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TWA 800 explosion may have been caused by electrostatic charge from fuel flow, professor says

A year after TWA 800's central fuel tank exploded shortly after takeoff, investigators still don't know why. But Stanford Professor Sullivan S. Marsden has an idea: flow electrification.

A professor emeritus of petroleum engineering, Marsden bases his theory on the similarities between the conditions surrounding the mysterious explosion that downed the Boeing 747 jumbo jet on July 17, 1996, and the factors that caused a series of explosions that wracked petroleum plants around the world in the 1950s. Those explosions finally were traced to the electrostatic properties of hydrocarbon liquids, including gasoline and jet fuel.

From the investigations of those explosions, petroleum engineers learned that the rapid movement of hydrocarbon liquids through metal pipes can build up large electrostatic charges. The effect is called "flow electrification," or streaming potential. Marsden and his students studied various aspects of this phenomenon in the 1960s.

After flow electrification was determined as the cause of the refinery explosions, the petroleum industry began to take a number of precautions, such as never pumping liquid hydrocarbons faster than one meter per second (2.2 miles per hour) and filling tanks from the bottom, rather than the top, to avoid splashing.

Based on the information about the TWA 800 crash that has been made public, Marsden says that there is a good chance that the explosion in the aircraft's central fuel tank was touched off by a discharge of static electricity generated by flow electrification. Large electrostatic charges could have resulted from pumping the fuel too rapidly between wing tanks to maintain the aircraft's balance. Fuel spraying from a leaking gasket or "o" ring in a joint between pipes running through the central tank is another possible cause. Or the charge could have been created by the flow of fuel through dirty filters, he says.

If Marsden's theory is correct, a number of measures could be taken to make certain that such an explosion does not happen again. One procedural step, which the Federal Aviation Administration has already implemented, is prohibiting 747's from running their air conditioners for long periods while they are on the ground. Another relatively simple measure would be to install temperature sensors in the central fuel tank. More expensive preventive actions include pumping nitrogen into the tank as the fuel is consumed, and increasing the diameter of the fuel lines, Marsden says.

Basis for theory

The pertinent facts about TWA 800 that have led Marsden to propose an electrostatic cause are: the plane was 25 years old; it refueled at Kennedy airport and spent about two hours on the ground with its cabin air conditioners blasting; there was very little fuel in its central tank, which is located in the body of the plane; the heat exchangers for the air conditioners are located under the central fuel tank; and some of the fuel lines used to pump fuel back and forth between the wing tanks pass through the central tank.

Jet fuel normally is not explosive at temperatures below 100 degrees Fahrenheit. But on TWA 800 the air-conditioner heat exchangers probably warmed the air/fuel mixture in the tank above that point. When the aircraft is flying, the energy given off by the heat exchangers is effectively dissipated to the outside air. But when the air conditioners are run while the aircraft is on the ground and the tank is nearly empty