Filters are used through the aviation fuel supply chain to ensure the health of the fuel reaching the aircraft engine - to make sure it is both clean and dry. In systems with microbial contamination, filters can be choke points for this contamination to gather, where it can cause premature failure. And contaminated filters can play a role in spreading microbial contamination throughout the system. People aren’t used to considering this, but it makes sense.
Take fuel-water separator units. Microbes need water to grow and thrive, and these FWS units are a ready-made source for that. If there’s water on the outside of the coalescer element, you’ll see the development of “leopard spotting” - formation of brown spots indicative of microbial growth. When this occurs, you’ve got two dangers. First, clean fuel passing through the filter will be inoculated with microbial growth as it passes through the coalescer. Second, the microbes can render the coalescer increasingly ineffective at removing water from fuel, due to the formation of biosurfactant molecules from the microbes.
It’s not just FWS units that have their performance microbially compromised. Particles of microbial biomass that break off (from tank turbulence) and get suspended in the fuel will make their way to the filter monitors and microfilters, clogging them. In more severe cases, it can magnify the fuel filter’s differential pressure up to an unacceptable level. Pilots and flight crew know that sudden significant increases in differential pressure like this triggers a bypass indication, most commonly around or just after take-off. The aircraft has to be grounded, to spare the possibility of engine failure if the fuel’s contamination happens to reach the engine and cause damage.
Filter health can be one of the indicators of microbial presence in the aviation fuel system. But it’s dangerous to rely on them alone, because once the filter problems are severe enough to be caught through inspection, the microbiological contamination is already well-established and more difficult to get rid of. And very likely, it will be a lot more expensive to remediate.
This is where microbial testing and monitoring of the fuel brings real value. Testing of fuel samples gives the aviation user the kind of window into their system’s microbial presence that would be impossible to get through filter inspection. This kind of testing allows the user to make decisions that will save them a planeload of time, money, and hassle later on.