Fuel Tanks – Modern Fuels – Thread Lockers and Sealants - In Need of Some Advice


I am building a CH750 in the UK –  and am ready to install the fuel tanks into the wings.   However, I am now full of doubt as to what ‘thread locker’ to use on the finger screen and nipple fitting and what ‘sealer’ to use on the screws that locate the fuel senders into the tanks.   I am thinking that any leaks in the wing tanks can only mean one thing - and that’s opening up the wings!   It seems that with modern fuels the way they are – with increasing amounts of ethanol/alcohol in them – most of the test results on sealants and thread lockers are suspect.   For the last few weeks I have been experimenting with two of the most well-known brands, Locktite and Hylomar, with various specimens in glass containers full of either MOGAS or AVGAS.   Interestingly, the thread lockers Loctite 577 and Hylomar 4430 seemed to be okay in both MOGAS and AVGAS, but the gasket type sealants, ie Loctite 5922 and Hylomar Universal Blue, totally disintegrated after a week’s immersion.  Interestingly also, when I had previously phoned up Loctite and Hylomar - I was assured that their products had been fully tested with modern fuels and all  their products were ideal for use in fuel systems.   As I see it - not quite true!   So, should I use the thread locker only and just rely in the rubber seal for sealing in the fuel tank sender units?   Is there a more suitable sealant out there?   Perhaps I ought to use Teflon/PTFE tape for the finger screen and nipple connection ...... at least I would get a good seal, if not locked threads?  I would be most grateful is any anyone who has successfully passed this building stage could tell me what they used/how they got over this problem.

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Afraid I know nothinig about the folding wings - never seen one, even. Although flexible, the braided stainless has a certain degree of rigidity. It would seem to me that if you wanted to keep the line intact, the excess would nearly have to slide into the fuselage when the wing is mounted. Perhaps a tray could be fabricated to accomodate this? I don't know if the resultant flexing and straightening of the tubing would adversely affect the integrity of the line? I've always seen these lines securely fixed and immoveable.  Alternatively, it's very easy to unscrew the fitting from the bulkhead fitting at the wing root and detach it and cap it, but, while the fittings are reuseable, I don't think they're intended for repetitive assembly/disassembly - you'd need someone with expertise to advise you on that - technical support at ANPlumbing.com could probably advise you. Finally, I don't think most would like the idea of such a critical connection having to be repetitively tightened and loosened - just another spot for a mistake to bite you!

Although you didn't ask - my take is that the folding wing option is not very "user-friendly" and I've seen others say maybe they'd use it once or twice a year, say at the beginning and end of a flying season, etc., but it's too clunky for regular use. Perhaps you already have, but I'd definitely talk to someone who's "been there, done that" before proceeding too far. Good luck and keep us posted!



Tony -- here's an idea. If you were to plumb from the tank outlet to the wing root with aluminum tubing that is beaded at the end, and do the same where it meets the fuselage, you could then connect them with a short piece of rubber tubing. This would be easy to remove and install. You could even cut it off and replace it each time, as it would only be a few inches long.

John – It’s me again, I’m afraid.   By the way, I didn’t realise that is was your beautiful green 750 that I commented on a few weeks ago.   It looks absolutely fabulous – a really nice colour and scheme....

I am still researching options and I managed to extract from the designer Caleb, at Zenithair, the fact that I should need an extra 6” or so, in the length of any fuel lines from my wing tanks to accommodate the wing fold mechanism.   I will probably give this an extra two or so  inches – but I am only looking at pictures at this stage.   This wing fold mechanism basically means that the wing pivots around a sort of universal joint.

 I have decided that I would like to follow your lead and put Teflon braided hoses in – with proper end fittings - in place of the supplied black  hoses with jubilee clips (as supplied by Zenith).   [I am slightly uneasy about putting these rubber hoses with just jubilee clips into the wing - where they won’t be seen in the life of the aircraft.]

