A high temperature in TWA Flight 800's center fuel tank made it much more prone to an explosion caused even by a low-level spark, a California Institute of Technology scientist testified yesterday.
Tests showed that increasing the temperature in the fuel tank from 86 degrees to 140 degrees magnified the risk of explosion by 100,000 times, Dr. Joseph Shepherd testified during the second day of the National Transportation Safety Board's hearings at the Baltimore Convention Center into the explosion aboard the Paris-bound flight in July 1996.
As the plane taxied down the runway, its fuel tank temperature reached 145 degrees, he said, then dropped to about 120 degrees as the jet climbed to 13,800 feet, where it exploded.
After the largest and most expensive probe in NTSB history, investigators have been unable to pinpoint the source that sparked the highly volatile vapors in the nearly empty fuel tank. But CalTech's research provides new information about jet fuel characteristics -- and could help to prevent future catastrophes.
"It's a very significant finding. It's new information," said Peter Goelz, director of the NTSB's office of government, public and family affairs. "No one has had any idea how little energy it took to cause an explosion."
Frustrated by an inability to identify an exact cause, the NTSB has urged the Federal Aviation Administration and Boeing Co., the Seattle-based manufacturer of the aircraft, to step up inspections on the nation's 747s and to focus on ways to reduce flammability of the fuel tank.
Shortly after the crash, at Boeing's request, airlines began inspections of 747 center fuel tanks. But only 52 of the 907 jumbo jets in use have been checked. It could take two years to complete the inspections, the NTSB was told yesterday.
NTSB Chairman James E. Hall said yesterday that he failed to understand why the FAA had not ordered the inspections, since a federal agency requirement would have carried more weight with the airlines than an advisory by Boeing.
"To me that's frustrating," Hall said. "It's 16 months since the accident occurred."
But he praised the FAA and Boeing for agreeing to concentrate more on how to reduce the flammability in fuel tanks. In what it termed "a major philosophical shift," Boeing said it is looking at issues surrounding flammability, not just what caused the spark that ignited the Flight 800 tank. "Five years ago, we never would" have done so, said Ivor Thomas, Boeing chief engineer of fuel systems and auxiliary power units.
In the past, Boeing and aircraft manufacturers had insisted that fuel tanks were designed to be protected from sparks and other ignition sources.
The new approach is a marked departure from Boeing's investigation of a 1990 crash involving a Philippine Airline Boeing 737-300 in Manila in which the fuel tank exploded on the ground in 97-degree weather. In that crash, Boeing did no investigation on the flammability of the fuel tank.
As part of its investigation into Flight 800, the NTSB last summer leased a Boeing 747 and conducted nine test flights under similar conditions to those seen when Flight 800 took off on a summer evening with temperatures about 86 degrees.
The temperature inside Flight 800's fuel tank is believed to have risen as the delayed flight sat on the ground more than an hour with its air-conditioning systems, located under the center tank, heating up. Investigators conducted a test flight under similar conditions to Flight 800, but without the runway delay. The temperature in the test flight's fuel tank was lower than Flight 800's, they testified.
The plane used for Flight 800 had flown to John F. Kennedy International Airport from Athens, Greece, where the fuel tanks were filled. The center fuel tank, one of several, was almost empty before taking off for Paris, and investigators have speculated that the small amount of fuel heated rapidly, causing a buildup of dangerous vapors.
But Shepherd of CalTech said the nearly empty fuel tank did not appear to be a significant factor, though the addition of more fuel could have cooled the fuel and thereby reduced the possibility of an explosion.
The NTSB is expected to produce a list of recommendations stemming from the Flight 800 investigation. With the airline industry likely to resist expensive changes, such as carrying unnecessary fuel, the NTSB is hoping to come up with more practical recommendations it will accept.
Ultimately, the question may be how the industry and federal regulators can keep warm temperatures, regardless of their source, from reaching the fuel tank.
Pub Date: 12/10/97