Explosion Research at Caltech and TWA Flight 800
1/4-Scale Experiments |
Aviation Kerosene (Jet A) Combustion
Unfortunately, misinformation about aviation kerosene combustion and
misrepresentations about the official investigation of TWA 800 and the
investigators have been published in print and on the worldwide web. The
most common of these erroneous claims are discussed below:
- You can put a match out in Jet A at room temperature so it
can't possibly explode inside an aircraft fuel tank.
- False. You can do this with many fuels that have
sufficiently high flash or fire points. At room temperature, a
combustible liquid fuel has enough heat capacity to absorb all the
energy and extinguish a small match flame without raising the fuel
surface temperature above the fire or flash point. A flame cannot be
sustained over a liquid fuel until the surface temperature exceeds the
fire point. A flash will not result unless the fuel temperature and
vapor space are above the flash point. [Kuchta and Clodfelter] This does
not apply to the case of TWA 800 because a) the explosion occurred in
the fuel vapor-air mixture and did not require a fire on the fuel
surface, b) the fuel and air inside the tank were hotter than the flash
point, and c) the effective flash point was lowered due to the lower
outside air pressure at the altitude of the explosion. The Jet A
involved in the Center Wing Tank (CWT) explosion had a flash point
temperature of about
115 F and the decreased air pressure at the explosion altitude of 13.8
kft lowered the effective flash point to about 100 F. [Exhibit20S] The temperatures inside
the tank were between 100 and 130 F, and at some points, as high at 140
F [Exhibit 23F].
- The CWT of TWA Flight 800 was completely empty of
- False. It is impossible to
completely empty the center wing using the fuel system due to the
location of the scavenge pump, the nature of the tank bottom surface,
the division of the tank into bays, and the arrangement of the fuel
probes that supply the fuel quantity instrumentation system. At least
50 to 100 gallons of fuel will remain even when the fuel quantity
system indicates no fuel is remaining. [NTSB Final report]
- The CWT of TWA Flight 800 did not contain enough air to
- False. This is
apparently derived from the notion that fuel tank ullages in
automobiles are generally too rich to support combustion. Jet A is a
much less volatile fuel than gasoline and the situation is very
different in an aircraft fuel tank than an automobile. The fuel tanks
in an airplane are vented directly to the outside atmosphere and each
time the plane descends from cruising altitude, a fresh supply of air
is brought into the tank ullage by the vent system. The amount of
fuel added to this air by evaporation is determined by the vapor
pressure of the fuel, which has a maximum level of about 1 to 1.5% of
atmosphere pressure for a tank containing 50-100 gallons of Jet A heated
up to 60C - a situation comparable to TWA 800. [Exhibit 20D] The rich flammability
limit for Jet A is probably above 6% by volume, far higher than any
possible conceivable fuel concentration that would have existed in the
TWA 800 situation. In contrast, the fuel vapor concentration in the
ullage of a gasoline tank can be as high as 40% on a warm day.
- The fuel in the CWT of TWA Flight 800 was cold.
- False. Fuel and fuel tank temperatures were
measured in flight tests with a Boeing 747. The temperatures at the
time of the explosion ranged up to 60 C in the fuel layer at the
bottom of the tank. [Exhibit
- The vents in the CWT would prevent the pressure from building up
during an explosion inside the tank.
- False. The effects of venting were
studied in the 1/4-scale testing program and shown to be insignificant.
The venting system area is too small to relieve the pressure from an
explosion, even one that takes more than several seconds to pressurize the
tank. [Exhibit 20E, 20O]
- The NTSB investigators were unable to make Jet A explode in any
- False. Several hundred combustion experiments with Jet A were
conducted to determine the peak overpressure, flame speed, and ignition
energy. The conditions of these experiments bracketed the range of
conditions observed in the flight tests. [Exhibits 20D, L, P, T]
- The NTSB investigators had to use an ultra-sensitive explosive
mixture of propane and hydrogen in 1/4-scale testing because they were
unable to get Jet A to explode.
