Explosion Research at Caltech and TWA Flight 800
1/4-Scale Experiments |
- Why carry out scale-model testing?
- The scale-model testing was
carried out to examine combustion issues that we could not study in our
laboratory testing. These were:
a. The motion of the flame between the bays of the CWT. b.
Acceleration of the flame by turbulence created by gas motion through
the passageways between the bays. c. Dispersal of the liquid Jet fuel
into a spray or mist of liquid droplets. d. Effect of the failure of
the partitions (FS, SWB3, and manufacturing access panel in SWB2) on the
motion of the flame.
The principal goal of the testing was to determine if the ignition
location would leave a "signature" that could be identified from the
wreckage of TWA 800.
- What features of CWT were modeled?
- All of the features considered
essential for understanding how a flame would propagate through the
center wing tank were modeled. This included:
- The 7 bays of the CWT.
- The partial ribs, beams and spars.
- The openings (passageways) in the ribs, beams, and spars.
- The water bottles on the front spar.
- The vent tubes and vent stringers.
- Vapor and liquid fuel (thin layer on floor).
- Structural failure of the spanwise beams, front and mid spar.
- What features of the CWT were not modeled?
- A number of features
were not modeled since these were either considered unimportant
(variation in height), modeled in other ways (heating), or else too
complex or expensive (failure of tank top, etc).
The top, bottom, rear spar and side of body ribs were modeled as rigid,
i.e., nonfailing, structural components. The detailed structure and
structural failure mechanism of the tank were not simulated. The only
aspect of the structural failure that was simulated was the gross
failure of the beams and spars due to pressure differences between the
- The variation in height from the back to the front.
- The fuel probes and associated wiring in the tank.
- The boost and scavenge pumps.
- The stringers and stiffeners.
- The variation in elevation of the bottom of the tank.
- The heating by the air packs.
- Failure of the top, bottom or sides of the tank.
- What type of fuels were used in the 1/4-scale tests?
- Two types of
fuels were used in the 1/4-scale testing.
The first set (1-40) of 1/4-scale tests used simulant vapor fuel rather
than hot Jet A. Laboratory experiments with Jet A and other fuels
determined that a mixture of hydrogen and propane could be used to
simulate Jet A combustion due to Jet A vapor resulting from liquid fuel
evaporation at 50 C. This was done to simplify the field testing and
enable an unheated tank to be used. The simulant fuel was also selected
to compensate for the difference in pressure and temperature between the
tests and the conditions in the CWT at the time of the explosion.
The second set of tests (41-70) used a heated tank and Jet A. The Jet A
was carefully characterized in laboratory combustion tests at Caltech
and chemical analyses at UNR. The tank was heated to temperatures
between 40 and 60C and evacuated to the pressure altitude of the
- How were the tests carried out?
- The tank was heated in some
cases, partially evacuated, filled with fuel, and mixed. A thermal
ignition source consisting of a rapidly-heated, exposed, light bulb
filament was used to start the flame. Laboratory tests have shown that
the ignition of the flame occurs in an essentially identical fashion for
both spark and thermal (filament-type) ignition sources. Video cameras,
high-speed (500 frames-per-second) cinematography, pressure, temperature
and light measurements are used to record the development of the flame.
A special photographic system using parallel light through the
transparent sides of the tank was used to follow the motion of the flame
through the tank.
- What were the results of the tests ?
The laboratory and scale-model experiments with Jet A fuel were carried
out under conditions simulating the CWT environment at the event
altitude: temperatures of 40 to 50 C (104 F to 122 F) and a pressure of
0.585 bar (8.5 psi). These experiments demonstrated that the fuel
vapor-air mixture was flammable over the range of temperatures (40 to 50
C; 104 F to 122 F) and fuel loading (50-100 gallons) present in the CWT
at the time of the accident. Furthermore, the elevated temperatures
inside the tank, caused by heating from the air cycle machines under the
tank, increased the explosion hazard over that posed by a cool tank by
substantially increasing the amount of fuel vapor and thus decreasing
the ignition energy. Flight and ground testing confirmed this finding.
The measured peak pressure rises recorded during our experiments were
between 1.5 and 4 bar (20 to 60 psi), sufficient to cause failure of
structural components inside the CWT of a B-747 aircraft. Scale-model
experiments, using both Jet A and a simulant fuel, and numerical
simulations reveal that the flame front can propagate rapidly between
the compartments of the tank once the flame reaches the passageways and
vent stringers connecting the compartments. In some cases, the flame
will be quenched as it passes through these openings or connections. The
motion of the flame through the tank follows a complex path that results
in pressure differences between compartments. Scale (1/4) model
experiments and numerical simulations have confirmed that the flame
path, and consequently, the pressure differentials generated across the
fuel tank structures, are dependent upon ignition location.
Our experimental measurements and numerical calculations have shown that
combustion-induced pressure differences produce forces on CWT internal
structures (span-wise beams and spars) that can cause deformation and
failure of these components. The magnitude and direction of these forces
determine which components fail, in what direction, and the sequence of
failure. The location of the ignition source is reflected in, but not
obvious from the examination of, the damage to the various internal
structures, since the forces created during the explosion depend on that
location, among other factors.
Test Number 69 (Nov. 3, 1998) This test used the full 6-compartment configuration with
simulated failure of the FS and SWB3 and open vents. The fuel was Jet A, the initial temperature was 39.9C,
and the initial pressure was 58.5 kPa (TWA 800 CWT conditions). The igniter was a light bulb filament
in the 2L position, with socket base oriented horizontally.
These movies were taken with high-speed (500 fps) pin-register cameras and transferred to video.
A. Combustion process in bays (AVI) or (MPG)
B. Exterior view showing expulsion of FS. (AVI) or (MPG)
Frames from combustion process film showing development of explosion within tank.
Frames from film of tank exterior showing expulsion front spar model from tank