California Institute of Technology

Explosion Dynamics Laboratory

Explosion Research at Caltech and TWA Flight 800

Background | Facts | Documents | 1/4-Scale Experiments | Misconceptions |

Jet A Explosions and TWA Flight 800 Investigation

The Facts:

What kind of fuel is Jet A?
Jet A is aviation kerosene, it is very similar to the common kerosene used in home lamps and heaters. It is a clear liquid that is a mixture of different kinds of fuel molecules, hydrocarbons, made up of hydrogen and carbon. Kerosene is similar to gasoline and diesel fuel in that it is a mixture of hydrocarbons of different sizes. The sizes of the molecules are measured in terms of the number of carbon and hydrogen atoms in the each molecule. Jet A liquid contains molecules with between 4 and 20 carbon atoms.
What makes jet fuel explosive?
The liquid fuel is not explosive by itself. Explosive conditions are created when the fuel evaporates and mixes with the air in a partially empty tank. The evaporated fuel is referred to as fuel vapor. The fuel vapor consists of fuel molecules that have escaped from the liquid fuel and form a gas in the volume (ullage) above the liquid in the bottom of the fuel tank.
Why don't explosions happen all the time in fuel tanks?
An explosion can only happen if there are certain conditions within the fuel tank and an ignition source is present. The combination of conditions that can allow an explosion are known as flammability. A mixture is flammable when there are the correct proportions of fuel vapor and oxygen in the tank ullage. If there is too much or too little of either fuel or oxygen, a mixture will not be flammable. There must bea minimum concentration of fuel molecules for a fuel vapor-air mixture to be flammable. This value is known as the lower flammability limit. This can also be measured in terms of the ratio of the mass of fuel vapor to the mass of air in the ullage. Even if the ullage is flammable, an explosion will not occur unless there is an ignition source, which can be a spark of sufficient energy or a sufficiently hot surface, within the tank. The goal of airplane manufacturers is eliminate any potential sources of ignition from fuel tanks.
What are the most important factors in the flammability of Jet A?
The most important factors in the flammability of Jet A are: 1) the temperature of the liquid fuel and the tank ullage. 2) the pressure of the air in the tank. 3) the amount of liquid in the tank. 4) how the fuel has been handled - known as "weathering". 5) The strength of the igniter.
Why is the temperature of the fuel and ullage important?
The temperature of the fuel determines how much fuel evaporates from the liquid. The temperature of the ullage determines how fast the vapor fuel condenses on the walls of the tank. When the liquid and ullage temperatures are equal, a condition of liquid-vapor equilibrium is said to exist. In equilibrium, the amount of fuel vapor is measured in terms of the pressure that the fuel molecules alone produce, this is known as the vapor pressure. The higher the liquid temperature, the higher the vapor pressure of the fuel. The fuel and ullage temperature were measured as part of the flight tests carried out in July 1997. The temperatures within the fuel tank at 14 kft ranged between 38 and 54 deg C (100 and 130 deg F), and the tank lower surface temperatures ranged between 38 and 60 C (100 and 140 deg F).
How does the evaporated fuel vapor mix with air inside the tank?
Flight test measurements indicate that the bottom of the fuel tank was slightly hotter (10-20 degrees Fahrenheit) than the top and the sides. This will produce upward motion of the gas in the center of the tank (similar to the heat waves observed above pavement on a hot day) and downward motion at the side of the tank. This circulation or convection within the tank stirs and mixes the fuel vapor with the air. Direct measurements of the fuel concentration and tracer gases in the flight test confirm the well-mixed nature of the tank contents.
Why is the pressure in the air important?
The pressure in the air determines the amount of oxygen present in the ullage. The pressure in the atmosphere decreases with increasing altitude. As the pressure decreases, the ratio of fuel to air increases and the lower flammability limit is exceeded. That is why fuel tanks are more hazardous during climb than on the ground. The pressure at 14 kft is lower by a factor of 0.585 than the standard atmosphere at sea level. Laboratory tests of flammability and explosion were carried out at this pressure.
Why is the amount of the liquid fuel important?
There was a very small amount of fuel, about 50 gallons or 300 lbs, compared to the 13,000 gallon capacity of the center wing tank of Flight 800. Evaporation will deplete the liquid, resulting in a smaller fuel vapor pressure than if a larger amount of liquid was present. This effect is more important for the small fuel molecules than the large ones. This means that the fuel vapor above a small amount of liquid will tend to heavier than above a large amount, for a given tank. In scientific terms, the composition of the vapor will shift toward the higher molar mass components as the amount of liquid fuel is decreased. The effect of the limited amount of fuel was studied in laboratory tests of vapor pressure and ignition energy. Tests were carried out using the same proportions of fuel to tank volume as was present in TWA Flight 800. Although the vapor pressure is reduced by approximately a factor of two over the full tank value, the tank was still flammable at 14 kft and the ignition energy was not significantly affected.
How does weathering affect flammability?
