AFRL researchers are developing an integrated oxygen sensor for aircraft fuel tanks.

Shortly after its takeoff from New York City on July 17, 1996, Trans World Airlines (TWA) Flight 800 exploded over the Atlantic Ocean and crashed. The accident investigation board determined that the center wing fuel tank caught fire and exploded. Although the ignition source remains unknown, it was unquestionably the presence of a combustible fuel/air mixture in the center wing fuel tank that caused the resulting
explosion.

Prior to the TWA Flight 800 tragedy, the world’s aviation experts theorized that the best way to avoid a fuel tank explosion was to minimize the number of ignition sources. Since the accident, however, the Federal Aviation Administration (FAA) has investigated ways not only to eliminate ignition sources, but also to reduce fuel tank flammability. As a result of these ensuing investigations, the FAA recently introduced a concept called fuel tank inerting. Already in use within some military aircraft, fuel tank inerting involves diluting the tank’s ullage (the airspace above the fuel) with an inert gas to the point it is no longer flammable. An onboard inert gas generation system allows an aircraft to maintain its fuel tanks in an inert status indefinitely. This type of system pumps engine bleed air into air separation canisters that filter oxygen from the air and leave a nitrogenrich mixture that dilutes the fuel tank ullage oxygen. When the oxygen in the fuel tank decreases to a level between 9% and 12%, the ullage has an inadequate level of oxygen to burn. Although this type of development is immensely helpful, it does not constitute a perfect system. For instance, the aircrew still needs a sensor to monitor the amount of oxygen in the tank. Currently, the crew depends on computational models that use the known tank ullage volume, temperature, and pressure to control the amount of nitrogen-enriched air needed to dilute the ullage below the flammability threshold. To develop a sensor capable of directly monitoring fuel tank oxygen levels, AFRL researchers, in conjunction with experts from the 516 Aeronautical Systems Group (formerly the C-17 Systems Group) and the Aeronautical Systems Center Engineering Directorate, have initiated several Small Business Innovation Research (SBIR) contracts. During Phase I of these various contracted efforts, the interdisciplinary government engineering team provided guidance and directed the technical efforts of Tau Theta Instruments, LLC; InterSpace, Inc.; Aviation Safety Facilitators Corporation; and Physical Sciences, Inc. These experts continued their collaborations throughout all project phases, providing relevant input from different viewpoints.