INTRODUCTION
A flammable mixture of fuel vapor and air can exist at times in a partially filled aircraft fuel tank containing jet fuel or much less so Avgas. Research has been done to develop methods to eliminate or reduce the risk of having an explosive condition in the fuel tank. There are a few different approaches to preventing fuel tank explosions. Explosions need three conditions to occur simultaneously: a flammable fuel source, sufficient oxygen to react with fuel molecules, and an ignition source to start the chemical chain reactions. Eliminating any one of these conditions will prevent a fuel tank explosion.
REDUCING OXYGEN CONCENTRATION
Recently, attention has been focused on developing a low-cost, low weight, high-efficiency fuel
tank inerting system for use in large transport airplanes. This system uses high temperature
bleed air from the engines to create nitrogen-enriched air (NEA) with as high as 98% nitrogen
concentration. The NEA is plumbed into the ullage space above the liquid fuel in the fuel tank,
forcing air out the vents and creating an atmosphere with a maximum oxygen concentration of
12%. This value has been shown to be the lowest oxygen concentration that will support ignition
of fuel vapors. This approach eliminates one of the key ingredients required to have a fuel
tank explosion (sufficient oxygen).
So now we have added a component to the aircraft to address what can't be addressed with a capacitive system in the fuel tank.
So now we have added a component to the aircraft to address what can't be addressed with a capacitive system in the fuel tank.
REDUCING IGNITION PROBABILITY
Ignition of fuel vapors can occur as a result of several different mechanisms. Voltage sparks, thermal sparks, and hot surfaces are the most probable ignition sources present in or around a fuel tank. Any of these ignition sources could occur due to lightning strikes, electrical faults in fuel tank electronics, or short circuits caused by cleaning debris, such as steel wool or other small conductive filaments that may have been inadvertently left within a fuel tank. Combined with fuel tank inerting, reduction or elimination of the likelihood of ignition sources could provide an additional safety factor to preclude virtually any fuel tank mishaps during the life of an aircraft.
Electrical spark has been the standard method of determining ignition energy required to ignite a flammable mixture. The generally accepted minimum ignition energy for a hydrocarbon/air mixture is around 200 micro Joules (μJ) for a specific mixture of fuel and air, usually at a stoichiometric mixture or slightly richer. The 200-μJ energy in most experiments is the energy stored in a capacitor and discharged across an electrode gap as a voltage spark. It should be noted that the stored capacitor energy is not the exact amount of energy deposited into the spark, as there are always losses between the capacitor and the electrodes. Nevertheless, the capacitor energy is a very good approximation of the minimum ignition energy of a mixture and the relative ignition strength of a voltage spark.
Ignition of fuel vapors can occur as a result of several different mechanisms. Voltage sparks, thermal sparks, and hot surfaces are the most probable ignition sources present in or around a fuel tank. Any of these ignition sources could occur due to lightning strikes, electrical faults in fuel tank electronics, or short circuits caused by cleaning debris, such as steel wool or other small conductive filaments that may have been inadvertently left within a fuel tank. Combined with fuel tank inerting, reduction or elimination of the likelihood of ignition sources could provide an additional safety factor to preclude virtually any fuel tank mishaps during the life of an aircraft.
Electrical spark has been the standard method of determining ignition energy required to ignite a flammable mixture. The generally accepted minimum ignition energy for a hydrocarbon/air mixture is around 200 micro Joules (μJ) for a specific mixture of fuel and air, usually at a stoichiometric mixture or slightly richer. The 200-μJ energy in most experiments is the energy stored in a capacitor and discharged across an electrode gap as a voltage spark. It should be noted that the stored capacitor energy is not the exact amount of energy deposited into the spark, as there are always losses between the capacitor and the electrodes. Nevertheless, the capacitor energy is a very good approximation of the minimum ignition energy of a mixture and the relative ignition strength of a voltage spark.
Flammable mixtures can also be ignited by means of thermal or friction sparks. Thermal sparks are different from voltage sparks; they are very small burning particles of metal that radiate bright colors due to high temperature burning. Thermal sparks are produced either by two hard surfaces sliding against each other creating a shower of sparks or a wire or filament making or
Currently, the Federal Aviation Administration (FAA), guidance for electrical systems that
introduce electrical energy into fuel tanks, such as fuel quantity indication systems (FQIS),
provided in draft Advisory Circular (AC) 25.981-1C, states a maximum steady-state current of
10 milliamps (mA) root mean square (rms) is considered an intrinsically safe design limit for
FQIS. It also states that current levels above 10 mA rms, particularly for failures and transient
conditions, could also be considered acceptable, provided that proper substantiation by test
and/or analysis justifies them as intrinsically safe. As an example, the AC states that for
transient conditions, it is acceptable to limit the transient current to 150 mA rms, and failures that
result in steady-state currents above 10 mA rms should be improbable and not result in steady-
state currents greater than 30 mA rms. These values were determined after a considerable factor
of safety was applied to the lowest values found from previous tests using Jet A vapors and steel
wool filaments as the ignition source. The experimentation presented in this work was
performed using a calibrated gas mixture with a predetermined minimum ignition energy to
solidify the confidence in the electrical current guidance in draft AC 25.981-1C.
Our senders do not have any electrical components in the tank, and no generated heat energy to ignite fuel in the tank. The system measures the position of a magnetic pair on a float arm located inside the tank from a location outside the fuel tank proper. This method of sensing is Anisotropic Magneto Resistive technology and is exclusive to CiES Inc.
The CIES Senders eliminate the in tank ignition sources that could occur due to lightning strikes or electrical faults in fuel tank electronics. As the sensor does not rely on the fuel interface for measurement, corrosion removal is not an issue. Cleaning materials like steel wool that are used to clean capacitive sensors are not required with the CiES sender design.
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