Intrinsic Safety for Aircraft Fuel Tanks

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.

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.

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

Spontaneous ignition of flammable vapors can also occur due to heat transfer from a hot surface to fuel molecules. A standard test method has been developed to measure the autoignition temperature of a liquid fuel by dropping a small amount of fuel onto a flat, heated surface and noting the temperature at which a flame is observed. It has been accepted that the autoignition temperature of jet fuel is around 450 ̊F and Avgas is around 536 ̊F , although these are not exact figures. Many factors can affect the ignition of the fuel vapors and the propagation of a flame front from the hot spot. The design of the test apparatus will determine the type of combustion that will occur. Cool flames can develop and propagate through a flammable mixture without creating an explosion as long as the rate of heat generated is not much greater than the rate of heat lost; explosions can only occur if significantly more heat is generated than lost.

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. 

CIES Inc FQIS SENDERS - 
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.

Made in the USA

One of the most clear moments of my youth was delivering newspapers - the Tribune in Chicago .   I remember one day carefully dropping the paper on the front porch at a home where the older couple were particular about their delivery.

They asked me to come in to their home and see men walking on the moon.

We were all proud of what we accomplished that day, a well delivered paper, lemonade, cookies and manned space flight.

My dad had gone to Grumman in Bethpage, NY a few weeks before and brought back a Lunar Module Model I was absorbed by the accomplishments around me.

I was immersed in aviation - I grew up in it, and I had aeroshell grease on my hands.   If I was good, I got to go flying.  My recollections of youth and aircraft pretty much trump anybody else I talk to.  From messing around in the DeHavilland Comet that sat idle for so long at O'Hare,  to a ride in the space behind the pilot in a Grumman Bearcat for teasers.

These formative experiences led me to pursue a life of working in and around aircraft.  This is (fuel level sensor) is the best accomplishment and neatest engineering work of my life in aviation...so far.

The fuel level sender program is a culmination of a career of aircraft experience, good fortune and a little of the space program all wrapped up into a convenient aviation system package.

I am waiting patiently (I am not capable)  for the time when what we have accomplished here at CiES will reach the light of day or more specifically the aviation press.

I am very proud of the group that created this a design.  From it's initiation by a former space program engineer, it's re-vision and re-creation by our customer and internal team, it's integration by the interface team that allowed display in a modern cockpit, and it's thorough evaluation and certification that allowed CIES to provide a better level of reliability and accuracy to aviation fuel level sending.  Something that may,  improve the safety of flight for general aviation aircraft.

We really feel at CIES that we are on the edge of delivering something new in aviation in an area that has really waited for a better technology to appear.

All of this effort to design, evaluate, test and manufacture can be stamped with a Made in America. (we had some Canadian assistance)

We are most proud that we initiated the American Dream here in Oregon.

We built a new company,  we put people to work,  we build a world class product.