Weathering is the net effect of the airplane flight on the liquid fuel in the tanks. The most important effect is that the fuel vapor initially present in the tank is vented to the atmosphere during the initial climb to cruising altitude. Venting during climb is a consequence of the expansion of the gas in the tank as the altitude increases and the pressure decreases. The vented gas flows out of the tank through the vent stringers to the wing tips and then out into the atmosphere. Both fuel vapor and air in the tank are vented in equal proportions. As the fuel vapor is vented, liquid fuel evaporates to replace it. Since the lighter molecules in the fuel evaporate faster than the heavier components, weathering tends to deplete the liquid fuel of the lighter components. This effect is more prominent for small amounts of liquid fuel in a large tank, which was the situation in TWA Flight 800. The effect of weathering was measured by taking liquid and vapor samples during the flight tests conducted last July. Chemical composition measurements by UNR and DRI show a slight shift in the chemical composition but no net effect on the fuel concentration. Laboratory measurement of flammability using weathered fuel confirm that no significant increase of the ignition energy took place as a result of the weathering.
How does the strength of the igniter affect flammability?
Flammability is measured by deliberately putting an ignition source into a mixture. If a flame is observed to move away from the igniter, then the mixture is flammable. The lower flammability limit is usually determined using a very strong spark or arc produced by a high-voltage transformer. The flammability limit is determined by finding the lowest concentration of fuel vapor that will result in flame propagation for a given spark. As the spark is made weaker, the concentration of fuel vapor has to be increased for the mixture to remain flammable. The strength of a spark igniter is measured in terms of the stored electrical energy (Joules) used to create the electrical discharge. The lower flammability limit is typically measured using a spark of energy 10 to 100 Joules. This is comparable to the arc created by a short circuit in household wiring. The lowest possible energy that will cause ignition of a flame is known as the minimum ignition energy. For most hydrocarbon fuels this has a value of about 0.25 mJ or about 100,000 times less energy than is used to determine the lower flammability limit. The minimum ignition energy is extremely small, comparable to the energy in an electrostatic spark created on bedding or carpets on a dry day. The minimum ignition energy occurs for a mixture with a large amount of fuel vapor in comparison to the lower flammability limit. For liquid fuels like Jet A, flammability is typically measured by finding the minimum temperature of the liquid (and ullage) that will result in flammability of the fuel vapor produced by evaporation of the liquid. Since the vapor pressure and fuel concentration increases with increasing temperature, ignition energy decreases with increasing fuel temperature. For Jet A, the lower flammability limit found using a 100 Joule spark is between 35 and 40 degrees Celsius, or about 94 to 105 degrees Fahrenheit. When the temperature is increased to 60 degrees Celsius (140 degrees Fahrenheit), the ignition energy decreases to less than 1 mJ. This decrease of a factor of 100,000 in ignition energy with a 20 degree increase in temperature indicates the extreme sensitivity of the explosion hazard to the temperature in the tank.
Does this mean that a spark always the most probable ignition source?
No, sparks are just a convenient method for determining flammability in laboratory experiments. Hot surfaces or particles are the other common source of ignition. The strength of such an igniter is somewhat harder to characterize and no single factor such as energy can be used to describe flammability limits with hot surfaces. In general, a surface has to have a minimum surface area and temperature to cause ignition. Experiments with rapidly heated lightbulb filaments show that Jet A can be effectively ignited with small (1/4-in long) hot wires. Arcing electrical contacts, short circuits in electical wiring, electrostatic discharges and hot particles are some of the many other ways that accidental ignition can occur.
Was the mixture in the center wing tank flammable at the time of the explosion of TWA Flight 800?
Based on the temperatures measured in the flight test and vapor pressures measured at Caltech, the fuel-air composition within the tank was in the flammable range with fuel-air mass ratios between 0.040 and 0.072 (mole fractions between .0089 and .015). These estimates are corroborated by the vapor sampling Sagebiel (1997), who measured fuel-air mass ratios between .048 and .054 (mole fractions between 0.010 and 0.012) at 14kft. These values should be compared with a lean limit fuel-air mass ratio of 0.030 (mole fraction of 0.007) and a stoichiometric fuel-air mass ratio of 0.070 (mole fraction of 0.015). Flammability measurements at Caltech verified the flammability of both fresh and weathered fuel under the conditions found at 14 kft in the flight test. The ignition energies could be as low as 1 mJ in the hottest portions of the tank.
What happens in the explosion?
The explosion occurs through the motion (propagation) of a flame, or combustion wave, through the ullage of the tank. The flame is a thin layer of intense chemical reaction in which the fuel vapor molecules and oxygen in the air combine to produce high temperature (3600 degree Fahrenheit) combustion products, water vapor and carbon dioxide. The motion of the flame sweeps up or lofts the liquid fuel in the bottom of the tank, creating a fireball of burning fuel with the remaining oxygen in the tank. Any liquid remaining on the tank bottom may also continue to burn after the fuel vapor is combusted.
How can the explosion destroy the tank?
