Turbine Technology
Fuel System Contamination
Tips for identification and prevention of fuel contamination
By Joe Escobar
September 2004
An Air Wilmington FBO technician inspects the difference between dark fuel infected with microbial growth (left) and bright clean uncontaminated fuel on the right.
Contamination in fuel systems can mean trouble for an aircraft and engine. Damage caused by contaminated fuel can range from corrosion in the fuel cells and fuel system components, clogging of fuel filters, failure of instrumentation, or even blocking the fuel supply to the engine. A little awareness of the causes of fuel contamination and identification can help keep it from damaging your aircraft.
The three basic types of fuel contamination are water, particulate, and microbial growth.
Water contamination
Jet fuel's composition allows water to be easily absorbed. Water
can be introduced in various ways including through humidity in the
air. Water present in the fuel is either suspended or is present as
a liquid. The degree of suspension is affected in part by temperature.
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 low points in the system. Warmer temperatures promote
the absorption of moisture from the atmosphere and suspension in the
fuel. There is a constant cycle of absorption, suspension, and accumulation
at low points.
If water is allowed to remain in fuel, it will promote the growth of microorganisms 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.
Microbial growth
Certain bacteria and fungi thrive in water where it interfaces with
jet fuel. These microorganisms feed off alkanes and additives in
the fuel. The by-product of these organisms is a sludge-like substance
that can cause corrosion on steel and aluminum surfaces and attack
rubber fuel system components. It can also foul filters and system
instrumentation.
Eliminating water from the fuel system is one way to control microbial growth. In addition, there are products that can be added to the fuel. These additives eliminate the growth of fungus and other microbes.
Particulates
Particulate contamination can be introduced in many different ways.
From dirt and sand getting in open ports to degradation of fuel system
lines, particulates are constantly being introduced to fuel systems.
Engine fuel filters and screens help trap particulates before they can damage the engine. Regular inspection ensures that any excessive particulate presence is investigated to the source of the contamination. Cleaning the filters 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 damage the filter element and possibly generate even more particulate contamination.
Protecting open fuel lines
There are several practices that can be incorporated to help prevent
fuel contamination. At the forefront of preventing contamination
is covering all open lines during maintenance. All open fuel system
lines should be capped or otherwise protected during maintenance
operations to prevent particulates and moisture from entering the
system.
Any time a fuel cell is opened up, there is a big potential for debris to be introduced into the system. Before closing a fuel cell access panel, check thoroughly for any foreign objects.
Fuel sampling
Regular fuel sampling can help reduce problems with microbial growth
and freezing associated with water in the system. It can also help
identify if particulate contamination is present.
Because water has a higher specific gravity than jet fuel, it tends to settle at the bottom of tanks. Sampling fuel at all the low point fuel drains helps remove any accumulated fuel.
During sampling, fuel is drained into a clear container filling it half way to two-thirds full. By holding it up to the light, any water or particulates present can be seen. Swirling the sample around to create a tornado-shaped vortex can also be helpful to spot contaminants. Any water or particulates will accumulate at the bottom of this vortex.
One way to tell if water is present in a sample is to add a few drops of food coloring to it. The food coloring mixes with any water present but does not mix with the fuel. If no water is present, the dye will just settle in the bottom of the container.
When sampling, fuel should be drawn 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 keep the fuel tanks topped off. As mentioned
earlier, jet fuel has a tendency to absorb moisture from the atmosphere.
With less air in the fuel cells areas, the rate of absorption will
be significantly lowered.
Good housekeeping
Good housekeeping can reduce contamination of the fuel system. Fuel
storage and dispensing equipment should be kept clean at all times.
Being aware of the factors that can lead to fuel contamination as
well as identification of any contamination present is vital to the
safe operation of the aircraft. Through proper prevention, identification,
and removal of contamination, we can help ensure that contaminated
fuel does not impact the aircraft we are working on.
