**7. Conclusions**

Microorganisms have been shown to be highly adaptable in their ability to use different food sources. It has been shown that several adaptations are needed for Pseudomonas aeruginosa to metabolize jet fuel [33]. When jet fuel is the available carbon source, the bacteria alter their metabolism through transcriptional regulation to favor the use of paraffinic hydrocarbons of the C11–C13 length as a food source [34]. These same transcriptional changes require biofilm

The introduction of ultralow sulfur and low sulfur fuels introduces new complexities in biofilm formation. It is known that sulfur compounds normally found in petroleum based fuels are natural lubricity improvers, antioxidants and antimicrobial agents [35]. Studies have shown that the removal of the sulfur compounds does not appear to alter the bio-corrosion properties of the fuel under anaerobic conditions [36], however microorganisms have also been shown to rapidly deplete the corrosion inhibitor/lubricity improver (CILI) additives

On approach to reduce microorganism growth in fuels is to incorporate biocides in the fuel formulation. There are very few biocides that have been approved for use in Jet A, but are not allowed in the military jet fuels JP-4, JP-5 and JP-8. In Jet A, the allowed biocides are Biobor™ and Kathon™. Kathon has a sulfur heterocyclic compound as the active ingredient [37] and

The aviation community uses huge amounts of fuel and requires a reliable source of high quality fuel. The uncertain markets for petroleum and the differences in fuel composition due to different crude oil feed stocks has led the industry to investigate alternate sources of fuel. In the United States, several alternative energy sources are possible, including natural gas, coal and biomass. Natural gas and coal are in abundant supply in the United States but are still non-renewable and have a large environmental footprint [39]. Biomass based sources, are potentially more environmentally friendly, although the life cycle analysis must certainly be taken into account [40, 41]. The United States military has estimated that at least 50% of its energy use would be from renewable sources by the year 2020 [42]. Several processes are under investigation, but considering current market conditions of low crude oil prices, pros-

Primary attention has been paid to processes which produce fuels that can serve as drop in replacements for petroleum based fuels requiring no modifications to the aircraft, and can be interchanged with conventional fuel depending upon local availability [44]. It is also important that the feedstock can be produced sustainably and are not either used in foods or raised on land that can also be used to raise food crops [45]. Two processes that are the most developed and can produce a synthetic fuel from a variety of feedstocks to provide an alternative fuel source are synthetic fuels through the Fisher-Tropsch process and hydroprocessed esters

formation for bacterial growth.

204 Flight Physics - Models, Techniques and Technologies

Biobor has a boron containing compound [38].

pects for widespread development are not promising [43].

**6. Alternative or synthetic fuels**

from the fuel [22].

and fatty acids fuels.

Civilian and military aviation requires an abundant, affordable, safe and storable source of fuel. The variable composition of fuel depending upon crude oil source and refining methods creates many challenges for fuel storage. Chemical deposit formation, which has been tied to oxidation reactions, followed by addition mechanisms is one source of fuel degradation. Other problems in fuel storage are related to the presence of water, both from contamination through leaks in the storage tanks and the general solubility of water in jet fuel can create icing problems and also provide a growth medium for microorganisms. Good water management can provide added safety in the fuel storage system. Finally, the development of renewable and synthetic fuels will change the problems of fuel storage, due to the more predictable composition of the fuel and the lack of heteroatom containing impurities associated with the fuels.
