Fuel, fuel additives, and fuel tank coatings

[Jim Lee]

There have been two different fuel tank coatings in the Phoenix to protect from ethanol fuel.  Phoenix 02 had it's tanks ruined due to ethanol, and the wings had to be returned to the factory for repair (they did a great job!).  This was not discovered until a few years had passed after delivery of the plane.  Consequently, a bulletin was issued to not use ethanol fuels, and to change the fuel tank decals.  I think that the bulletin should have also included any fuel additive, but it did not.  So if you use any fuel besides 100LL or greater than 91 octane mogas without ethanol, beware that you won't know the impact for several years, and then it will be too late.  Late signs of "tank poisoning" are small bubbles in the gel coat in the wing area of the fuel tanks.  Pipistrel (and other companies with composite wet wings) also had massive problems, and had to repair many wings because of it.

Phoenix Air also changed the type of fuel tank coating, trying to be able to approve ethanol fuels because of their predominance and less expense in Europe.   But the problem is that any crack, chip, or uncoated area of the tank (this is done by hand) will allow the ethanol (or other additive) to penetrate the substrate into the foam core of the wing, and then the game is over.  One problem that I pointed out to Martin and his boys was this.  There is a wire running along the inner surface of the fuel tank out to the fuel cap for static prevention.  It is a bare wire, not encapsulated by the fiberglass plies.  The tank coating is then brushed directly on top of the wire.  You can see this with a borescope inside the tank.  Well, nothing sticks well to metal, so it is only a matter of time before a crack develops along this wire, and there is the route for any ethanol (or whatever) to penetrate the coating, and enter the wing structure.  So it really doesn’t matter what coating is used; over time, cracks will develop, and the wing will be susceptible to damage.

The first coating used was Synpo Veropal NB 531-1203.  It is orange in color, and is visible in some of the photos in your Maintenance Manual.

The second coating (and current, as far as I know) is GRM Systems LGC1.  It is a clear coating, but will look slightly green with the fiberglass behind it if you shine a light into the tank.

I don’t think one is better than the other.  My plane has the Veropal coating.  As I have described, it is probably not the coating, but the long term resistance to voids in the protection which can get us in trouble.

Even if the coating is 100%, the manufacturer of the coating has not tested it against impermeability of every substance, since new additives appear on the market regularly.  I have talked extensively with the makers of Swift fuel, and was close to signing an agreement with them many years ago to test the Synpo coating with their fuel and run Swift fuel in my plane.  But the more I learned about the situation the more it became apparent to me that it isn’t the coating, it’s the guaranteed 100% coverage.  Anyway, as Swift explained it to me, they would introduce the coating (or any other substance) to Swift fuel, seal it for a length of time, and then test on the molecular level whether any of one had migrated into the other.

I get 91 octane no-ethanol fuel from a local gas station and fuel my Phoenix with that.  When I am on the road I use 100LL because most of the mogas at airports is 89 octane, which is not high enough for the 912ULS.  I think that gas isn’t used much, so I don’t know how long it has sat in the tank at the airport.  So I won’t use it even if it is 91 octane.