Ramon Lopez/WASHINGTON DC
On 23 August, the US National Transportation Safety Board (NTSB) concluded the longest and most costly inquiry in its 33-year history when it ruled that the 1996 crash of a Trans World Airlines Boeing 747-100 was caused by an explosion of the centre wing tank (CWT) resulting from ignition of the flammable fuel/air mixture in the tank.
The NTSB says a contributing factor to the fatal accident was design and certification criteria which assumed that fuel tank explosions could be prevented solely by precluding ignition sources. Another fatal flaw was the existence of heat sources beneath the CWT, with no means to block heat transfer into the tank or to render the fuel vapours in the tank non-flammable.
The safety board urged the US Federal Aviation Administration to give "significant consideration" to nitrogen-inerting systems for fuel tanks in existing and newly certificated civil transports, with such a rule change representing a fundamental shift in FAA philosophy.
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 first believed. Thomas McSweeney, the FAA's associate administrator for regulation and certification, says: "It is clear where we're heading on inerting. There could be a big payoff. We are very encouraged by the research, but we are not at a point where we can issue an airworthiness directive. But I'll say it is around the corner, not long-term."
Computer modelling shows that a flammable fuel/air mixture exists in CWTs for 30% of flight time on average, fleet-wide. McSweeney says ground inerting could reduce the flammability exposure to just 2%.
The FAA will form an Aviation Rulemaking Advisory Committee (ARAC) to advise within a year whether a "practical" means of inerting fuel tanks can be found for in-service and new-production civil transports. In 1998, another ARAC concluded that on-board inerting would improve safety, but at a cost of at least $20 billion over 10 years. At that time, ground inerting was deemed an option that needed future study to determine the ultimate costs.
A new FAA report pegs the cost of ground-based inerting of fuel tanks, which provide protection during ground operations and during the initial phases of flight when the chances of an explosion are greatest, to be just $1.6 billion. This is based on inerting the tanks of all passenger aircraft with over 19 seats at the 400 largest US airports.
The FAA notes that on-board inerting of fuel tanks with nitrogen has been practiced by the military for years, but the weight and dispatch reliability of on-board inert-gas generating systems (OBIGGS) have made on-board inerting less than practical or cost-effective for commercial aviation. The ARAC said OBIGGS causes performance penalties, but they could be minimised in new aircraft designs. Mandating OBIGGS on current transports could make them uneconomical to fly, it warned.
Lockheed Martin, which provided the C-5A/B Galaxy heavylift military transport with a liquid nitrogen system for fire protection and for inerting the fuel tanks, says these systems are difficult to operate and maintain due to the extremely low temperature of liquid nitrogen. It says the systems have also proven to be safety hazards to maintenance personnel. As a result, company officials say: "The usefulness of a liquid nitrogen system on commercial transport aircraft has not been identified, and may even increase the hazards of total aircraft operations."
While continuing to research OBIGGS technology with NASA, the FAA is more hopeful about ground-based inerting. It consists of displacing most of the oxygen dissolved in the fuel with nitrogen by a process called fuel scrubbing. It also requires displacing the air in the fuel tank empty space, also known as ullage, with nitrogen-enriched air (NEA) in a process called ullage washing.
In a 'pre-emptive' strike prior to the NTSB meeting, aviation industry groups released a three-year study that concludes that "the fuel tank systems on the world's fleet are soundly designed and do not tend to degrade as airplanes age." The study does recommend, however, a handful of changes to enhance fuel system safety. "The design philosophies that we've used over time do not show evidence of systemic issues," says Robert Robeson of the Aerospace Industries Association. He adds that the issue of nitrogen-inerting was not addressed in the study.
Research and lobbying over the coming year will determine what price the industry must pay for increased safety. NTSB chairman Jim Hall says: "It is healthy that the industry engage in a thorough examination of the practicality and benefits of safety recommendations....What I don't want to see, though, is a reflexive action by industry that leads to our recommendations being rejected out of hand."
Source: Flight International
