Frequently Asked Questions: Microbial Growth and Biocide
In response to COVID-19, a large number of passenger aircraft have been grounded with fuel left standing in aircraft wing tanks. During these unprecedented times, it is important for airlines to develop a strategy to avoid any unnecessary risks so aircraft can fly safely again in the future.
On 3 June, Rob Midgley, Global Quality and Technical Manager with Shell Aviation, hosted a webinar for customers addressing the technical challenges and developments in the industry around the problem of microbial contamination of fuel in grounded aircraft. The webinar covered the science of microbial growth and available options to help anticipate, prevent, and treat contamination. This is a transcript of Rob’s Q&A with customers on the webinar. It has been lightly edited for clarity.
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If a test result indicates that an aircraft is severely contaminated, which removal approach would Shell advocate for - defuelling or applying a biocide treatment?
The advice on actions following detected contamination will be contained within the Aircraft Maintenance Manual (AMM) for the aircraft in question. However, the testing limits are designed in such a way that, when used as part of an ongoing and structured microbial testing program, the first time an Action or Severe level indication is given, then treatment with biocide without tank entry should be effective. Of course if it is the first test that has been taken that gives this level of indication, then it is possible that an infestation has developed considerably and defuel plus tank entry may be required.
Other indications are also useful to consider when trying to asses severity of infestation. Testing of the acidity of this water is also a useful indication and results in the pH3 to pH5.5 would be symptomatic of microbial growth. If there is a discoloured water phase in a tank low-point sample, especially where it contains visible particles, then this is an indication that microbial growth is active within the tank.
Attention should especially be given to any samples that have a smell of hydrogen sulphide (H2S), which is the smell of rotten eggs, as this is commonly associated with anaerobic bacteria, which often develop after other forms of microbes and can in their own right produce aggressive corrosion issues.
The best advice is to consult your the Aircraft Maintenance Manual (AMM), to have a structured program of regular microbe testing in place, and to plan to act with biocide treatment either to prevent microbes from forming or to add biocide no later than the first incidation of an Action level microbe test result.
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While comparing the defuelling vs biocide approach, what are the advantages and the disadvantages of these methods?
The biocide route is generally preferred as there are a number of advantages to this:
- If the correct biocide is selected (approved for use within the engine) then the treated fuel can be burned in the engine (and so is not a financial loss to the airline, unlike downgraded fuel).
- Aircraft tank entry for cleaning is a time-consuming and potentially hazardous activity.
- Due to accessibility and complex structure, tank cleaning is not in itself totally effective and so is often also followed up with biocide treatment.
- Defuelling of contaminated fuel may not be easy to arrange. There is not much capability within fuel-provider networks to offer defueling of contaminated fuel – not only are there not many vehicles, but also the disposal of any contaminated fuel is not easy to arrange as it is a non-routine activity.
Therefore we would strongly recommend that airlines conduct a program of either biocide injection to prevent microbial growth in tanks or have in place a robust program of tank draining and microbial testing to identify microbial contamination in a timely manner so that biocide injection can be performed on a needs basis.
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Are there specific aircraft models that are more receptive to microbial growth?
Yes, certainly we see issues with certain aircraft types which tend to accumulate water in their wing tanks, possible due to draining points being not quite at a low point of their tanks or ineffective scavenger systems. During operation we have noticed that some airline operators have challenges with water accumulation within the fuel tanks of Airbus A330/A340 aircraft. However, aircraft which have challenges with water drainng during operation may not be the same types that are at risk during periods of inactivity.
All commercial aircraft use scavenge systems to draw fuel (and any water) from inaccessible low points within the wing tanks and deliver this to the engine to be consumed during flight. While these scavenge systems can be effective in scavenging water during normal operation, the effectiveness of this process requires the engines to be running. So even aircraft that might not have microbial issues during normal operation may start to develop microbial issues once the aircraft is dormant. So it is important not to only look for what might be perceived to be higher-risk aircraft types: a program of analysis and / or inhibtion should cover all airframes in a fleet.
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Microorganisms need three things to cultivate: water, oxygen and food source. While specifically referencing the food source, are there any differences in how microorganism react to JETA1 vs Avgas?
In principle it is true that the three elements exist for microbes in Avgas too, but we tend not to see many issues with microbial growth in avgas. There are several theories around why this might be, including the toxicity of the lead that is in avgas 100LL and the higher vapour pressure of the fuel (resulting in the hydrocarbon vapour concentration being high in any air accessible to the microbes). Water also settles out of avgas much easier and so there tends to be less water in suspension or dissolved in the fuel which then has the potential to settle and form a water layer. This will also combine with the fact that the fuel tanks on avgas aircraft tanks are much smaller than those in commercial turbine-engined aircraft and so there is less total water in the fuel and that water is easier to drain. Perhaps the reality is that all of these factors contribute to microbial growth being a less prevalent problem in avgas aircraft tanks. For bulk storage tanks, of course, we should continue to conduct daily draining to remove water from the tanks.
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In the webinar, it is shared that a sample needs to be tested within 48 hours but to gain the most accurate result, a sample should be tested within 4 hours. If you aren’t able to test your sample within the next 4 hours, what would Shell advise to do while refrigerating the samples?
