This is interesting - under "Inspection and Maintenance of Flush-Mounted Fuel Caps"
I change the outer o-ring every year. It's fairly cheap from Socata and anyway the size is easy to establish as a 49mm ID x 3mm thick 61D6-material o-ring from Busak/Shamban/Trelleborg.
I have some parts listed here.
But there is an inner o-ring also. This one is not documented, so the only apparent way to service is is to replace the whole filler cap! The ID of this o-ring is going to be about 9mm. Has anyone got a spare one of these filler caps which could be opened up to measure it up?
IMHO, the pilot of that ditched plane didn't drain his tanks before flying. I know quite a lot of people who never drain their tanks...
Found the part numbers...
N7423015002 Fuel cap outer o-ring
M25988/1-338 3-1/8 ID X 3-1/2 OD X 3/16 W
Material Fluorosilicone 70A
N7423015001 Fuel cap inner o-ring
M25988/1-010 1/4 ID X 3/8 OD X 1/16 W
Material Fluorosilicone 70A
These mil-spec o-rings are so common you can even buy them from amazon.com, which unfortunately won't usually ship to Europe, and certainly won't give you a CofC.
This is quite interesting. It's funny how one often finds others have been around the same block before you 
It's a bit less funny how often the same poor design is recycled in one aircraft type after another even though it has given trouble for decades. There is a 1400psi hydraulic pressure switch which has a poor reliability record but which is used in generations of retractables. I think there is a group of designers in aviation who just move around from one project to the next and specify the same bits, regardless of whether they work or not.
Peter
Some interesting stuff that you've dug up. I don't have a problem with water ingress into the fuel tanks but now if I do I'll know where to investigate.
However, as you say, such a problem should manifest itself with the daily fuel drain check. I do this always. Maybe I wouldn't be surprised if a few pilots didn't do it but 'quite a lot' beggars belief.
I would recommend doing the inner o-ring because it is highly likely it has never been touched - especially on an EASA-reg.
I am doing the Annual on my plane today and this weekend (at a hangar at Biggin - if anybody is after hangarage at Biggin try EA Aviation - the former Air Touring place - as it is really good, and crucially allows maintenance to be done) and we did the filler caps today.
It took about 1hr to do both. The adjustment of the castellated nut is fairly critical, to get it just right. Too tight and you can't lift the "flap" with 2 fingers (which thankfully can be tested before you fit the cap into the tank) and too loose and the big o-ring doesn't get compressed.
The good news was that the steel "spindle" in the middle, which that PDF shows as heavily corroded, was spotless.
Parts section updated.
I think anybody who finds theirs corroded ought to have a little look at other parts of their plane... some parts probably haven't seen a lube in years if not decades. But parking outdoors will make things worse because the inner o-ring seal will be exposed to water a lot of the time.
The official line from certain sources is that since the filler cap manufacturer has not released a CMM (component maintenance manual) it is illegal for an A&P to undo that one nut and replace the inner o-ring - notwithstanding the fact that Socata publish its P/N in their IPC. Under FAA rules this is rubbish; AC43-13 covers such a trivial "repair".
I also think that most if not totally all cases of water in tanks is due to filler cap leaks - not condensation as one is taught in the PPL. Somebody (bookworm, in his less busy days?) once calculated the maximum possible amount of water which can condense from that volume, given the worst likely TP/DP spread on the earth etc, and it was only a few cc. In my PPL training, I once drained approximately 1 litre (about 5 jars) of clean water from a Tomahawk (no suprise, hey) tank, after a rainy night. I would have flown after the first one because it looked just like avgas but somebody spotted it. Now, to guard against that, I drain all 3 drain points into the same jar, without emptying it, because having 100% water in all 3 is highly improbable.
However, as you say, such a problem should manifest itself with the daily fuel drain check. I do this always. Maybe I wouldn't be surprised if a few pilots didn't do it but 'quite a lot' beggars belief.
If your aircraft is hangared, is isn't necessarily beyond belief.
If you have flown and put the aircraft away and it hasn't been refueled, why drain it?
If you have flown and put the aircraft away and it hasn't been refueled, why drain it?
Because the hangar owner often moves your plane outdoors, for various reasons, for a whole day, when you are not there
I also believe it's possible for water dissolved in avgas to come out. Is that true? It certainly is true in avtur.
Sure but if you have your own hangar I know several pilots who won't drain as there is no possibility of meaningful water.
Obviously if it is outside your control it is a different matter.
I also believe it's possible for water dissolved in avgas to come out. Is that true? It certainly is true in avtur.
I think that is a Jet-A1 problem rather than an avgas one. Hence prist etc.
No; there are definitely cases of avgas fuel systems freezing solid, well above the freezing point of pure avgas which is something very low like -59C.
One uses Prist/EDME or IPA in avgas. I use IPA, about 1%, for flights which are expected to be in -30C or below, though I have been to -39C without any (for hours) and was fine. IPA is not carcinogenic but on a long trip the amount that needs to be carried is a hassle.
But it's a different Q whether water can come out of solution in avgas, and create a puddle of water in the bottom of the tank. Or indeed in avtur. I don't know.
We had -35-40 all the way across the North Atlantic without any additive.
