One-way valves are installed in process piping to the tanker spaces to prevent volatile hydrocarbon vapors or mist from entering other equipment.
The International Maritime Organization (IMO) publishes technical standard IMO-860 describing the requirements for inert gas systems.
Early applications using nitrogen were on the Handley Page Halifax III and VIII, Short Stirling, and Avro Lincoln B.II, which incorporated inerting systems from around 1944.
The FAA did not formally propose lightweight inerting systems for commercial jets until the 1996 crash of TWA Flight 800, a Boeing 747.
The explosion of a Thai Airways International Boeing 737 in 2001 and a Philippine Airlines 737 in 1990 also occurred in tanks that had a small amount of residual fuel.
NTSB identified "Elimination of Explosive Mixture in Fuel tanks in Transport Category Aircraft" as Number 1 item on its Most Wanted List in 1997.
[citation needed] After the TWA 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 such explosions.
Also, the FAA determined that the fuel tank could be rendered inert by reducing the ullage oxygen concentration to 12% rather than the previously accepted threshold of 9 to 10%.
The new, simplified inerting system based on membrane gas separation technology was originally suggested to the FAA through public comment.
Inerting in military aircraft is typically accomplished by ventilating fuel-vapor laden ullage gas out of the tank and into the atmosphere.
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.
Additionally, regional jets and smaller commuter planes would not be subject to the rule, because the FAA does not consider them at high risk for a fuel-tank explosion.
[9] The order affects aircraft whose air conditioning units have a possibility of heating up what can be considered a normally empty center wing fuel tank.