Because aircraft fuel tanks are vented to allow for equalization of pressure at different altitudes, the nitrogen must be constantly fed into the tanks to displace the outside air that freely enters. Pressure valves regulate the flow so that the fuel tank
isn’t over-filled, and FAA ground tests of the prototype showed that the nitrogen is quickly distributed throughout the tank, so no fans are needed to circulate the gas. The system operates in two modes: low-flow and high-flow. When the airplane has taken off, instruments signal OBIGGS, and the system enters the low-flow mode. OBIGGS remains in that mode through climb and cruise. Since air pressure is much lower at cruising altitudes, the system can displace all but two or three percent of the fuel tank’s oxygen.
Conversely, during descent, air pressure rises, so more outside air flows into the tank, increasing the oxygen content. To compensate, OBIGGS goes into high-flow mode, pumping in additional nitrogen at a faster rate. Even so, the concentration of oxygen jumps from a few percent at cruising altitude to between nine and 12 percent at landing.
Airlines and manufacturers are not required to adopt the FAA prototype, Hickey notes. Boeing recently announced it will use its own system, which is based on the FAA prototype, and has committed to deploying it on the new 7E7. According to Hickey, FAA regulations probably will require installation of some fuel-inerting technology on all large jets in the U.S. fleet within five to seven years, at which point—in FAA’s Blakey’s words—“we can close the door on fuel tank explosions.”