The center fuel tank, about the size of a bedroom, can hold 12,890 gallons of jet fuel. Six other tanks in the wings hold 34,320 gallons total. Fuel tanks are more volatile empty, when they're filled with fumes and oxygen, than when they're full of kerosene-based fuel and deprived of the air needed for a fire or explosion. But airlines tell pilots to take only as much fuel as they need to make the trip safely. A lighter plane uses less fuel.
Less fuel saves money.
In 1967, an industry panel rejected the thought of pumping fumes out of empty tanks since in-flight problems with empty tanks were not a major safety issue. The pump system was considered too complex, too heavy and unnecessary. But the same panel urged that planes install flame arresters, safety devices that prevent fire from traveling into fuel tanks. Now common on Boeing aircraft, the device was never put on the 747 that exploded near New York's Long Island. Boeing says that even the more than 100 older 747s still flying without flame arresters are safe because the company designed the fuel system to be protected from outside energy.