Inerting system
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An inerting system is a device that attempts to increase the safety of a fuel tank, ball mill, or other sealed or closed-in tank that contains highly flammable material, by pumping nitrogen, steam, carbon dioxide, or some other inert gas or vapor into its air space in order to displace oxygen. With no oxygen (or an insufficient oxygen level) in the tank, the fuel cannot ignite, and explosions cannot occur.
Inerting systems have been used in many military aircraft, starting with a 1950 version of the B-47 bomber jet which sublimated dry ice to produce gaseous carbon dioxide and pump it into the fuel tanks whenever the fuel pumps were active or whenever in-flight refueling was in process. This system was implemented largely over concern over static electricity discharges during in-flight refueling.
Research intensified on lightweight inerting systems for commercial jet aircraft after the explosion of TWA Flight 800, which was blamed on an explosion in the center wing fuel tank. The exact cause was never determined, but investigators concluded there was an electrical fault inside the fuel tank which caused a spark, which set off an explosion. This may have been exacerbated by high temperatures caused by the airplane's air conditioning unit. If no oxygen had been present in that fuel tank, it has been argued, the explosion would not have occurred.
After the Flight 800 crash, one FAA committee studying inerting systems issued a negative report, stating in 2001 that the airlines would have to spend $35 billion to retrofit their existing aircraft fleets. However, another FAA group created a comparatively inexpensive nitrogen-based inerting system prototype that weighed only 200 pounds, utilizing compressed air from the engines to displace some oxygen with nitrogen, and Boeing commenced testing a derivative system of their own, performing successful test flights in 2003 with several 747 aircraft. The FAA then announced in February 2004 that within the next two years, it would probably require an inerting device in all airliners whose air conditioning units had a possibility of heating up the center wing fuel tank. This requirement would phase in over the next seven years.
The FAA stated that there had been three center wing fuel tank explosions in the previous 14 years -- two on the ground, and one in the air -- and that based on this statistic, the fuel tank inerting system requirement could prevent around 4 explosions in the next 25 years. The proposed retrofit would require a retrofit of about 3,800 current airliners as well as all future airliners built. The total cost for the U.S. fleet is now estimated at $600 million to $700 million.
Boeing has stated that it will install its own inerting system on airliners it manufactures beginning in 2005.