Very much appreciate the discussion and the feedback, here.
Please allow me to provide a response to some of the messages.
1) I am a big Mooney fan. Flew a 67 Mooney Super 21 for nearly 2,000 hours. Commuted to and from law school in Norman, Ok. to the family Ranch (100miles each way) on a daily basis for 3 years.
2) Part I - - Some aviation fuel chemistry history:
A- During WWII fuels with very large amounts of aromatics were adopted. Arguably, the Air War would have been much more costly in terms of losses for the fighter aircraft without the introduction of aromatics to aviation gasoline. The fuel approved for D-Day was "100/150" - - and used a lot of different aromatic components;
B- In the ranking of the levels of "aggressiveness" as solvents, the ranking starts (most severe) with benzene. Then add a single CH3 group and you get toluene. Yes. Paint stripper you buy at the paint store.
C- Add a second Ch3 group and you get xylenes (three isomers - ortho, meta, & para) Xylenes are significantly less aggressive as a "paint stripper" than is toluene. <== THAT is important. Keep that in mind.
D- Many fuel bladders were developed during WWII and are labeled "approved for aromatics" or something similar. If you look closely at some of the P-51s and other planes at Oshkosh, you will see a placard that states "approved for aromatic fuels."
Part II - - Post WWII.
A- We had "Green" 100/130 with LOTS and lots of lead. And even purple 115/145 (with lots of aromatics) At the end of the piston airline and going into the late 1970s and 1980s, the ASTM folks decided to drop the lead content and created BLUE 100Low Lead. Still lots of lead. But much less than GREEN 100/130.
B- Some of the refiners had very good "aviation alkylate" (~ 70% of some 100LL fuel formulations) with relatively high MON values for that aviation alkylate (a less than pure form of isoctane). Those refineries could make 100LL with relatively low levels of aromatics (almost always "toluene" from the paint store). Maybe 10-20%.
C- Other refineries (P66 at Borger, Tx, for example) had lower MON quality "aviation alkylate" and the had to use a LOT of toluene. I can show you detailed hydrocarbon analysis (GC-FID) test results for local (Ada, Ok. FBO) P66 Borger, Tx 100LL that has 29% toluene and another 4% of other aromatics for a total of about 33% aromatic content.
3) Fleet Experience during the transition to 100LL - -
A-Starting shortly after the introduction of 100LL, with high levels of aggressive toluene, A LOT of Mooney, Piper, and other airplane owners that had aluminum integral fuel tanks (no bladders) began to leak fuel out of lots of rivet holes. That started a whole new business for G.A. with companies initially specializing in re-sealing those tanks. Later, companies developed retrofit fuel bladders for those problematic "integral fuel tanks".
B- A big part of the problem was the aircraft manufacturers did an "inconsistent" (careful choice of words) job of applying the polysulfide sealant to the interior (rivets and seams) of the aluminum integral wing tanks as they were manufactured. [As we have learned during material compatibility testing, the devil is in the details when it comes to the proper application of sealants to fuel tanks. ]
C) Over the decades, the level of toluene from most refiners has decreased due to better quality aviation alkylate, but some refiners - - for at least some production runs - - still (from looking at their data sheets) still have a lot of toluene.
4) G100UL Avgas - -
A- Uses a very high quality aviation alkylate (2-4 MON numbers higher than the alkylate used for 100LL) and then uses xylenes rather than toluene in order to achieve the 100/150+ octane/supercharge rating for G100UL avgas. The right choice of xylene isomers will have higher octane blending value than does toluene. Using "xylenes" also has the advantage of being less chemically "aggressive" than is toluene. But the high quality alkylate and the premium xylene isomers also "cost more" than the related components in 100LL. BUT - - there ends up being no lead.
B- We did extensive material compatibility testing, including a whole variety of older bladders and a whole range of tests for sealants applied to aluminum. All of that supervised (in person) by multiple FAA engineers and managers and then later approved by the FAA.
5) The 100LL we have at the airport in Ada (typically Phillips from Borger, Tx) still tends to have a lot of toluene in that fuel.
