Although particulate contamination is relevant to both jet fuel and aviation gasoline, entrained water and microbial contamination are issues that are inherent to jet fuel.
Jet fuel's composition allows water to be easily absorbed and held in suspension. Water can be present as suspended particles in the fuel and in liquid form. The amount of suspended particles varies with the temperature of the fuel. Whenever the temperature of the fuel decreases, some of the water particles that are suspended in the fuel are drawn out of the solution and slowly accumulate at the bottom of the fuel cell. However, whenever the temperature of the fuel increases, it draws moisture from the atmosphere to maintain a saturated solution. Therefore, temperature changes result in a continuous accumulation of water.
Water can promote corrosion in fuel system components. If enough water is present, it can form ice crystals in low temperatures and clog fuel lines, filters, or components. This could disturb or even stop the fuel supply to the engine.
Some fuel systems employ heated fuel filters or fuel heaters to eliminate the problem of ice crystal accumulation. Others rely on anti-icing fuel additives.
If water is allowed to remain in fuel, it will culture micro-organisms or bacteria that feed on the hydrocarbons in the fuel, thereby degrading the fuel quality. These organisms are the next topic of contamination - microbial growth.
Certain bacteria and fungi are capable of existing in the water where it interfaces with the fuel. These microorganisms use alkanes and additives in fuel as foodstuff. These microbes can propagate rapidly. The by-product is a sludge-like substance. In sufficient quantity, this can cause corrosion on steel and aluminum surfaces and attack rubber fuel system components. It can also foul filters and system instrumentation.
The most destructive of the microbes that grows in the aircraft fuel environment is the fungus Hormoconis resinae. One reason is because of its size. Compared to single-cell yeasts and molds, it produces far more biomass. Secondly, it is the most common cause of microbial corrosion in aircraft fuel tanks.
Elimination of water from the fuel system is one way to control microbial growth. In addition, there are additives that can be added to the fuel such as Biobor®JF. These additives eliminate the growth of fungus and other microbes. Products like these are soluble in both fuel and water.
As far as microbe detection goes, Conidia Bioscience has introduced a new test kit called FUELSTAT™ resinae. It detects active H. resinae fungus in the fuel system and gives immediate results to the tester.
Cap those fuel lines
There are several practices that can be incorporated to help prevent fuel contamination. First of all, all open fuel system lines should be capped or otherwise protected during maintenance operations. This can prevent particulates from entering the system.
One story on fuel system contamination sticks in my mind. It is that of an IA who found a dried-out, cracking fuel line during an annual inspection. He started the fuel line replacement on a Friday, stopped for the weekend, then finished the installation on Monday and approved the aircraft for return to service. The pilot of the aircraft, his daughter, and two guests were killed when the engine quit after takeoff. In their investigation, the NTSB found a bee in the recently replaced fuel line. The mechanic had left it uncapped over the weekend.
Almost anything can cause particulate contamination from rags and bugs to deterioration of fuel system components like corrosion of metal parts or deterioration of rubber fuel cells and lines. Rust can be introduced through pipelines, storage tanks, fuel trucks, and drum containers. Dust and sand can be introduced through openings in tanks and from the use of fueling equipment that is not clean.
Fuel system screens and filters help collect particulates. These should be inspected and cleaned on a regular basis. Regular inspection ensures that any excessive particulate presence is investigated to the source of the contamination. Regular cleaning ensures that the filter elements do not become clogged. Two possibilities exist with clogged fuel filters. In filters with a bypass system, once the filter is clogged enough to cause the differential pressure to activate the spring mechanism, the fuel will no longer be filtered, but will instead bypass the filter altogether. This can cause failure of components downline. In non-bypass filters, the differential pressure that is built up could rupture the filter element and possibly generate even more particulate contamination.
Fuel cell maintenance
Any time a fuel cell is opened up, there is a large area for debris to be introduced into the system. Extra care should be taken when working on fuel cells.
Before closing a fuel cell access panel, check throughly for any foreign objects. It is a good idea to have another person take a look as well - the old four eyes are better than two. In fact, many facilities have a mandatory call point in their procedures prior to closing a fuel cell.
Be especially aware of paper towels and rags. If left in the fuel cell, they have the capacity to easily clog fuel lines and cause subsequent fuel starvation.
Regular fuel sampling can help reduce problems with microbial growth and freezing associated with water in the system and can also help identify particulate contamination.
Jet fuel has a lower specific gravity than water. Because of this, water will tend to settle at the bottom of tanks. Sampling fuel at all the low point fuel drains can help remove water from the system.
The actual process of fuel sampling is quite easy. Fuel is drained into a clear container filling it half way to two-thirds full. Holding it up to the light, one can see any water or particulate contamination in the bottom. Swirling the sample around to create a tornado-shaped vortex in the container can also help isolate any contaminates. Any water or particulates will accumulate at the bottom of this vortex.
An easy way to tell if water is present is to add a few drops of food coloring to the sample. The food coloring will not mix with the fuel but will mix with water. If water is present, the coloring will mix with it. If no water is present, the dye will just settle in the bottom of the container. This is a good test to ensure that the whole jar is not just full of water.
Fuel samples should be taken until you have a clear, clean sample. Never take a fuel sample immediately after an aircraft is fueled. The fueling action causes the water and particulates to become temporarily suspended in the fuel. A good time to take a fuel sample is prior to the first flight of the day.
Top off tanks
Another good practice is to top off aircraft tanks at the end of each flying day. As mentioned earlier, jet fuel has a tendency to absorb moisture from the atmosphere. With less air in the fuel cells, the rate of absorption will be significantly lower.
Total elimination of fuel contaminants may not be a realistic goal, but they can be controlled by the use of good housekeeping habits.
Storage and dispensing equipment should be kept clean at all times. They should be free from dirt and other foreign matter.
Avoid refueling from drums or cans. Containers like this can pose a high risk of introducing water and particulates.
Know your fuel supplier
It is good to know your fuel supplier. Become familiar with the procedures that they implement to control contamination. Do they perform regular fuel samples? Are quality control procedures in place? It is good to have a good line of communication with them in order to address any contamination problems that are encountered during sampling. This can help to quickly discover the root cause of any problem.
Being aware of all the factors that can lead to fuel contamination is important. This can help ensure that the proper procedures are in place to help prevent contaminated fuel from causing problems to your aircraft.
Advisory Circular 20-43C
Aircraft Fuel Control
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