The high-temperature combustion products try to occupy much more volume than the original contents of the tank. Unless the products can be rapidly released through the vent system, the pressure in the tank will increase as the flame moves through the tank. The pressure reaches a maximum value and then falls as the hot products cool off and are released through the vent to the atmosphere. The speed of the flame and the rate of venting from the tank determine the maximum pressure reached inside the tank. In all cases that we have studied, the pressure inside the tank increases quickly enough that the strength of the tank, known as the failure pressure, is exceeded before the gas produced by the explosion can be vented out of the tank. This causes the outer walls of the tank to be forced outward and to rupture. The force of the explosion can be strong enough to tear the walls of the tank and propel the pieces outward at high speed.
What determines the maximum pressure in a fuel tank explosion?
The maximum pressure is determined by the amount of fuel vapor in the tank. For Jet A liquid between 40 and 60 degrees Celsius (102 to 140 Fahrenheit), the maximum pressure is about 4 to 4-1/2 times the normal atmospheric pressure of 14.7 psi. This is 2 to 3 times higher than the pressure needed to cause the failure of the front (spanwise beam 3) portion of the center wing tank. The maximum pressure was measured in experiments at Caltech using 1180 liter (300 gallon) capacity explosion vessel, Jet A liquid in the same proportions as in the CWT and temperatures between 100 and 140 degrees F. This tank is about 1/50-th of the CWT volume.
Will the peak pressure found in the laboratory experiments be the same as in the center wing tank?
The peak pressures produced in a center wing tank will be different than that observed in the laboratory experiments. The size of the tank, venting of gas to the atmosphere, and the heat transfer (cooling) during the combustion event are three causes of this differences. Most importantly, the peak pressures in the different bays of the center wing tank may be very different and also depend on the location of the ignition source and how the center wing tank structure fails. These issues have been examined in our scale-model testing and using numerical simulations.
How fast did the explosion occur?
The speed of the explosion is measured by the rate at which gas is being burned inside the tank. This is determined by several factors:
  • A property of the combustion wave, known as the laminar burning velocity.
  • The flow or motion of the gas within the tank.
  • How the flame propagates through the passageways and vents connecting the bays in the fuel tank.
The first factor, the laminar burning velocity, was investigated in our laboratory tests. The laminar burning velocity is how fast the flame approaches the unburned fuel vapor-air mixture. For Jet A at the conditions at the time of the explosion of TWA 800, this value is about 1-to-2 ft-per-second, less than 3 mph. Inside of a closed tank, the increase in volume of the high-temperature combustion products shoves the flame ahead even faster, and the flame appears to move even faster, 5 to 10 feet-per-second, up to 15 mph. The second two factors are very significant and can increase the rate of combustion by a factor of up to 100. Different bays may also burn at very different rates. The details of the construction of the tank (location and size of the bays, passageways and vents), the location of the ignition source, and the failure of the tank structure will determine how the combustion occurred in the CWT of Flight 800.
What was learned in the scale-model experiments? See the separate page on the 1/4-scale experiments.
How were numerical models used?
Numerical models, validated against laboratory and scale-model experiments, simulated combustion within a full-scale CWT. Ultimately, 32 full-scale scenarios were calculated. These scenarios were created by parametrically varying the ignition location (8 points corresponding to fuel quantity instrumentation system (FQIS) components), fuel vapor concentration (2 temperature levels), and time delay (0-24 ms) between failure of the front spar (FS) and the manufacturing door in span-wise beam 2 (SWB2). Each of these scenarios was analyzed using the rule-based analysis method to quantify its consistency with the observable damages.
How were the results from simulation and experiments used to analyze ignition location possibilities?
The rule-based analysis employs a set of rules derived both from the physical damage of the wreckage and structural failure estimates. Key elements in the establishment of these rules were that the wreckage indicated a particular sequence of events involving the failure of the two forward structural components (FS and SWB3), while the rear components (SWB2, MS, SWB1, and RS) probably remained essentially intact during the early explosion event. A total of 15 key observations were considered in the rule-based analysis method as probabilistic constraints or rules. In order for an ignition location to be considered plausible, the associated scenario must show a high degree of consistency between the observed and predicted damages. The rule-based system evaluation of these numerical simulations found several ignition locations that would produce propagating flame fronts within the tank volume and pressure differences on the structural components that were consistent with the observed damages. Although the evaluation considered a limited number of specified points as the ignition locations, the uncertainties associated with this analysis preclude making a determination of ignition location with a high degree of certainty. In particular, substantial uncertainty exists in extrapolating the combustion behavior from 1/4-scale to the full-scale (actual) CWT. As a result, a unique and most probable ignition location could not be identified.
What were the conclusions?
The conclusions from this research are that the Jet A vapor-air mixture present in the center wing tank of TWA 800 was flammable at the time of the accident, and that a propagating flame resulting from a localized ignition of this mixture can generate overpressures which are consistent with the observed damages in the wreckage of TWA 800.
Copyright © 1993-2016 by California Institute of Technology, Joseph E. Shepherd