PANEL SUMMARY: Flammability

To aid in its study of the apparent explosion that erupted in the center wing fuel tank of TWA 800, NTSB investigators worked with scientists from a number of research organizations, experts in the field of fire and explosion. At the panel devoted to these efforts, Dr. Merritt Birky summarized the progress so far. Birky, National Resource Specialist in the NTSB’s Office of Research and Engineering, said that while the source of ignition for the explosion has not yet been identified, "there is much that we do know."

First, he noted, research has already shown that vapors in the fuel tank were flammable, capable of supporting an explosion. "We can lower this flammability significantly," he continued, "with added fuel and by not using the air conditioning packs while at the gate. We can increase the energy for ignition dramatically by lowering the temperature. And we can further reduce the temperature and flammability by adding a radiation shield between the packs and tank bottom." Such a shield would help prevent heat from radiating from the packs to the tank.

Birky and other senior NTSB scientists outlined the characteristics of Jet A fuel flammability and then detailed a series of flight tests conducted by the Board to learn more about the conditions on Flight 800 and also to explore potential solutions.

Birky explained that before a fire can occur, three elements are needed: fuel, oxygen and an ignition source. To prevent a fire, he said, one of these must be removed.

In the case of TWA 800, the NTSB has determined that the center fuel tank exploded, leading to the breakup of aircraft. But the Board has not come up with the source of the spark that ignited the vapors in the center fuel tank.

Birky, a specialist in chemistry, fires and explosions, said that in Flight 800, the vapors from the fuel were generated when the fuel tank, containing approximately 50 gallons of Jet A fuel, was heated by the air conditioning packs under it.

He said the flash point for Jet A is above 100 degrees and usually about 115 to 125 degrees at sea level. But, he added, for auto-ignition of Jet A, that is for the fuel to ignite on its own without a spark, the vapor must reach about 450 degrees.

Detailing flight tests in an Evergreen International Airlines 747 similar to the accident aircraft, Dr. Daniel Bower, an Aerospace Engineer, said nine tests were conducted, one of which attempted to emulate Flight 800.

He said that the aircraft was instrumented with over 150 sensors to measure temperature, pressure and vibration in the center fuel tank.

Bower said that the test which emulated Flight 800 remained on the ground for 3 and 1/2 hours, with 2 of the 3 air conditioning packs running, before it took off. It also carried the 50 gallons of fuel from Athens, Greece, in the center tank, the same as Flight 800.

Temperature Comparisons (60K)

He said readings showed that the temperature of the vapor in the center fuel tank was 127 degrees at an altitude of 13,800 feet. On a second flight, in an effort to quanitfy the Board's recommendations, the center tank was loaded with an additional 12,000 pounds of fuel and with the air conditioning packs running 90 minutes, the temperature of the vapor at 13,800 feet was only 84 degrees.

Dr. Joseph Shepherd, Associate Professor of Aeronautics at the California Institute of Technology, said that studies CalTech made with scientists from other institutes showed that 50 gallons of fuel is sufficient to create a flammable mixture, that high temperatures in the tank drive evaporation and that the energy needed for ignition is greatly reduced with high temperatures in the tank. His research indicates that a 50 degree decrease in temperature increases the energy needed for ignition by a factor of 100,000. Therefore, Shepherd later said, one of the most significant contributions that could be made to aviation safety would be to help keep those temperatures low.

QuickTime 1/4-Scale Explosion (1.6M)

The flight tests with the Evergreen 747 produced the first studies of fuel vapors taken from the tank of an aircraft in flight, reported Dr. John Sagebiel, of the Desert Research Institute, University of Nevada. In addition to those tests, researchers associated with the investigation used analysis by computer modeling, as well as explosion tests in a one-quarter-scale model of the 747’s fuel tank. The scientists who testified noted the complex interplay of forces in an explosion and the contributions this research is making to the understanding of those dynamics.

Addressing the families of the victims, Chairman Hall pointed out that this research team is doing all that is humanly possible to determine what caused the loss of TWA 800. While all involved may wish that results could come sooner, he said, the day’s testimony shows the thoroughness of the effort and the difficulty of the task.