Clean, dry aviation fuel is a basic prerequisite for aviation safety. Only the most highly specified products, manufactured to the highest quality by specialists in the field, provide the necessary assurances. Contaminants that can be typically found in aviation fuels include free water, particulates, microbiological growth, and surfactants. In addition, distribution systems can be contaminated by chemicals. These contaminants can be introduced at any stage of the distribution system and can have a negative impact on fuel management, which in turn affects aircraft engines and control systems on board aircraft.
Provided that the delivery of fuel is restricted to a single specification, aircraft refueling company or airport storage and distribution system, proper and effective use of filtration and sensor technology significantly reduces the risk of contamination.
The Energy Institute (EI) is a registered professional association that brings together global energy know-how and is committed to providing and maintaining uniform safety standards. EI and JIG jointly issue JIG standard 1530 "Quality assurance requirements for the manufacture, storage and distribution of aviation fuels to airports". It covers all aspects of fuel quality prior to delivery to an airport and includes both mandatory requirements and best practice guidelines. While this standard has been prepared under the European legal and regulatory framework, it is noted that its provisions may be applied in other countries, provided that national and local legal requirements are met. Cleanliness specifications are cited in EI 1550 "Handbook on equipment used for the maintenance and delivery of clean aviation fuel". This standard provides an overview of the technical requirements from the series of EI aviation fuel filter specifications, which is specifically intended for filter users and goes on to define good practice in the implementation and operation of filtration systems.
The Joint Inspection Group (JIG), an association of the world's major oil companies, publishes a global standard and regulatory framework for monitoring and maintaining aviation fuel quality. Standards undergo a quality assurance review process at regular intervals which validates their application. JIG 1 covers operating standards for refueling services in airplanes whilst JIG 2 covers operating standards for airport depots. Both are endorsed by IATA, who are an associated member of the group alongside other industry stakeholder (non-mineral oil companies). The JIG 1 and JIG 2 standards are supplemented by EI 1530, which covers operating standards prior to delivery to airports. Recently, an additional standard, JIG 4, was issued, covering operating standards at smaller airports.
A4A (Airlines for America) publishes the ATA Specification 103 - Standard for Jet Fuel Quality Control at Airports. A4A endorses the development of key policies and courses of action on behalf of its members to promote high safety standards in the US aviation industry.
Water may be absorbed into aviation fuel at any point during its storage, transportation and delivery. Water present in aviation fuel exists in two states: free water and dissolved water (water solubility). Free water may be introduced by impurities from an external source during a refueling operation, rainwater that has passed seals, or venting. Dissolved water indicates the saturated water content of aviation fuel at a given temperature, pressure or humidity. Any surfactants present in the fuel reduce the water’s ability to settle in the sump and so to be drained off. Due to the low temperatures at high altitudes, ice crystals can form and block fuel lines.
Water present in aviation fuel enables the growth of microorganisms or bacteria that feed on the hydrocarbons in the fuel, thereby compromising its quality.
The main sources of particulate contamination are rust, metal and sand. Rust can be introduced through pipelines, storage tanks, tankers and drum containers. Other sources include equipment components such as damaged hoses or filters and catalysts from manufacturing processes. Dust and sand can be introduced through openings in tanks and by using unclean tank equipment. Particulate filtration must therefore be as refined as possible to prevent filters becoming blocked during operation. Blocked filters cause the aircraft engines and fuel components to be bypassed which may lead to them failing.
The main source of this type of contamination is free water, as it is at the interface between fuel and water that microbiological activity takes place. The resulting slime corrodes and impacts aircraft fuel systems, meaning it is important to keep fuel systems as dry as possible by frequently draining the accumulated water. Microbiological contamination can cause severe equipment damage and blockages in fuel filters and fuel lines. In cases of proven microbial contamination, considerable downtime is often required for post-treatment. The basic method for assessing the presence of microbiological growth in storage tanks and filters is the daily clear and light test of a sump sample. In addition, there are additives available which prevent the growth of fungi and other microbes.
A surfactant is a substance that leads to a significant reduction in the interfacial tension between liquids. Surfactants in fuel therefore cause the fuel and any water to mix more readily, making separation much more difficult. Surfactants disperse both water and dirt in the fuel and sometimes form very stable emulsions or slimes.
Surfactants can be present in crude oil as naturally occurring substances, or as a residual product from refinement processes. These materials collect and concentrate in the coalescer elements of the filter/water separator, thereby compromising their ability to combine and remove water from the aviation fuel. It is known that a surfactant concentration of less than 1 ppm in aviation fuel can cause the coalescer elements to malfunction.
Sampling probes of aviation fuels
Aviation fuels have a lower specific density than water. For this reason, water settles at the bottom of the filter vessel in the water sump. By sampling fuel at all low fuel runs, water can be removed from the system. Regular fuel sampling can help reduce problems with microbial growth and water freezing in the system, as well as with the detection of particulate contamination.
The fuel is sampled by draining aviation fuel into a clean bucket until it is half full. If you then hold the bucket against the light, you can see if water or particulate contamination is present in the fuel. Contaminants can be isolated by swirling the sample to create a tornado-shaped vortex in the container.
Fuel samples should be taken until you have a clear, clean sample. Never take a fuel sample immediately after refueling an aircraft. A good time to take a fuel sample is before the first flight of the day.
Housekeeping and maintenance
Although the complete elimination of fuel contamination is not a realistic goal, it can be controlled by applying good housekeeping and maintenance practices. Storage and dispensing equipment should always be kept clean. They should be free of dirt and other foreign substances. Avoid refueling from barrels or other containers. Filter vessels like these pose a high risk of introducing water and particulates into the fuel.