- False. As discussed above, explosions were obtained
in numerous experiments with Jet A in the laboratory and in the
1/4-scale tests. Several hundred experiments with Jet A were carried
out in laboratory tests [Exhibits 20T,
L]. The first series of experiments in the 1/4-scale facility used
a hydrogen-propane mixture to simulate Jet A. The purpose of using
this mixture was to simulate flame propagation in a tank that was at a
different temperature and pressure than the explosion altitude. The
way in which this was accomplished and the choice of the simulant
mixture are documented in Exhibits
20E and O. Subsequent
experiments in this facility used Jet A at the temperatures and
pressures corresponding to the conditions in TWA 800 at the explosion
altitude. A movie from a Jet A experiment was shown at the August 22,
2000 board meeting illustrating the explosive combustion of Jet A
vapors in the CWT model. This experiment demonstrates that sufficient
pressure was created to reach the failure pressure on SWB3 despite the
presence of venting and the failure of the model SWB3 and FS
panels. [Exhibits 20O, P]
- The NTSB investigators had to use excessively large spark
energy in their 1/4-scale testing.
- False. A spark was never used to ignite the
1/4-scale tests and ignition energy studies were not a part of the
1/4-scale testing program. Extensive results are available on ignition
energy in separate laboratory tests [Exhibits 20T, L]. The ignition system
used in the 1/4-scale was actually a hot filament ignition system and
no sparks were created. [Exhibit
- The NTSB investigators had to use a torch in order to start
combustion in Jet A laboratory experiments.
- False. Jet A vapor-air
mixtures were ignited by many different ignition sources in the Caltech
experiments. Experiments were conducted with spark ignition sources
[Exhibits 20T and L] with energies ranging between 1 mJ and 100 J and the
1/4-scale experiments simply used a hot filament [Exhibit 20O]. The
preliminary experiments discussed in Exhibit 20D used a flame jet from a
1/4-inch diameter nozzle, comparable to some of the smaller openings in
the spanwise beams and spars within the tank. Other experiments with
pools of liquid creating vapor-air mixtures in the entire ullage with
spark ignition in both the laboratory testing [Exhibit 20L and T] and
hot filaments in the 1/4-scale experiments [Exhibit 20O, P] demonstrate
that the explosion of Jet A vapor mixtures in an ullage over a thin
layer of fuel is associated with a propagating flame (deflagration) in
the vapor-air mixture rather than a pool fire on the liquid layer. The
combustion time is sufficiently short so that the combustion is completed
before appreciable venting occurs. [see 1/4-scale experiment number 67 -
- Jet A has to be violently shaken and lofted as a mist in order
- False. Hundreds of experiments documented in
and the Caltech studies [Exhibit 20D, L, T, P] demonstrate that an
explosion can be produced in a quiescent mixture of Jet A vapor in air.
- The studies that Caltech carried out on Jet A explosions refute
the NTSB crash investigation conclusions.
- False. Caltech's studies
support the NTSB findings of probable cause.
[NTSB Final Report]
- The combustion of Jet A inside the CWT was not a "real"
- False. An explosion is a vague term used to describe an event
associated with rapid energy release (see the glossary). The type of
explosion considered in our studies is technically a deflagration or
propagating flame occurring in the premixed volume of Jet A vapor and
air within a multi-compartment vessel (the CWT). The deflagration
pressurized the CWT until the failure pressure of Spanwise
Beam 3 (SWB3) was reached. When
SWB3 fails a blast wave is created. The combination of this blast and
the rotation of SWB3 into the Front Spar (FS) cause the failure of the FS and
ultimately, the fractures in the fuselage that destroyed the hull.
- The peak explosion pressure was too small to rupture the CWT of
- False. Boeing estimated the failure pressure of the components of
the CWT. [Exhibit 20E and
Report on Observable Early Event Damages in
Exhibit 18] The peak explosion pressures were measured in tests,
including 1/4-scale experiments with Jet A, venting, and simulation of
structural failure of SWB3 and FS. [Exhibit 20E, O, P] The peak
explosion pressures significantly exceed the failure pressure of SWB3
and the FS.
