Click on image to enlarge.

Coverage of TWA Flight 800 Iran Owned TWA jet same year one of its other 747s exploded Blast sound questioned Memos warned of 747 fuel leaks

Federal aviation and Boeing service records contain extensive documentation about where the wing and belly tanks on 747s are susceptible to leaks that can cause fire.

NTSB Vice Chairman Robert Francis, who is heading the investigation into the crash of TWA Flight 800, has declined to compare how much credence investigators are giving to the bomb vs. leaky fuel theories.

ut investigators have been piecing together the section of the plane they believe will provide them with the most clues on what caused the belly and right wing tanks to rupture and burn, a sequence that most likely began about a half a minute after things began to go wrong shortly after it departed New York's John F. Kennedy Airport bound for Paris.

The center section, between rows 17 and 28, is where the blast is believed to have originated, a source said. Those seats are near where the central fuel tank is located and where the wings are attached to the plane.

An explosive device precisely placed in the passenger cabin above the belly tank or in a cavity adjoining the belly or wing tank would punch a crippling hole in the airplane's most vulnerable, volatile area.

Another way things could have gotten started is if fuel or fuel vapors leaking from the belly or wing tanks accidentally ignited in an adjacent cavity.

It may have been a two-staged event," said Mike Hynes, an independent crash investigator from Frederick, Okla. "You could've had a malfunction of some kind . . . followed by a big splash of fuel going off."

Federal avaiation and Boeing service records on the 747 show several ways that the wing and belly tanks on 747s are susceptible to leaks that can cause fire.

-- Fuel and fuel vapors can seep past each of the thousands of fasteners used to form the front wall of the belly tank. To account for this design characteristic, Boeing applied a rubber-like sealant, called CAT-A-LAC, on the heads of each fastener. As an extra measure of protection against leaks, Boeing also called for application of the sealant on the other end of all the fasteners.

But in 1989 Boeing sent a service document to 747 operators warning that "the secondary fuel barrier may not have been properly applied to some areas . . . An inspection of an airplane in production revealed that the back side of fasteners and brackets may not have been coated as these areas were shielded from the spray. This coating is required to prevent fuel or fuel vapors from entering the cargo and passenger compartments in the event of a failure of a primary fuel seal or a crack in the center section structure."

In 1991, the FAA gave airlines 30 months to inspect for this production oversight and apply the secondary barrier.

TWA officials have said that the Flight 800 jet complied with this and other federal directives.

-- Another FAA directive, last updated in 1994, instructs 747 operators to repeatedly inspect hundreds of fasteners in a 10 foot length of the wing tanks above each of the inboard engines - an area in close proximity to the fractured and scorched parts of Flight 800's right wing. Fastener holes on jets with more than 13,000 flights are prone to begin cracking and leaking near the inboard engines and into cavities where electrical pumps and wiring are housed.

-- Most recently, an airline discovered that when electrical insulation on a 747's fuel pump failed (the plane was on the ground at the time), electricity burned a hole in the pump's case, letting fuel leak and then igniting it.

On Aug. 7, in a move officials said was not related to Flight 800, the FAA proposed a directive requiring regular testing and possible replacement of 747 fuel pumps. Following a 90-day comment period, the FAA will decide whether to issue the order.

An extensive account of how a small fire or explosion near the 747's fuel system during flight can escalate in disaster was detailed in the late 1970s when the safety board concluded a 30-month investigation into the 1976 fire and explosion of an Iranian air force 747-100 near Madrid.

In that case, investigators believe lightning struck a wing, causing a fuel sensor to emit an electrical spark that ignited fuel vapors inside the wing.

An initial flash fire and explosion breached the wing skin above the engine and sent flames licking forward in the slip stream to consume a small section of the wing's front fiberglass edge.

Once breached, the Madrid jet's wing skin began peeling back, causing the wing frame to twist and bend. The buckling of the wing caused the outboard engine to begin oscillating wildly from side to side.

The engine snapped off leaving its fuse pins - fasteners designed to break if the aircraft pitched up or down too drastically - intact.

Flight 800's two right engines were found separated from the right wing, with the fuse pins from both engines still in tact on the right wing. The left engines were also recovered separated from the wing.

As the Madrid jet's wing disintegrated, thousands of gallons of jet fuel spilled into the air, creating a highly combustible cloud of fuel vapor, which fed and multiplied the original small explosion.

Officials have declined to say whether they are discussing the possibility of something akin to the Madrid disaster in the TWA 800 crash.