Eleven years after TWA 800, FAA to act on fuel tank rule

Paris Air Show » 2007
June 14, 2007, 9:29 AM

The U.S. Federal Aviation Administration (FAA) is expected to issue fuel tank inerting rules in September in a bid to reduce the risk of explosions. In 1996, just such an explosion caused the in-flight break-up of a TWA Boeing 747, and the new FAA mandate will target both new and in-service airliners.

The heart of the fuel tank problem is the flammability of the mix between air and fuel vapor inside a tank. This mix can become highly flammable when the temperature inside the tank increases. This happens, for example, when the aircraft is parked for several hours in a hot, sunny area–just one spark could cause a catastrophe. In the TWA 800 accident, investigators suspected that a faulty fuel quantity indication system created the spark.

The U.S. National Transportation Safety Board (NTSB) has long been urging the FAA to take preventive action. According to FAA figures, fuel tank flammability has caused four accidents–with a total of 346 fatalities–since 1989. The FAA’s notice of proposed rulemaking (NPRM) had a comment period, which ended in May 2006. FAA rules only apply to the U.S. but usually a lot of countries follow them.

The FAA’s proposal focuses on wing center tanks, which are located under the aircraft’s belly. They are the most vulnerable tanks and, once heated, they take a long time to cool down.

In the years after the TWA 800 tragedy, the FAA issued several series of rules to eliminate potential spark sources. For example, wiring is now better protected, with the SFAR 88 rule pointing out 200 potential ignition sources that had previously been unknown.

Further safety progress involved reducing the flammability of the tank’s vapor mix. In May 2002, the FAA came up with a prototype for an on-board inerting system.
The idea is to make the air leaner in oxygen before it enters the tank by making it richer in nitrogen. By cutting the air’s oxygen content from 21 to 12 percent, flammability is drastically curbed. The system uses engine bleed air.

Playing the unusual role of system designer, the FAA has certainly introduced innovations with this solution. The agency took advantage of the recent discovery that decreasing the air’s oxygen content to 12 percent was enough to cut flammability. Previously, it was believed that getting down to 10 percent was necessary and this development made a significant difference in the practicality of such an on-board system. The FAA’s system separates nitrogen from oxygen using a membrane.

The FAA has proved that such a system is feasible in civil aviation. Military aircraft have long been using inerting systems, but neither the weight nor the safety levels of those systems make them suitable for commercial aircraft, the FAA emphasized.

The industry is now moving to the production phase. Early last year, Honeywell completed certification of its inerting system on the Boeing 737 and 747. Hamilton Sundstrand is to install its own product on the 787. In December, Parker Aerospace announced it will provide Airbus with inerting systems for the A320 family, the A330 and the A340 beginning in 2009.

Airbus has told the FAA that developing a retrofit installation would take another 12 months. However, according to the airframer, the absence of a center wing tank on the A380 obviates the need for an inerting system on this model.

In its public comments, Airbus has criticized the FAA’s logic. “Inerting improves fuel tank safety but we do not agree with the FAA’s cost-efficiency analysis,” Airbus safety experts said. Moreover, Airbus is concerned about job safety for maintenance technicians. Even though system faults occur rarely, they will sometimes create spaces where oxygen is rarefied in the aircraft’s hold and bays. Technicians working there should take precautions, but, inevitably, fatal accidents will occur, Airbus asserted.

The FAA acknowledges that an inerting system is costly–$140,000 to $225,000 per aircraft–and relatively heavy–100 to 250 pounds. However, the agency’s statistics predict that as many as nine aircraft could be written off over the next 50 years as a result of a fuel tank explosion if nobody takes action. Ignition-source protection measures should prevent four accidents and inerting could save another four.

The European Aviation Safety Agency (EASA) intends to issue rules in the coming months, too. However, the agency officially has hinted that the retrofit objective is too ambitious and expensive. According to EASA, the FAA wants to go beyond the standard of one accident per million flight hours.

The current level of fuel tank safety is 10 accidents per million flight hours, but the FAA wants to divide this by 100 or 1,000 (instead of just 10), according to EASA. In other words, the European agency is suggesting that the industry might be content with a smaller increase in safety, for a more reasonable cost. Its preferred option is to install inerting systems on in-production aircraft only.

The FAA is considering offering manufacturers several options. Inerting will be the preferred one but other flammability reduction means–such as cooling the fuel tank–could be deemed
acceptable.   

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