I am very sorry to hear of the loss of your aircraft. For what it is wo= rth, I present my suspicions as to the causes of this explosion. From t= he information provided, the source of ignition was almost certainly electro= static discharge resulting from the use of compressed air to dust off the wi= ng. The sanding process may also have contributed somewhat to the event.

The interesting part is the fuel for the explosion. The interior space=20= of the fuel tank should not be combustible 4 hours after the fuel transfer w= as made (assuming the fuel was gasoline) as the ullage space should be=20= fuel rich by then.

Some dusts can be explosive when suspended in air. I don't know, off ha= nd, if the paint/primer/filler used in this application are susceptible to e= xplosion, but it seems unlikely that they would have been suspended inside o= f the wing structure.

I would suggest the most likely fuel for the explosion was vapor f= rom the solvent used to soften the paint that seeped into the void spaces wi= thin the wing. The spark probably arced between the filler and the dust clou= d or possibly between two closely spaced, electrically isolated, conductive=20= components in the wing.

Grounding the plane might have helped the situation, as long as ALL con= ductive components were tied to the ground circuit. If just one ungrounded,=20= conductive component is in close proximity to a grounded component then tyin= g the aircraft to ground would actually increase the risk. This is due=20= to an increase in electrical differential between the components.

Transferring fuel to the opposite wing was a little questionable w= ith regard to safety in this incident. On the positive side, the tank w= as probably ruptured in the explosion, and the resulting fuel spill would ha= ve added a significant hazard of secondary fire after the explosion occurred= if the fuel hadn't been transferred.

On the other hand, fresh air is drawn into a tank as it is emptied, cre= ating an explosive atmosphere. This hazard gradually dissipates as fumes evo= lve from the residual gasoline until the tank becomes fuel rich and inc= ombustible. Assuming the painter started work on the wing shortly after= the fuel transfer was made, he was working on a bomb.

A second reason not to transfer the fuel is that liquids do not expand=20= as much as gasses for a given increase in temperature. Thus, as the aircraft= warms a nearly full tank will vent a smaller amount of flammable vapor= than will an empty tank.

Of trivial interest, while their are numerous negative aspects to the a= ct of smoking, a lighted cigarette cannot ignite gasoline vapors. Yes, you c= an smoke to your heart's content around all of the gasoline you like without= fear of blowing yourself up. The problem comes from the act of lighting up.=

Rob Harshman, PE

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