilled them to fit the copper tube. You can get quality brass barbs from any pneumatics supplier - and also some lovely high pressure pneumatic push together fittings that are wonderful for hydraulic brakes as long as you use transmission fluid and not brake fluid.

 

So as far as the cost savings are concerned by using plastic I don't think they exist, take a bit more time and do the job properly, you're talking about fuel here. Imagine an unfortunate incident on landing, didn't realise how soft the ground is, nosewheel digs in, plane flips, engine still running, broken fuel line fitting, pump running, gotta get out, which way is the door, smell fuel, whoooooosh - another Darwin award. if there's one thing in a plane I'll spend a couple of extra dollars on, and the extra time to do it right, it's the fuel lines, that's the fittings, the hoses, the clamps, the routing, the insulation, the protection of them. I'd hate to burn ...

 

 

[skippydiesel]

At last!! a well reasoned response with a dash of science, real world observation & comment, rather than the ultra conservative stick with the herd mentality.

 

Thank you "Head in the clouds" I was starting to think the rec aviation crowd might be addicted to banjo playing (with the exception of Gnu & Koreelah who at least left the door of inquiry open

 

One or two points of order -

 

- I don't think I mentioned or expected any time saving in plastic V brass.

 

- The $$$ savings are undeniable with the plastic fittings being less than 1/4the price of brass.

 

- Weight savings are similarly undeniable. Haven't actually weighed the Acetal or brass, but I would guess the Acetal to be in the realm of 1/8 the weight of brass, may be better.

 

- My rubber hoses (fuel, oil, coolant) & other rubber fittings are all replaced every 400 hrs - may be just as easy to replace the cheap connecters at the same time.

 

- The last paragraph did stray into the realm of over kill - I can not afford to be risking my aircraft (let alone myself) by voluntarily landing on an unknown surface - this is the stuff of Carbon Cub jockeys with their over sized equipment blowing in the wind.

 

Perhaps its time to call it quits on this little question - there may be a cost effective durable light plastic substitute for brass just around the corner but I dont think its "Virgin Acetal Plastic" - anyone know what sort of plastic is being used to make induction manifolds on cars engines these days??

 

 

[Old Koreelah]

Thanks for the mention, Skippy (my ego needed that).

 

I too was informed by the post from HITC.

 

Although I love innovation, hard experience has taught me to listen to the "banjo-playing" conservatives. At least some of them were like me once...

 

But. If we lucky few are to fly for fun in the coming decades of scarce resources, we need to adapt fast, and get the weight down.

 

In the decades since people like Burt Rutan revolutionised aeroplane design, amazingly strong new materials have been developed.

 

We all need to keep an open mind.

 

 

[Pilot Pete]

I too have often thought about cheaper alternatives that are safe enough and have the quality needed to stand up to the rigours of flying. I have been reading these posts with interest.

 

As a point of interest, what would be wrong with using airlines as used on trucks, I have used and installed these lines before( on trucks that is ) and have noted that most times they get covered in all sorts of gunk from deisel to corrosive chemicals, get banged arround, shaken, rubbed and pelted with rocks and stones and still have to hold air pressure.

 

A cheaper alternative with a proven track recored?????

 

 

[Head in the clouds]

- I don't think I mentioned or expected any time saving in plastic V brass.

True, but it does take a lot longer to make up copper and brass silver soldered fuel lines than it does to push a few plastic barbs into flexible lines, that's all I meant.

- The $$$ savings are undeniable with the plastic fittings being less than 1/4the price of brass.

Yes, but it would be a tiny percentage of the total cost of the project, and is it worth cutting corners on the potentially most dangerous system on the aircraft? Keep in mind you don't have to have a crash to catch fire, and a fire while airborne isn't a matter of jumping out of the plane and grabbing a fire extinguisher. It would only take a small leak in the engine bay to end up with a small exhaust-ignited fire and once it starts it'll burn through the fuel lines and you've got a big fire on your hands. Or - if you're going to save every possible cent on your fuel fittings you're probably going to do the same on your electrical wiring, an electrical short could set that tiny leak alight somewhere in the hidden recesses.

- Weight savings are similarly undeniable. Haven't actually weighed the Acetal or brass, but I would guess the Acetal to be in the realm of 1/8 the weight of brass, may be better.

Yes, but how much acetal or brass are we actually talking about? My present project has all brass fittings and all copper lines. The only rubber lines (and they'll be high pressure race-car style fibre re-inforced, double skin with stainless external braiding protection) are where I need flexible lines for wing folding and to allow engine vibration/movement. Even so my entire length of fuel lines and fittings would only weigh about 500g. And cost - $10 for the 3m of copper tube, $8 for the two double ended barbs and $20 for the brass pipe fittings to solder to the lines and screw into the fuel valves and fuel pump. And $10 for the silver solder and flux. Then it'll be about $100 for the braided flexible lines. So that's about $150 and 1kg (incl the braided lines) for a bulletproof fuel system. As a percentage of the $45,000 the project is costing me it's probably the cheapest and best value part of it.

 

- My rubber hoses (fuel, oil, coolant) & other rubber fittings are all replaced every 400 hrs - may be just as easy to replace the cheap connecters at the same time.

