Inerting System - Inerting Gas Systems in Aircraft

Inerting Gas Systems in Aircraft

Fuel tanks for combat aircraft have long been inerted, as well as self-sealing, but those for transport planes, both military and civil, have not, due to considerations of cost and weight.

Cleve Kimmel first pitched an inerting system to passenger airlines in the early 1960s. His proposed system for passenger aircraft would have used nitrogen. However, the Federal Aviation Administration refused to consider Kimmel's system after the airlines complained it was impractical. Indeed, early versions of Kimmel's system weighed 2,000 pounds—which would have probably made an aircraft too heavy to fly with passengers on it. However, the FAA did almost no research into making fuel tanks inert for 40 years, even in the face of several catastrophic fuel tank explosions. Instead, the FAA focused on keeping ignition sources out of the fuel tanks.

The FAA did not consider lightweight inerting systems for commercial jets until the 1996 crash of TWA Flight 800. The crash was blamed on an explosion in the center wing fuel tank of the Boeing 747 used in the flight. This tank is normally used only on very long flights, and little fuel was present in the tank at the time of the explosion. A small amount of fuel in a tank is more critical than a large amount, since heat entering the fuel tank with residual fuel causes the fuel to increase in temperature faster and evaporate. This causes the ullage fuel air ratio to increase rapidly and the ullage fuel air ratio to exceed the lower flammability limit. Large quantity of fuel (high mass loading) in the fuel tank retains the heat energy and slows down the fuel evaporation rate. Explosion of Thai Airways International Boeing 737 in 2001 and Philippine Airlines 737 in 1990 also occurred in a tank that had residual fuel. All the above three explosions occurred on a warm day, in the Center Wing tank (CWT) that is within the contours of the fuselage. These fuel tanks are located in the vicinity of external equipment that heats the fuel tanks. The National Transportation Safety Board's (NTSB) final report on the crash of TWA 747 concluded “The fuel air vapor in the ullage of the TWA flight 800 CWT was flammable at the time of the accident.” NTSB identified “Elimination of Explosive Mixture in Fuel tanks in Transport Category Aircraft” as Number 1 item on its Most Wanted List in 1997.

After the Flight 800 crash, a 2001 report by an FAA committee stated that U.S. airlines would have to spend US$35 billion to retrofit their existing aircraft fleets with inerting systems that might prevent future such explosions. However, another FAA group developed a nitrogen enriched air (NEA) based inerting system prototype that operated on compressed air supplied by the aircraft’s propulsive engines. Also, the FAA determined that the fuel tank could be rendered inert by reducing the ullage oxygen concentration to 12% rather than previously accepted threshold of 9–10%. Boeing commenced testing a derivative system of their own, performing successful test flights in 2003 with several 747 aircraft. The new, simplified inerting system was originally suggested to the FAA through public comment. It uses a hollow fiber membrane material that separates supplied air into nitrogen-enriched air (NEA) and oxygen enriched air (OEA). This technology is extensively used for generating oxygen-enriched air for medical purposes. It uses a membrane that preferentially allows the nitrogen molecule (molecular weight 28) to pass through it and not the oxygen molecule (molecular weight 32).

Unlike the inerting systems on military aircraft, this inerting system would run continuously to reduce fuel vapor flammability whenever the aircraft's engines are running; and its goal is to reduce oxygen content within the fuel tank to 12%, lower than normal atmospheric oxygen content of 21%, but higher than that of inerted military aircraft fuel tanks, which is a target of 9% oxygen. This is accomplished by ventilating fuel vapor laden ullage gas out of the tank and into the atmosphere.