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Microbes in the form of bacteria, yeasts and fungi can grow when water finds its way into fuel tanks. The heavier water sinks to the bottom of the tank, and the zone where it meets the fuel provides an ideal environment for microbes, according to Rob Midgley, Global Technical and Quality Manager for Shell Aviation.

鈥淭hese organisms require three things to be able to survive. One is a water phase in which to live. They tend not to live in the fuel itself but in the water. Secondly, these organisms would need a food source to survive 鈥� in the form of fuel. And they also need access to oxygen. If you have those three things in combination then we have the conditions in which we can start to see microbial growth,鈥� Midgley said on a recent Shell Aviation webinar devoted to the issue and its solutions.

The wide variety of environments and microbes means every infestation is different and can cause a wide range of problems, Midgley said. Bacterial films can interfere with sensors. Microbial mats can clog filters and pumps. Microbial growth can extract the plasticiser contained in seals, making them less flexible and leading to leaks. Fungi can spread filaments under the epoxy layer that lines the bottom of some fuel tanks, breaking it apart and creating debris that can block fuel filters.

鈥淎ll of these (microbes) tend to form by-products of metabolism that are generally acidic. So that water, if you tested it, would be quite acidic. It鈥檚 quite common for the water to be somewhere in the pH of about 3-5.5. Those organic acids are capable of attacking aluminium structures, and of course, that is what aircraft are made of,鈥� Midgley said, adding that other microbes can create sulfuric acid and sulphide ions capable of eating away at steel and copper.

Unlike fuel storage tanks, aircraft fuel tanks are designed principally to function as a wing and then as a fuel tank. The wing structure and design does not permit a single, simple sump but creates lots of undrainable water traps. When an aircraft is in regular operation, a system of 鈥渟cavenge rakes鈥� -- pipes in the fuel tanks that mix any water back in with the fuel, is designed to ensure that microbes can鈥檛 get a foothold.

Over the last couple of months, while over 80% of the world鈥檚 fleet is grounded³, conditions have been created where water may be accumulating in the tanks of many planes. As the approach of summer sends temperatures rising across the northern hemisphere, that is creating ideal conditions for microbes to grow.

鈥淲ith COVID-19, we are on the ground all the time. And so, the fuel system, the fuel, and the water get to ambient temperature, which in most parts of the world in this season is in that 20-35 degrees (Celsius) range. So, we see low utilisation as being a trigger for this increase in microbial growth,鈥� Midgley said.

鈥淎necdotally, we鈥檝e been seeing that in some fleets that have not been treated with biocide, somewhere around 50% or more of those aircraft are starting to show signs of microbial growth after two to three months of storage,鈥� Midgley said.

Airlines should test their aircraft for signs of microbial contamination in the fuel, Midgley said. That poses its own challenges. Microbe levels are highest at the interface between water and fuel and decline farther away from the water. Microbe populations can fluctuate over time. Sampling and testing equipment must be kept sterile to avoid
accidental contamination.

Most microbial contaminations can then be treated with one of two biocide chemicals. However, those pose their own unique challenges, such as not being compatible with certain engines or not having approval in all countries. In the most extreme cases, a plane may need to be defuelled, a complicated process that not all airports can handle,
Midgely said.

鈥淯ltimately if we get to the point where we need to defuel 鈥� especially if it鈥檚 for disposal because it鈥檚 been contaminated, then we are going to need some warning if we are to have the potential to help with that because disposal of contaminated fuel is not something we routinely do at airports. It may not even be possible,鈥� Midgley said.

鈥淵ou really need to have a strategy to treat the aircraft that have a high level of contamination and how are you going to do that. It鈥檚 the availability of an injection cart, availability of the additive, and also simple things like being able to access to aircraft that are parked nose to tail on taxiways.鈥�

Ask Shell Aviation

At Shell Aviation, we understand that while the industry is facing various challenges as a result of the pandemic, enabling and maintaining efficient and safe operations will be crucial for Aviation鈥檚 recovery.

With most of the world鈥檚 aircraft fleet grounded, our industry is presented with an unfamiliar challenge: the increased threat of microbial contamination. Shell advises to proactively monitor and test your fleet to manage and prevent the risk of microbial contamination. Airlines without a microbial prevention or removal strategy, are more exposed to develop microbial contamination over the next few months, which, unchecked, can lead to significant cost and operational issues.

If you are concerned about microbe contamination of your aircraft and would like advice from Shell Aviation, please contact us using the contact form. Details of situation and location will dictate the level of support that Shell Aviation may be able to provide. Once your request is received, we will reply as soon as possible. To make it easier for us to assist you, please kindly provide as much information as possible when completing this form.