The complication here is that I need to order flexible braided fuel lines (of the type you describe), with end fittings on them – and fit one end of each to each my wing tanks.   Then, I need to 'close up', ie top skin, my wings, and store them away.   Unfortunately, the amount of space I have precludes me leaving the top skins off until I have built my fuselage.

Like you – I should like to end up with a line going from the tanks to a point at the wing root (where the pipe could be disconnected , if required), and then have a second line to the ANDAIR valve.   This second line I can measure up when I building built my fuselage and cockpit area.

So, what I should like to do – is purchase ‘made up’ hoses from my local hose specialist with the required end fittings.    However, before I talk to a hose specialist, I will have to get the lengths right.   The hoses can be a little longer but they can’t be shorter!   I have assessed that I need about 24 inches (measured on the wing) to get from the tank to the inside edge of the trailing edge (with relaxed bends), and about 12 inches to straddle the gap between the wing and fuselage (this allowing about 8 inches to accommodate the pivoting action of the wing around its universal joint - and again with relaxed bends, and 4 ins to cover the basic wing gap).   As I say and copying your solution, I need the hoses to run to a fitting near the wing root.    It’s at this stage that I rather lose it.

 I have studied your pictures and over and over again, and I cannot identify where exactly your “bulkhead fitting at the wing root” is .    I presume this is the picture of the single pipe /single fitting (Picture 1).    Is this on the side of the fuselage OPPOSITE the wing’s trailing edge?

[I am interpreting the picture of the white unit with three pipes going into it as the back of the ANDAIR fuel valve, mounted on the rear of the centre console (Picture 2).    Looks neat.    The one I cannot understand is the one with the white air brake, green, and black corrugated tubes – which I assume is the protection for the aluminium from the steel braiding (Picture 3).   Is it the cockpit side of the Picture 1?    However, where are the green and white hoses going to?]

So, what I am struggling with - is to get a really accurate assessment of the length of hose that I will need to take me to from the tank to the wing root fitting.  So far, my assessment is about 24 in + 12 in = 36 ins.  Given this I can talk to a hose specialist, order hoses, put my fuel tanks in, and skin the top side of my wings.   If you can help, I need to know the exact location of the fitting near the rear rib.  If I know a location, I can at least estimate the total pipe length required.    When I have built my fuselage – and assuming my fuel hose is the right length, I can exactly place and attach my fitting.

Can you help?   If you can, I should be most grateful.  

Tony Oliver, Essex, United Kingdom

Tony - Two concerns:

One thing to consider about trying to use hoses that are already made-up/prefabricated ... IF you already have fittings on both ends, it will require a much larger hole in the rear spar to allow the line to pass from the tank to the wing root. Caleb can give you the limits of the hole size, but as I recall, it was "iffy" that a fitting would pass through the hole. I highly recommend passing the line through the hole, THEN attach the fitting! Remember, you need to allow for a grommet to protect the line passing through the hole, too. It's tougher to attach a fitting with the line on the wing rather than the bench, but it can be done and I've done it!

Secondly, the inherent rigidity of the braided stainless will make it difficult to "stow" the excess fuel line when the wing is in position. IF you attach to a bulkhead fitting, by necessity, the excess line will have to retract/slide back into the wing's trailing edge. That line is stiff enough that it will not easily flex and slide back beyond the point it comes through the rear spar. I really think it will work better to have the excess slide into the fuselage rather than the wing through a grommeted hole rather than use a bulkhead fitting. Alternatively, one could use the bulkhead fitting, but you would have to detach it each time you folded the wing down - as I said before, I don't know if that's a good idea or not - perhaps your fuel line specialists could tell you. I don't know what the length of the fuel line in the wing ended up - I just trimmed it to where it barely protruded beyond the wing so the fitting could be snugged-up just as the wing was brought home. (Another advantage of cutting and installing the fitting on the wing and not the bench!) That way, it only had to retract a couple of inches into the trailing edge as the wing skin came home to the fuselage - there was enough flexibility to accomodate that easily.