- Lightning strikes frequently occur on aircraft without causing
explosions, proving that Jet A must never be explosive.
- False. Lightning strikes do occur on most
commercial aircraft on the average of about one per year. Airplanes are
designed to accommodate these through special design and engineering
safety features (explosion suppression systems located at the wing
tips on the vent lines, bonding of components) that prevent the stray
electrical energy from reaching the interior of the fuel tanks. These
features are not designed to prevent or suppress explosions that start
within the fuel tanks. [See the Fischer and Plummer reference in Exhibit 20T
- Why doesn't the FAA do something about fuel tank
flammability if this is a real problem?
- They are. The FAA has
Special Federal Aviation Regulation
(SFAR No. 88) to minimize the
potential for failures that could cause ignition sources in fuel tanks
on new and existing airplanes.
They have issued (as of 2002) over 40 Airworthiness Directives (AD) related to issues
derived from TWA 800 and related fuel tank hazards. The FAA initiated a
research program on
fuel tank protection and
are carrying out research and proposing regulations for
fuel tank inerting,
which is being carried out with
industry participation. See also
Advisory Circular AC 25.981-2 Fuel Tank Flammability Minimization.
The FAA announced on Feb 17, 2004 that it is considering issuing a
Notice of Proposed Rulemaking (NPR) requiring
a fuel tank inerting system to be installed on existing aircraft with
center wing tank flammability hazards. The FAA issued the proposed rule in November 2005, the final rule in July 2008, and the NTSB closed the issue
in October 2008.
- Why didn't the industry identify the problem earlier?
- The industry
was not required to consider this as part of the certification process.
SFAR No 88 and
Advisory Circular AC 25.981-1B FUEL TANK IGNITION SOURCE PREVENTION GUIDELINES
now require that "The design approval holder would
be expected to develop a failure modes and effects analysis (FMEA) for
all components in the fuel tank system. Analysis of the FMEA would then
be used to determine whether single failures, alone or in combination
with foreseeable latent failures, could cause an ignition source to
exist in a fuel tank. A subsequent quantitative fault tree analysis
should then be developed to determine whether combinations of failures
expected to occur in the life of the affected fleet could cause an
ignition source to exist in a fuel system."
- If this is such a problem, why aren't airplanes falling out of
the sky all the time due to fuel tank explosions?
- You wouldn't expect them to. The frequency of
commercial aviation fuel tank explosions worldwide is estimated by the
industry to be about one every 5 years without any preventative
measures (see the FTHWG
ARAC report for a complete discussion). Given that major airplane
accidents are relatively rare events, the observed and predicted
frequencies of major accidents due to fuel tank explosions are
consistent. (PAL 1991, TWA 1996, Thai Air 2001) In the last two
decades, there has been an average of about 1.5 major accidents per
year in the USA (due to all causes) with relatively large fluctuations
from year to year. Of these, one (TWA 800) has been a fuel tank
Accidents have been reduced to this very low rate
in commercial aviation through careful attention to design,
engineering safety features, and redundant systems. This applies to
fuel tank explosions just as it applies to any other mode of
failure. The fact that a failure mode or safety hazard is identified
by an accident investigation does not mean that it has a high
probability of occurring. On the other hand, the fact that a system
has engineering safety features does not mean that accidents can never
occur. Accidents in complex engineering systems like aircraft are
frequently due to a combination of failures that were never considered by
the original designers.
In the interest of safety, the NTSB does investigate every accident in
commercial (and general) aviation and attempts to find a probable cause.
The purpose of this is not to fix blame but to improve safety.
Identification of a potential safety hazard requires that the
investigative and regulatory agencies respond with proposals to reduce
or eliminate the problem. The burden of evaluating the solutions and
carrying out a cost-benefit study (required by federal statute) falls to
the FAA - who relies almost exclusively on the industry to make the
evaluation of technical feasibility and economic viability of any
- Cars contain fuel pumps and wiring inside the fuel tanks - why
don't they blow up more often?
