The ARAC studied use of inert gases (including nitrogen and exhaust gases), fuel "scrubbing" and "washing" the ullage space above the fuel, which are methods of purging oxygen from the tank. Scrubbing fuel at the airport fuel farm offered little added protection while use of foam would reduce aircraft payload and available fuel volume, causing severe economic impact for operators. Using exhaust gas from turbofans was deemed unfeasible primarily because the exhaust contains too much oxygen.
The study costed options, including:
ventilating fuel tanks to reduce heat generated under them from air conditioning units would cost $3.5 billion over 10 years; adding gases to partially filled fuel tanks while aircraft are on the ground would cost $3 billion; reformulating aviation fuel to reduce its flammability would cost $15 billion in the USA and $60 billion elsewhere. Raising the minimum flash point could significantly increase the manufacturing cost and decrease the availability of the modified jet fuel, significantly affecting the price of jet fuel; carrying nitrogen on commercial flights would cost more than $30 billion over the next decade and could be risky to maintenance personnel. On-Board Inert Gas Generating Systems (OBIGGS) are used on military aircraft to separate nitrogen from engine bleed air. OBIGGS causes performance penalties, but they could be minimised in new aircraft designs. Making OBIGGS mandatory on current transports could make them uneconomical to fly.Source: Flight International