That's good to hear but is probably poor scheduling. I think you mentioned that you're origins were GA and that's not how they do it in GA. Rubber components age with time rather than engine/flight hours. Given that the average utilisation of recreational aircraft is something less than 40hrs/yr it means you'd typically be replacing your rubber and plastic fittings every ten years and that's way past their use-by date. The far bigger problem comes when you sell your plane to someone who doesn't realise that those items needed replacing years earlier. Frankly I think this whole plastic fuel fittings thing is a false economy. Whichever way you look at it flying is an expensive pastime and if people can't afford to make a safe fuel system, at the very least, then perhaps they should find a more affordable hobby.

 

- The last paragraph did stray into the realm of over kill - I can not afford to be risking my aircraft (let alone myself) by voluntarily landing on an unknown surface - this is the stuff of Carbon Cub jockeys with their over sized equipment blowing in the wind.

Australia has some of the most rapidly changing weather on the planet due to our global location, topography and massive inland deserts. No flyer in this wonderful land can consider themselves immune from sudden weather changes which may alter their intended flight regime. So if you spend any significant time airborne you will most certainly have to land somewhere unexpected on occasions.

 

Perhaps its time to call it quits on this little question - there may be a cost effective durable light plastic substitute for brass just around the corner but I dont think its "Virgin Acetal Plastic" - anyone know what sort of plastic is being used to make induction manifolds on cars engines these days??

The exact composition used by each manufacturer is a well guarded secret but essentially they are fibre (not glass) re-inforced Polyamides (the nylon family).

 

I've attached a couple of pics of my fuel lines in case they might be helpful to anyone - the red insulation tape is covering the brass barbs and blocking the fuel lines to keep ants and wasps out.

 

 

 

 

 

 

[Old Koreelah]

You've convinced me, HITC. Well-presented argument, neat installation.

 

 

[skippydiesel]

I second Koreelah -

 

Your comment on weather change - I have lived in N America & the UK - if you want rapid weather change go there. This is reflected in the number of IFR rated pilots (private & commercial) in these parts of the world. Our flying conditions are amongst the best you will find anywhere. This said I did take your original point, but I am not about to fit balloon tires in the off chance of an emergency landing on an unsuitable bit of earth.

 

Grams turn in to kilos cents into dollars, why would you not explore options to save? I recently emptied my aircraft of all loose items, accumulated over about 10 years, not required for legal flight. Each item only weighed a few grams - total weight a whopping 20 kg of wasted performance. Not strictly related to fuel delivery systems, but illustrates how weight, a few gm's at a time, can easily add up if not careful monitored.

 

My apologies for abbreviating - I should have said 400 hrs or 5 years for the rubber components.

 

In the last three years I have averaged about 80 + hrs / year.

 

All my fuel lines are "rubber" as per manufactures specifications. On my last rubber change I upgraded all fuel lines to (Gates) EFI automotive standard, giving Ethanol compatibility (no I will not intentional use E10) higher burst & temp resistance & lower porosity. Previous hoses (as supplied by factory) were standard grade that you might find on a mower, stationary engine or old car.

 

Other rubber components have been replaced with manufactures parts or higher (than original) spec oil & coolant lines

 

I tell you these things so as to demonstrate my commitment to safety - this does not prevent exploration, investigation and a little trial, hopefully without to much error, from time to time

 

 

[Downunder]

For what it is worth, I've machined some small parts from acetal rod.

 

Found it quite soft and would not recommend it for fuel purposes.......

 

 

[Methusala]

Experimentation and innovation are good. When researching a job that must be done on my aircraft I, (as with Skippy) ask questions of all whom I believe may have an informed opinion. Armed with this knowledge and my own findings I make a decision and go ahead carefully. This 'works for me'. As for the claim that GA always get it right, I remember a case of a Piper that had landed at Temora with a strong smell of avgas in the cockpit. We investigated and found an aluminium fuel line that connected the two wing tanks through the rear of the luggage area had developed a radial crack due to vibration. Being a plumber I used a mini tube cutter to complete the cut and we performed a temporary repair by sleeving with a piece of efi fuel hose and a pair of pipe clamps. This was only to allow the aircraft to return to its base. Absolute safety is an illusion, Don.

 

 

[turboplanner]

Absolute safety might be an illusion, but it is prudent that one of the highest risk areas gets the nest design and materials.

 

The downside of something failing could be a spray of fuel under pressure leading to a fire in the air.

 

In building and racing cars, I've seen line failures and fires as a result of corners being cut with a lot of laughs all round since no one was hurt and the operations were ground based.

 

Aside from the vibration and heat issues, another consideration is the relatively high pressure, where the elasticity of plastic fittings over time leads to cracks. My experience is this occurs usually on the inside of 90 degree bends.

 

As a minimum I always fitted reinforced lines, but in drag racing where the fuel line is about the side of a garden hose, and to make 12,000 hp the flow is about the same, lines are reinforced by woven steel to take care of the elasticity.

 

Because of the potential for an airborne leak, I would be recommending steel reinforced lines and threaded steel/brass fittings.