- Gasoline tank vapor spaces are almost
never flammable while Jet A tanks in airplanes will always pass through
a flammable regime during normal flight operations.
The ullage of gasoline fuel tanks in automobiles is almost always too
rich to be flammable except at very low temperatures. This is due to the
much lower flash point (about -40 C) of gasoline in comparison to Jet A.
The vapor space in a partially-filled gasoline tank does not become
flammable until the temperature has dropped below about 10F and a
serious hazard will exist below 0 F down to about -40 F for a typical
gasoline (Reid Vapor Pressure (RVP) of 9.5 psi, flammability limits between 1.4 and 7 % by
volume). [See W.F. Marshall and G. A. Schoonveld, SAE Transactions, Vol. 99,
No. 4, 594-617, 1990]
For this reason, the probability that a fuel tank containing liquid
gasoline has a flammable vapor space is extremely small in most climates
except in the artic regions. The exception to this is when the tank is
removed for servicing and the fuel is drained from the tank. Since
gasoline has such a high vapor pressure, the tank can have a flammable
vapor space even if there is no liquid fuel visible inside that tank.
For example, the complete vaporization of about 1-2 tablespoons of gasoline
will result in a flammable mixture inside a 15-gallon capacity
automobile tank! This is the reason why welding on or near "empty" gas
tanks is extremely hazardous and thorough purging of the tank with
steam, carbon dioxide, nitrogen, or other inert gas is required before
repair work is started.
The in-tank fuel pumps in modern fuel-injected vehicles are designed to
run submerged in fuel, as are the fuel pumps in aircraft, and the fuel
circulates through the pump, including the sparking brush-commutator
system. Unless the gasoline tank is almost completely emptied of fuel,
this will not pose a hazard since there will be only fuel and not a
fuel-air mixture within the pump. So in order for an in-tank fuel pump
to pose an ignition hazard, the tank must be "run dry" under very cold
conditions so that flammable vapor surrounds the commutator.
Fire and explosions occasionally (about 3 accidents out of 1000) do happen
when gasoline-powered vehicles crash. Post-crash fires are a serious
safety issue for automobiles and are the subject of ongoing study, legislation,
and litigation. Fires occur after the gas tank is ruptured and the
accident results in an ignition source, often arcing electrical wiring
or exposed hot lamp filaments. It is actually much harder to ignite
gasoline by spilling it on a moderately hot surface that it is to ignite
Jet A so that tailpipes and exhaust manifolds are not good ignition
sources for gasoline. On the other hand, gasoline makes a large vapor
cloud very quickly and if a high-temperature ignition source is
introduced, a very rapid or "flash fire" will be the result. This can
serve as an ignition source for the puddle or pool of fuel under the
leaking tank, causing a pool fire that may destroy the vehicle.
- If a fuel tank temperature is above the flash point, then an
explosion will always occur
- False A potential
explosion hazard exists when a liquid fuel is above its flash point but an
explosion occurs only if an appropriate ignition source is present in
the flammable portion of the fuel vapor-air mixture within the tank.
Some possible ignition sources include a hot surface, an electrical
arc or spark, static electricity discharge, open flame, or burning
metal fragments (e.g. from grinding or incendiary devices). Unless
the surface temperatures of a fuel tank containing kerosene exceed the
minimum AIT of 190 C, ignition will not occur due to the elevated
temperature alone. The term "flash" in flash point temperature refers to the
measurement technique, in which an open flame is introduced into the vapor space above
a warm liquid fuel. The flash point is the minimum liquid temperature for which a "flash" is
observed by the operator when the flame is inserted in the vapor space.
In general, in order for a explosion to take place, three elements are
required: fuel, oxidizer, and an ignition source. This is usually
refered to as the "fire triangle" by fire and explosion investigators.
In addition, two conditions are needed before an explosion actually
occurs: 1) The fuel and oxidizer must be molecularly mixed in the
correction (flammable) proportions, 2) An ignition source of the appropriate size and
duration must be present in the flammable portion of the mixture.