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, at least, an insufficient oxygen level) in the tank, the fuel cannot ignite, and explosions cannot occur.
Use in military aircraft
Inerting systems have been used in military aircraft starting with a 1950 version of the United States' 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, and over fires that might start during aerial combat.
During the Vietnam War, it became painfully apparent that fuel tank inerting ought to be a requirement for military jets, as thousands of aircraft were lost due to enemy ground fire; analysis indicated that fuel system fire and explosion was the major cause of aircraft loss from enemy gun and artillery fire.Template:Ref
Military inerting systems have used nitrogen gas (on experimental versions of the F-86 and F-100), liquid nitrogen (on the C-5A and XB-70), and halon gas (on the F-16 and F-117). The inerting system on the F/A-22 pumps the air used to pressurize the fuel tank through a filter which extracts oxygen from the air. In addition to the active inerting system that displaces oxygen, the fuel tanks of some aircraft, like the F-15, are filled with a special foam that adds weight but reduces flammability. The goal is to reduce oxygen content of the air in the tank to below 9%, down from the normal atmospheric oxygen content of 21%.
Inerting systems for most military aircraft are activated with a switch or button. Standard operating procedure is to activate the inerting system only when flying in a combat zone, as most inerting systems only have enough inert gas for about 30 minutes of use. In addition, the inerting systems of some aircraft use halon gas, which is an ozone depletion agent, so use is discouraged unless actually in combat.
Use in commercial aircraft after Flight 800
Research intensified on lightweight inerting systems for commercial jet aircraft after the 1996 crash 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 the fuel tank, it has been argued, the explosion would not have occurred.
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 created a comparatively inexpensive, simplified nitrogen-based inerting system prototype that did not require an air compressor, and weighed from 100 to 250 pounds (45 to 113 kg), utilizing air from the engines to displace some oxygen with nitrogen, and costing from US$140,000 to US$225,000, depending on fuel tank capacity. 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 nitrogen-enriched air from oxygen. This technology had already been in use for generating oxygen-enriched air for medical purposes.
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.
After what it said was seven years of investigation, the FAA proposed a rule in November 2005, in response to an NTSB recommendation, which would require airlines to "reduce the flammability levels of fuel tank vapors on the ground and in the air". This was a shift from the previous 40 years of policy in which the FAA focused only on reducing possible sources of ignition of fuel tank vapors.
Although the airlines could technically comply with this rule in a number of ways, the rule was aimed at getting the new inerting systems installed on large aircraft. The proposed rule would affect all future large airplane designs, and require a retrofit of more than 3,200 Airbus and Boeing aircraft with center wing fuel tanks, over seven years. The FAA had initially planned to also order installation on cargo aircraft, but this was removed from the order by the Bush administration.
The FAA estimated the cost of the program at US$808 million over the next 49 years, including US$313 million to retrofit the existing fleet. It compared this cost to an estimated US$1.2 billion "cost to society" from a large airliner exploding in mid-air. The proposed rule comes at a time when nearly half of the U.S. airlines' capacity is on carriers that are in bankruptcy.Template:Ref
The order affects aircraft whose air conditioning units have a possibility of heating up the center wing fuel tank. Some Airbus A320 and Boeing 747 aircraft are slated for "early action". Regarding new aircraft designs, the Airbus A380 does not have a center wing fuel tank and is therefore exempt, and the Boeing 787 has a fuel tank safety system that already complies with the proposed rule.Template:Ref
The FAA has stated that there have been four fuel tank explosions in the previous 16 years -- two on the ground, and two in the air -- and that based on this statistic and on the FAA's estimate that one such explosion would happen every 60 million hours of flight time, about 9 such explosions will probably occur in the next 50 years. The inerting systems will probably prevent 8 of those 9 probable explosions, the FAA said.
Before the inerting system rule was proposed, Boeing stated that it would install its own inerting system on airliners it manufactures beginning in 2005. Airbus had argued that its planes' electrical wiring made the inerting system an unnecessary expense.Template:Ref label
If the rule goes into effect after a four-month period of public comment, it will have taken nearly a decade since the explosion of Flight 800 for the new safety rule to go into effect.
References
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- "FAA to Order Long-Delayed Fixes To Cut Airliner Fuel-Tank Danger", Wall Street Journal, November 15, 2005, page D5
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