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 nitrogen-rich 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.
Three of the companies— Tau Theta Instruments, InterSpace, and Physical Sciences— subsequently received SBIR Phase II contracts. Aviation Safety Facilitators Corporation is proceeding with independent research in tandem with these laboratory-funded follow-on efforts. Meanwhile, Advanced Projects Research, Inc.—a company that completed a Phase I SBIR effort for the National Aeronautics and Space Administration and went on to earn a "congressional add" contract—will continue related work under AFRL administrative oversight.
Tau Theta Instruments' SBIR Phase II effort will continue the company's initial development of a ruggedized luminescent oxygen sensor compatible with aircraft fuel tanks and other harsh and flammable environments. The microprocessor-based sensor also monitors environmental variables such as pressure and temperature and can perform system control functions through digital and analog interfaces.
InterSpace will use Phase II funding to build on its successful Phase I oxygen sensor to develop a rugged field sensor that is intrinsically safe to operate in the harsh fuel tank environment. The sensor requires neither consumables nor vulnerable membranes and does not react chemically with the fuel/air environment.
Physical Sciences will continue working to develop a sensor based on near-infrared Tunable Diode Laser Absorption Spectroscopy (TDLAS), an optical technology in which only passive materials contact the fuel. The technology is essentially an adaptation of the low-cost, reliable, and robust single-board turnkey TDLAS platform that the company produces commercially for municipal gas pipeline leak inspection.
Concurrent to these Phase II SBIR efforts, Advanced Projects Research will use its congressional add contract to proceed with developing a diode-laser-based sensor that provides real-time monitoring of oxygen concentration. The company's method for obtaining oxygen concentration data involves the temperature-insensitive measurement of oxygen concentration over a wide range of pressures, complete with self-compensating capabilities.
Aviation Safety Facilitators will continue its work towards achieving a safe fiber-optic oxygen measuring system having no electrical or moving parts at the measurement site nor any need to draw a sample from the point of measurement. Throughout the numerous SBIR Phase II and concurrent efforts, AFRL's establishment of close ties with industry will facilitate the future demonstration and testing of approaches that could lead to the technology's transfer into military and commercial aircraft.
Ms. Lois Gschwender, Mr. Carl E. Snyder, Jr., and Ms. Mindy Cooper (General Dynamics), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, visit http://www.afrl.af.mil/techconn_index.asp . Reference document ML-H-05-47.