Hormoconis resinae in Aviation Kerosene
Micro organisms can grow in certain fuels by using the alkanes in the fuel as a foodstuff. In some cases, they may be able to utilise some of the additives in the fuel. Mid to light distillate fuels are particularly susceptible to contamination by micro-organisms, although more recently there are increasing reports of gasoline being affected, particularly some of the more "environmentally friendly" products.
The type of organisms and the damage inflicted depend upon on the fuel and the additives. All contamination is important when considering the quality of a fuel, particularly when monitoring stored products and reserves. However, whilst a wide range of micro-organisms can be found in fuels in aircraft tanks and, if left unchecked, can cause damage to the tanks, the most serious organism is the filamentous fungus H. res.. This is for a number of reasons:
Firstly, its size and bulk. When compared to single cell yeasts and moulds, H. res. produces far more biomass and is thus more likely to cause blockage problems.
Secondly, it is by far the most common cause of microbial corrosion in aircraft tanks. Other organisms are more important in other circumstances, for example, in some ship fuels and in long-term storage. Other important corrosive organisms are the anaerobic bacteria, collectively known as Sulphate Reducing Bacteria (SRB) or, more accurately, Sulphide Generating Bacteria (SGB). These are not common in aircraft wing tanks because of the high level of aeration produced during flight and refuelling. Other filamentous fungi may be emerging as important, but these tend not to occur without H. res. being present and, in any case, are not currently very common.
Thirdly, because of the way H. res. grows between fuel and water, it usually starts on small water droplets. It then covers the droplet, holding it in place, and continues its growth, actually generating more water under the mat due to its metabolism. In the process, it firmly attaches itself to the tank. Bacteria and yeasts require free water and are found, mainly, floating in the water phase. This means that they are less likely to adhere to surfaces and will, therefore, be significantly reduced at each water drain. H. res., once established, continues to multiply in situ.
In aircraft, high levels of bacteria and yeasts tend to indicate that you have picked up poor quality fuel and, as such, are useful indicators. However, this does not necessarily mean that they are causing any problem in the tank, and they will probably be significantly reduced at the next drain. High levels of H. res., however, indicate that there is, potentially, a serious problem.
Currently available fuel tests involving total microbial counts simply tell us that there are micro organisms present in the fuel and that they are alive. Conidia Bioscience's new Fuelstat® resinae diagnostic test goes beyond this. Because it detects only active H. res., it tells us not only that the fungus is present and alive but also that it is growing and, therefore, the potential for damage is real.