PANEL SUMMARY: Flammability Reduction

Aircraft are designed so that their fuel tanks are safeguarded against conditions that could trigger an explosion. But if a failure occurs and the contents of a tank are subjected to an ignition source, what can be done to still prevent a catastrophe? The last panel of the NTSB’s public hearing into the crash of TWA flight 800 addressed measures to reduce the flammability of aircraft fuel tanks.

Much of the research in this area has been conducted by the military to support the survivability of combat aircraft. The panel included testimony from: Dr. Robert Ball, professor at the Naval Postgraduate School; J. Hardy Tyson, mechanical engineer, Naval Air Warfare Center; and Ralph Lauzze, director, live fire test and evaluation, Air Force Research Laboratory.

They described the various ways military fuel tanks have been made resistant to explosion, using nitrogen inerting and foam systems inside the tanks.

Nitrogen is an inert gas widely used to inert fuel tanks by rendering the vapors non-explosive. It can be carried on board the aircraft or it can be generated there with an on-board inert gas generating system (OBIGGS).

Another technique is the installation of special rigid foam into a tank. The foam prevents the passage of flame, and when moistened by the fuel, it acts as a heat sink, keeping the mixture too cool to explode. "The bottom line is that foam works," Lauzze concluded.

Thomas McSweeny, the FAA’s director of aircraft certification, said his agency had looked into using foam in the 1970s, but had not pursued that research since. He indicated that the use of a less volatile fuel, such as JP-5, was being examined as a possible means of reducing flammability. McSweeney said these issues will be taken up by the FAA’s Aviation Rulemaking Advisory Committee (ARAC) on an urgent basis. He said an ARAC working group would be initiated within two months, and he expected a committee recommendation within six months of that date.

Boeing said that since TWA 800 exploded in July 1996, it too was looking into ways to prevent fuel tank explosions. Ivor Thomas, Boeing’s director of 747 support, said its studies covered the use of foam, inerting, the use of JP-5 and the relocation of the electrical fuel quantity probes to positions outside the fuel tank. NTSB investigators are looking into the possibility that deteriorated fuel gauge wiring inside the center fuel tank of the 25-year-old accident plane might have caused the spark that touched off the explosion.

George Anderson, an NTSB aerospace engineer, asked McSweeny which designs or concepts, preferably already being used by the military, are most applicable and can most quickly adapted into commercial aircraft use. McSweeny said it would be a mistake to make that decision now, and that the FAA is looking into various areas, including the fuel, the oxidizer and the ignition, and until all are weighed against each other it would be premature to make any decision about which of the ideas are best for civil aviation. Asked further by Anderson if the FAA was just beginning the process of evaluating possible solutions, McSweeny said that the agency had looked into inerting many years ago and that it had been looking into the fuel triangle since the accident.

Dr. Bernard Loeb, director of the NTSB’s Office of Aviation Safety, said studies showed that foam would be helpful in both post-crash fires and to prevent fires in the fuel tank and asked why foam was not considered another anti-fire possibility. McSweeny said he was not in the decision-making process at the time but he thought that it was not a safety improvement considered warranted at the time.

McSweeny said military technology cannot be easily retrofitted into civil aircraft. The Air Force’s Lauzze agreed that fresh engineering studies may be needed to adapt the systems for use in civil aviation.

Chairman Hall asked why the FAA did not start to look at the possible military solutions in the summer of 1996, right after the TWA crash. McSweeny said that was because the cause of the crash was not immediately known and it was some time before it was determined that the explosion was in the center fuel tank.

McSweeny said the FAA had asked the American Petroleum Institute to assist with an evaluation of the feasibility of fueling jets with the less volatile JP-5 fuel. He claimed that JP-5 has a significantly lower flammability range and that its use would thus increase safety many fold.

Captain Steve Green, a member of the Accident Investigation Board of the Air Line Pilots Association, said he supported ways to pump inert gas into the fuel tanks, but he added that if the inerting system failed, the pilots needed to be assured that there were no ignition sources inside the fuel tanks. Green said there had to be a certainty that any new technology introduced to reduce flammability did not lower aircraft reliability.

McSweeny indicated that no FAA research on Jet A flammability is under way or planned and that his agency intended to wait for the NTSB’s studies of Jet-A flammability to be completed. Asked by Dr. Vernon Ellingstad, NTSB Director of Research and Engineering, whether sufficient information exists on the explosive characteristics of the Boeing 747 center wing tank, Dr. Joseph Shepherd, Associate Professor of Aeronautics at the California Institute of Technology stated that much remains to be learned about the explosive environment in these tanks.

In the course of its investigation into the TWA accident, the NTSB has already issued recommendations to the FAA calling for measures to reduce or eliminate the explosive potential of fuel-air mixtures in transport aircraft. McSweeny discussed one part of the FAA’s response to those recommendations, its request for comment on the recommendations. He said the request garnered more than 1,000 public comments, reflecting widely varying views. These, he said, would not only provide a substantial body of opinion on which to base further FAA action, but also add needed data. This use of request for comment, McSweeny also pointed out, is fairly unusual for the FAA.

Chairman Hall asked how JP-5 fuel compares in cost to the current, widely used Jet-A. McSweeney replied that comparisons are difficult, because JP-5 is now produced only in small batches, while Jet-A is produced in vast quantities. He concluded that the price differential would be "not trivial, but also not enormous. It’s something we have to look at."

Questioning of the witnesses from representatives of parties to the investigation explored potential problems in the commercial application of flammability-reducing technologies. For example, the Air Line Pilots Association asked the experts from the military about who is responsible for monitoring the systems in daily operations. Foam, they were told, requires no checking. It is simply installed in fuel tanks and remains a component there. OBIGGS may require a check at the beginning of a flight, but after that it is a hands-off system.

At the conclusion of the panel, Thomas of Boeing commented on the achievements of this accident investigation. We may not be able to determine the ignition source, he said, but this has prompted us to reconsider many of the principles on which we operate, including a careful consideration of the flammability of fuel tanks. So even if the exact cause of the accident remains unknown, the result will still be safer aircraft.