Now that the four-year investigation into the crash of Trans World Airlines Flight 800 is officially over, the real challenge begins - maintaining the momentum behind efforts to improve aircraft fuel tank safety in the face of regulatory inertia and industry self-interest.
TWA 800, a Boeing 747-100, broke up in flight and crashed into the Atlantic Ocean off Long Island on 17 July, 1996, killing all 230 people on board. The lengthy investigation has concluded the most likely cause was not a bomb or a missile, but an explosion of the flammable fuel/air mixture in the centre wing tank.
The ignition source has not been determined, but was most likely a short circuit outside the tank which allowed high voltage to enter it through the fuel quantity indication system wiring.
During the National Transportation Safety Board's investigation, much has been learned about fuel flammability, possible ignition sources and wiring in ageing aircraft. As a result, the Federal Aviation Administration has issued nearly 40 airworthiness directives (ADs) related to fuel tank safety.
The NTSB, so often at odds with the FAA, has praised the aviation agency's rapid response to its safety recommendations. And undoubtedly the ADs have improved fuel tank safety, but none have tackled what the NTSB believes is the root cause of the crash - a "fundamentally flawed" design and certification philosophy that assumes fuel tank explosions can be prevented solely by precluding ignition sources.
Arguing that TWA 800 has demonstrated that all possible ignition sources cannot be predicted and reliably eliminated, the NTSB is calling for the installation of nitrogen-inerting systems in fuel tanks. Two years ago, the FAA was opposed to such action, questioning its effectiveness and cost, but further research has indicated ground-based inerting could be a more cost-effective solution than at first believed.
Computer modelling has shown that a flammable fuel/air mixture exists in centre wing tanks for 30% of flight time on average, fleet-wide. The same model shows that ground inerting could reduce the flammability exposure to just 2% of flight time. Such a dramatic reduction in the risk of an explosion would seem worthwhile, but inevitably there is a cost.
In ground-based inerting, nitrogen would be pumped into the empty spaces in fuel tanks of all aircraft before take-off, to provide protection during the initial phase of flight when the risk of an explosion is highest.
The alternative, on-board inert gas generation, would protect throughout the flight, but is costly, heavy, difficult to maintain and can only be considered for new aircraft.
There is disagreement on just how much nitrogen inerting would cost. While one industry projection has put the cost to US airlines alone at $19 billion, the FAA now estimates the cost of ground-based inerting at no more than $1.6 billion. This is based on inerting the tanks of all passenger aircraft with more than 19 seats at the USA's 400 largest airports.
That a potentially explosive condition exists for a third of each and every flight would be an extremely worrying statistic were it not for the fact there have been so few fuel tank explosions over the years. Of the 16 instances identified worldwide, only TWA 800 and Philippine Airlines Flight 1990 are believed to have resulted from a wiring short circuit.
Now that the TWA 800 investigation is complete, and the pressure of public scrutiny is relieved, there is a risk that the NTSB, FAA and airlines will revert to their traditional roles in the safety debate: the NTSB wielding its political power to goad the FAA into acting on its recommendations; the airlines arguing the cost of safety actions must be justified by their effectiveness; and the FAA trying to satisfy both from within a cumbersome regulatory environment.
The advisory committee convened to recommend rulemaking on fuel tank inerting has been given 12 months to report, but any rulemaking will take longer. Delay would not be wise. While another fuel tank explosion is highly unlikely, it would return the glare of the public spotlight to an issue which must be resolved fast.
Source: Flight International
