Most Wanted
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Eliminate Flammable Fuel/Air Vapors in Fuel Tanks on Transport-Category Aircraft
Objective
Importance
Center wing fuel tank explosions have resulted in 346 fatalities. Operating transport-category airplanes with flammable fuel/air vapors in fuel tanks presents an avoidable risk of explosion. A fuel tank design and certification philosophy that relies solely on the elimination of all ignition sources, while accepting the existence of fuel tank flammability, is fundamentally flawed because experience has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated. As a result of the TWA 800 accident that occurred in July 1996, the Safety Board asked the FAA to address both long-term and short-term solutions to the fuel tank issue. Previously, fuel tank explosions occurred somewhere in the world approximately once every 52 months, but in the 11 years since the TWA-800 accident, there have been three additional fuel tank explosions, illustrating the continuing need for reforms in this area.
Summary of Action
In response to the long-term solution-preventing flammable fuel/air vapors in fuel tanks-the FAA commissioned an ARAC to evaluate design modifications, such as inerting, that would satisfy this recommendation. In its July 1998 final report, the ARAC concluded that inerting would achieve this goal, but at a cost of over $20 billion. The ARAC also concluded that inerting systems would be very difficult to retrofit into existing airplanes and therefore recommended that the FAA continue to investigate a more cost-effective approach to reducing explosive vapors. A 2001 followup study also concluded that the benefit of inerting could not be reasonably balanced by its cost. In May 2002, in contrast to the ARAC’s reports, the FAA developed a prototype inerting system that required no moving parts, weighed less than 200 pounds, and could be retrofitted onto existing airplanes at a fraction of the industry-estimated cost: the cost of the prototype system was only $100,000. The system has been flight tested by the FAA, NASA, Boeing, and Airbus, and the results indicate that the system is both practical and effective. Although this system reduces flammability, rather than inerting the vapors, the reduction in flammability is substantial, and the system constitutes a major advancement in air safety. The FAA currently intends to use this system only for some, not all, fuel tanks on an aircraft, and not on cargo aircraft, a reduction in scope from what the Board recommended.
Although 11 years have passed since this recommendation was issued, the FAA’s more recent actions indicate positive movement, particularly in the development of a practical fuel tank inerting system. Boeing is making a flammability reduction system a basic feature in the design of the new 787 Dreamliner aircraft. Boeing has also designed a flammability reduction system and delivered these systems on production models of the 747 and 737 NG. Although the first B-737 equipped with a flammability reduction system was delivered on December 8, 2005, to Southwest Airlines, this system is an option, and many 737’s currently being delivered are not equipped with this system. The next design to receive a flammability reduction system will be the B-777.
A notice of proposed rulemaking (NPRM) was published in the Federal Register on November 23, 2005 to require the installation of the flammability reduction system in commercial aircraft. The NPRM closed in May 2006. In testimony before Congress on September 20, 2006, and June 6, 2007, the FAA stated that a rule concerning flammability reduction means, although not specifically inerting, would be issued by the end of 2007. The draft final rule was sent to the Office of the Secretary, Transportation (OST), on July 13, 2007, for executive review. After the draft rule clears OST, it must still be reviewed by OMB.
Action Remaining
Complete rulemaking efforts to preclude the operation of transport-category airplanes with flammable fuel/air vapors in the fuel tank on all aircraft.
Safety Recommendation
A-96-174 (FAA)
Issued December 13, 1996
Added to the Most Wanted List: 2002
Status: Open—Acceptable Response
Require the development of and implementation of design or operational changes that will preclude the operation of transport-category airplanes with explosive fuel-air mixtures in the fuel tank. Significant consideration should be given to the development of airplane design modifications, such as nitrogen-inerting systems and the modifications should apply to newly certificated airplanes and, where feasible, to existing airplanes. (Source:Letter of recommendation dated December 13, 1996; based on the investigation of the Trans World Airlines flight 800 crash near East Moriches, New York, on July 17, 1996)
November 2007
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