The bulkhead fitting is a 90 degree fitting with a nipple on the outside of the fuselage (enclosed by the trailing edge of the wing). The hole for this is located just under the longeron (dont' notch the longeron!). The green line is the pitot and white is the static. They go through grommeted holes in the upright and down inside of the upright, just like the fuel line (you're right, the fuel line has black plastic loom on it for abrasion protection. From there, they run under the baggage floor and pilot's seat then exit on the left sidewall to come up behind the left side of the instrument panel. The fuel line on each side goes under the baggage floor and up the central tunnel to the Andair in the console and then one line to the firewall.

I'm not quite sure of what you mean by "the exact location of fitting near the rear rib?"

 (P.S.: Those grommets for the pitot and static turned out to be lightly pressing against the top skin of the trailing edge. If I had to do it again, I'd drop them a few mm lower - otherwise plenty of room for the bulkhead fitting and wiring. I just eyeballed these locations from looking at Jerry Bryan's photos.)

Hi John - Thanks again for the great write up and the pictures.  Armed with all the information I have received over the last week or so - and having at last seen a route through the problems, I visited PIRTEK, my local hose and fitting specialists, in Cambridge.   They can make me up some braided hoses of the same size as those supplied, and some fittings that are small enough to pass through the holes in the rear spar and fuselage.  Also, I am going to make it so the 'excess' hose slides in and out of the fuselage (as you suggest).   I will also give myself an escape route in so far as I will make them of a length that should there be a problem, I can saw off the ends and put my own fittings.   Sorry, I was a bit dumb not to realize that the other lines were the pitot and static lines.  Now all I have to do is pass it all by my inspector and make it!    Many thanks for all your help.   Best Regards.   Tony Oliver, Essex, United Kingdom.  

John - Thanks for this (again).   As this is the first build of a CH750 in the UK, I don't even have a fuselage to look at - so, your advice had been most useful.   I am probably overstating the number of times that I will fold and unfold the wings - but all I can say is that when I built a wing-folding FRED aircraft many years ago, the trailering and wing-folding options proved most useful.   My next stage is to go and talk to PIRTEK, my local pipe experts.   Meanwhile, many thanks for the tremendous help.   Tony Oliver, Essex, United Kingdom

Hi Ken - Thanks for your interest and taking the time to put this to me.   It's an interesting Idea.  However, I do see a problem at this end  in having to persuade my Inspector and the Light Aircraft Assocation (LAA) (who function rather like a delegated FAA - only smaller) that this sort of solution could be undertaken without accompanying 'Modification' action.  So, I was trying to find a solution that avoided any cutting into primary structure.   The LAA - who have to answer to Europe's EASA and the UK's CAA - are quite strict in these matters.   So, I was trying to find a solution that kept within the original WFO design and concept.   However, its a good idea and I worth thinking about.  Many thanks.   Tony Oliver, Essex, United Kingdom

Hi, Rubber gaskets on fuel senders do occationally leaks so we have always used a very thin layer of loctite 515 (still transparent).  You can also use it just fine on tapered thread fittings although most engineers use a pipe thread sealant for this purpose can't remember the brand though.  I wil mention though that this experience has all been on GA aircraft running of 100LL so not sure how it would handle ethenol.

Hi Jacob - Thanks for this.   Yet another product. ...  The discussion was primarily aimed at identifying what thread lockers and sealants operated with modern vehicle, garage forecourt fuels.   We would call it "petrol"  in the UK: you, I think, will call it "gasolene".   The problem comes with the higher alcohol/ethanol content that have been introduced to modern fuels, today - in Europe we have legislation demanding the inclusion of same - as they can be extracted from 'renewable' technologies, for example wood, grain, vegetable oils, etc.   These fuels, ie modern MOGAS, may now have compatibily problems with older thread locking and sealants that were tested on 'yesterday's' fuels.   I don't think that AVGAS is affected in this way.   The engine in my CH 750 will be a Rotax 912 - using MOGAS - so hence my questions.   Many thanks for your contribution - it has turned out to be quite an extensive debate.     Best Regards,  Tony Oliver, Essex, United Kingdom.


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