Fuel Tank Flammability
Intrinsically Safe Current Limit Study for Aircraft
Fuel Tank Electronics
This technical note describes research performed to determine the ignition hazard presented by small fragments of superfine steel wool that contact energized direct current wires in aircraft fuel tanks. Several different methods of shorting a circuit with steel wool were explored. An ignitable mixture of hydrogen, oxygen, and argon, calibrated to have a minimum ignition energy of 200 micro Joules, was used as an ignition detection technique. The electrical currents at the ignition threshold were recorded to determine safe maximum allowable current limits for fuel tank electronics. The lowest current found to ignite the flammable mixture was 99 milliamps (mA); the lowest current found to ignite a steel wool wad in air only was 45 mA.
Download the Technical Note (DOT/FAA/AR-TN05/37)
Evaluation of Fuel Tank Flammability and the FAA
Inerting System on the NASA 747 SCA
Extensive development and analysis has illustrated that fuel tank inerting could, potentially, be cost-effective if air separation modules, based on hollow-fiber membrane technology, could be packaged and used in an efficient way. To illustrate this, the Federal Aviation Administration (FAA) has developed a prototype onboard inert gas generation system that uses aircraft bleed air to generate nitrogen-enriched air (NEA) at varying flows and purities during a commercial airplane flight cycle. A series of ground and flight tests were performed, in conjunction with National Aeronautics and Space Administration (NASA) aircraft operations personnel, designed to evaluate the FAA inerting system used in conjunction with a compartmentalized center wing tank (CWT). Additionally, the flammability of both the CWT and one inboard wing fuel tank was measured. The system was mounted on a Boeing 747, operated by NASA, and used to inert the aircraft CWT during testing. The inerting system, CWT, and the number 2 main wing tanks were instrumented to analyze the system performance, fuel tank inerting, and flammability.
The results of the testing indicated that the FAA prototype
inerting system operated as expected. Using a variable-flow
methodology allowed a greater amount of NEA to be generated
on descent when compared to the simple dual-flow methodology,
but it had no measurable effect on the resulting average
ullage oxygen concentration after each test, while improving
inert gas distribution by decreasing the worst bay oxygen
concentration when three similar tests were compared.
The highest average ullage oxygen concentration observed
on any flight test correlates directly with the worst
bay oxygen concentration, illustrating the importance
of maintaining a low average ullage oxygen concentration
in good inert gas distribution. Oxygen diffusion between
the bays of the tank was relatively rapid, and overnight
dispersion of the ullage oxygen concentration was measured
to be very small. Flammability measurements showed trends
very similar to what was expected based on both experimental
and computer model data. The equilibrium data agreed
favorably with data from both the Fuel Air Ratio Calculator
and the Condensation Model, while transient data trends
matched closely with the Condensation Model with some
discrepancies in total hydrocarbon concentration magnitude
Download the Final Report (DOT/FAA/AR-04/41)
Limiting Oxygen Concentration Required to Inert
Jet Fuel Vapors at Reduced Fuel Tank Pressures
A simulated aircraft fuel tank containing JP-8 fuel of an amount equivalent to a mass loading of approximately 4.5 kg/m3 was used to determine the limiting oxygen concentration (LOC) at pressures corresponding to altitudes ranging from 0 to 38 kft. In addition, the peak pressure rise was measured at various altitudes (pressures) due to ignition occurring at O2 levels approximately 1% to 1.5% above the LOC.
A wide range of ignition sources were used throughout the testing. An oil burner transformer connected to an analog timer provided a low power arc of both short (0.1 second) and long durations (1 second), a spark igniter taken from a J-57 engine provided a very short duration (175 mseconds) high powered spark, and a heated metal block was used as a hot surface ignition source. These varied capabilities allowed for an evaluation of the variation in the LOC due to a specific type of ignition source.
From these tests, it was determined that the LOC at
sea level through 10 kft is approximately 12% O2, while
exhibiting a linear increase from 12% at 10 kft to approximately
14.5% at 40 kft. Tests with various sparks/arcs as ignition
sources at sea level showed little variation in results,
with the LOC ranging from 12.0% to 12.8%. Also, a heated
surface capable of igniting a fuel air mixture proved
insufficient for ignition in a tank inerted to just
14%. Peak pressures resulting from ignition at oxygen
concentrations 1% to 1.5% above LOC values decreased
as the altitude was increased to 30 kft, while the duration
to reach the peak pressure increased.
Download the Final Report (DOT/FAA/AR-04/8)
Mass Loading Effects on Fuel Vapor Concentrations
Experiments were performed within a simulated fuel tank approximately 1/20 the size of a typical B-747 center wing fuel tank (CWT). The vapors generated within the ullage of this tank were analyzed under different mass loadings in an effort to determine the effects of the mass loading and fuel distribution. It was determined from these tests that in order to have a substantial effect on the flammability of the vapor within the CWT, the mass loading would have to be somewhere between 0.08 and 0.15 kg/m3. A Substantial effect was defined as a minimum 20% decrease in the maximum hydrocarbon count when compared to the average of all tests conducted with larger mass loadings. In addition, it was found that while the distribution of the fuel has no effect on the peak flammability (vapor composition) that is reached, it does have a significant effect on how long it takes to reach the final state. The less dispersed the liquid fuel is, the longer it will take the vapor to reach its maximum flammability point.
Download the Technical Note (DOT/FAA/AR-TN99/65)
Cold Ambient Temperature Effects on Heated Fuel
Tank Vapor Concentrations
Experiments were conducted within a simulated aircraft center wing fuel tank (CWT) to qualitatively analyze the effects of decreased ambient temperatures, such as might occur at increased altitudes, on the vapor concentrations found in a typical CWT ullage. A small quantity of fuel in the CWT test article was heated to 125°F for two hours, corresponding to a temperature approximately 10°F above the flashpoint of the fuel. The tests were conducted at sea level, however, the wall temperature of the tank was cooled to a temperature corresponding to a pre-determined altitude. From these tests, it was determined that the ambient temperature does indeed have a significant effect on the vapor concentrations formed in the fuel tank ullage at small fuel mass loadings. As the ambient temperature is decreased, the rate of decrease in the fuel-air ratio increases.
Download the Technical Note (DOT/FAA/AR-TN99-93)
A Review of the Flammability Hazard of Jet A Fuel
Vapor in Civil Transport Aircraft Fuel Tanks
A Fuel Flammability Task Group, made up of recognized fuel and combustion specialists, was formed to investigate the flammability and explosiveness of fuel within an aircraft fuel tank. The task group reviewed all available reports on the subject and met and discussed the data with technical experts from Boeing Commercial Airplane Co., California Institute of Technology, and the National Transportation Safety Board. A report was published by the task group which includes jet fuel definitions and specifications, jet fuel flammability data, influences of various factors on fuel flammability, and predictive analyses and models for flammability. The report discusses the impact of this knowledge on the needs for in-flight fuel fire prevention.
Download the Final Report (DOT/FAA/AR-98/26)
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Fax: (609) 485-5785