A- Late October of 2023, AOPA brought their 1965 demonstration Baron to Ada. Two freshly overhauled IO-520s. One fuel bladder was 46 years old and the other ~50 years old. They were supposed to have been replaced with new, prior to the start of testing, but the bladders were on "back-order".
B- G100UL avgas was exclusively kept in the LH bladder and 100LL was exclusively in the RH bladder.
C-Within 30 days, we noticed fuel "spots" on the hangar floor. Investigation - - we found fuel leakage and staining on the bottom of the RH wing - - which had ONLY ever had 100LL. See photographs.
6- Oshkosh, the leakage from the LH wing tank (G100UL Avgas) was ( based on Eagle Fuel evaluation) caused by pre-existing damage to the tanks and the gaskets associated with the access panels. (In addition, there were pin-hole leaks in both fuel bladders).
7)-G100UL Avgas is NOT a threat to normal aircraft paint.
A- Embry Riddle did extensive certification testing of G100UL avgas. They used G100UL in their C-172s for over a year, at Daytona Beach. They had no evidence of any fuel leaks and they had zero staining on the wings.
B- As part of our due diligence, wee have deliberately spilled a LOT of G100UL Avgas onto various painted components from the many Bonanza aircraft that come through the TAT shop for turbo systems. That includes LOTs of wing tips and the lower cowling access panels, both of which are removed and replaced (with tip tanks and newly louvered lower access panels.)
C- Some of that has been allowed to dry on those painted surfaces and then the process was repeated. NO SIGN OF ANY LOSS TO THE INTEGRITY OF THE ADHESION OF THE PAINT.
D- We have even soaked a couple of those side panels in G100UL for a week or more. The paint was fine at the end of that.
E- What does and will happen - - is if you do not properly clean up the spilled fuel, and allow it to dry, it can and will leave a light tan stain on the paint. If you do properly clean it up, it will not stain the paint - - even after repeated spills in the same location. But, to date, we have never seen any evidence of any type of "paint striping" activity to any of the dozen or more aircraft parts that were removed from our customers Bonanzas and used as "test articles." See https://g100ul.com/dl/Refueling Hygiene G100UL Avgas.pdf
F- An aside - - some of the recent crystal and graphene coatings improve the resistance of aircraft paint to any damage from any variety of 100LL or G100UL - - but that is not a cure all. See the link Refueling Hygiene!
8. So "what happened" on the bottom of the Mooney wings ?
A- In one case, the sump drain appears to be leaking. Likely the fittings inside the wing are also leaking.
B- In the other case, there appears to be some kind of leakage from inside the wing. Not sure from the photographs.
C- Almost certain that old drain valve has a "nitrile" set of O-rings.
D- The paint on the bottom of the wings has likely had many months of exposure to steady drips of 100LL. In California, probably with only ~ 15% toluene in the 100LL . . . maybe.
E- No information about the age of the paint on the bottom of the wings, but from its overall appearance and the multiple rivet holes that are bare and which have lost their paint - - it is likely a they are rather old paint jobs. The paint on my personal Bonanza, N11RT is about 35 years old and it is in much better condition than the appearance of the paint (away from the damaged paint).
F- It would be good to know the age of the paint for each of those two aircraft.
9. What to do ?
A-There is a long standing FAA / maintenance bulletin (20 + years old ???) that tells mechanics that when they find fuel leaks or deteriorated fuel lines or gaskets or O-rings, they should replace those components with parts that are made from chemically resistant materials (viton, fluorosilicones, etc.) Those types of materials are specifically stated to be for use with a wide variety of fuels, including aromatic based fluids. A lot of mechanics have ignored that recommendation over the years.
Last . . .
10. I would like to have the chance to borescope an older Mooney fuel tank that has not been converted over to a bladder.
A- If anyone on this forum would like to bring their plane to Ada we can do that here. You might find it interesting. If you have the STC you can probably leave with some G100UL in your tank, if you want.
I hope this information helps to bring some clarity and understanding to these issues that will be the subject of conversations during the transition away from leaded avgas.
Regards,
George Braly gwbraly@gami.com
