Mr. Fleming,

 

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

 

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

 

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

 

I would suggest the most likely fuel for the explosion was vapor from the solvent used to soften the paint that seeped into the void spaces within the wing. The spark probably arced between the filler and the dust cloud or possibly between two closely spaced, electrically isolated, conductive components in the wing.

 

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

 

Transferring fuel to the opposite wing was a little questionable with regard to safety in this incident. On the positive side, the tank was probably ruptured in the explosion, and the resulting fuel spill would have 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, creating an explosive atmosphere. This hazard gradually dissipates as fumes evolve from the residual gasoline until the tank becomes fuel rich and incombustible. 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 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 act of smoking, a lighted cigarette cannot ignite gasoline vapors. Yes, you can 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