Either a refrigeration unit if the sample is to be held in a lab, or alternatively placing the sealed sample bottle in a cooled ice box would be a suitable alternative if the sample is in transit.
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If fuel is treated with biocide, does a microbial test still work?
It is generally recommended to confirm a biocide treatment's effectiveness by testing samples after the biocide treated fuel is no longer present in the fuel tank. For aircraft, this means testing the sample after 10 days or after 5 flight cycles. Shell supports this recommendation. In part, there are some complications related to testing for microbes in fuels containing biocide.
As biocides inhibit the growth of microbes, the presence of biocide could affect those techniques that rely upon incubation. There is evidence available from the supplier that suggests that the MicrobMonitor2 test kit is still effective at detecting microbial contamination in fuel containing biocide if no more than the recommended concentration of biocide is present in the sample. It has been reported to Shell that testing by the manufacturer has shown that the biocide is diluted in the nutrient gel. Therefore, it does not adversely inhibit the result. However, this does require no more than the permitted concentration of biocide to be present in the sample, dependent upon correct dosing and mixing of the biocide within the tank.
Other tests that do not rely on incubation, such as the Merck HY-LiTE test, should not be unduly affected by the presence of biocide. In the HY-LiTE test, an indication of microbial growth depends on the presence of Adenosine Triphosphate (ATP) to indicate the level of biological activity. As ATP is only present in living cells, it should continue to give the correct directional indication of the level of viable growing cells present in the sample, independent of any biocide in the fuel. However, it should be noted that spores, the pre-cursor to future microbial growth, are not detected by this method, and any users of HY-LiTE should refer to ASTM D7463 to ensure the correct protocol is being employed when using this method.
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Is the risk of microbial contamination greater in high-temperature countries?
Yes, it is – but only up to a point. Once above a temperature of around 35oC, other microbes can prefer the higher-temperature regimes and start to be more competitive at the same time as the more typical microbes are becoming less viable. However, there aren’t a lot of locations in the world that are routinely above 35oC and typically temperatures don’t stay this high for long. This means that long-term growth of these higher temperature-preferring microbes becomes limited. As fuel temperatures are lower than this for longer, it means those species that prefer the lower temperature are the ones that tend to proliferate.
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Are there any concerns relating to long-term storage of aircraft containing biocide-treated fuel?
No. There is no limitation for long-term stored aircraft that contain fuel treated with biocide. However, aviation fuel itself is not always stable over time, so it is recommended that fuel is tested to ensure that it continues to meet the specification requirements after a period of six months.
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What are the industry developments regarding the development of additives to impede microbial growth? Will there be a unique blend of fuel developed in the future explicitly designed for aircraft that don’t often fly to avoid microbial contamination issues?
It is very doubtful that this would happen. The market for such a fuel is small during normal times and it would be difficult to know where the demand might be and therefore where to supply such a special fuel. It is much more practical to use an additive approach to this – to add something to normal fuel to inhibit microbial growth when an aircraft might not be in operation.
This question leads to another: can we approve more biocide additives so there is more flexibility and also potentially more options should resistance start to build within the microbial population to existing biocides? The industry currently only has Biobor JF and Kathon FP1.5 that have been approved for use in fuel that will subsequently be burned in aircraft. Approving new additives is a long and costly process as it demands that evidence is built to demonstrate that the use of such an additive is safe and would not impact equipment or operations. The cost and complexity of this approval process is one factor in why more biocides are not currently approved for use in aviation fuel.
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How often are the fuel tanks at the fuel storage areas tested for water and contamination, including underground piping that leads to the fueling port at the aircraft bay?
Water is drained daily from supply tanks and fuel filters in the supply chain. This, combined with the design of the tanks and filters to ensure that draining is effective in eliminating water, is seen as an adequate control and microbial testing is not routinely carried out in the supply chain. It is better to prevent than to detect a problem. There are some exceptions to this general rule and it woud be normal for a supplier to conduct routine microbial testing if, for example, the tank design may have a compromise in its ability to totally eliminate water, or it may be included as part of the oversight controls required to extend the period between routine tank-cleaning activities.
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Does daily water draining solve the issue of microbial growth in long-term parked aircraft?
Not necessarily. There are two primary strategies for removing water from within an aircraft wing tank: physical draining of low points using external drain points and scavenging of other areas to the engine fuel inlet. Not all areas of the tank water traps can be accessed by an external drain point and so scavenger systems are used to draw fuel (and any potential water) into a fuel feed to the engine. Obviously for this to work the engine needs to be running, which is why we can see that the water management of aircraft will be compromised now that they are not in operation.
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In tropical countries, it is advised to redo the water drain task after a severe rain shower. Would this guidance also apply for the current situation?
How and when to drain aircraft fuel tanks is not something that Shell is able to dictate. To understand the requirements for how and when to conduct aircraft tank drains we would have to refer you to your aircraft’s maintenance manual (and any local agreement that your airline has with the regulator for aircraft fuel tank water management). However, it would seem sensible to increase the frequency of sampling and draining if the risk of water ingress to the tank increases.
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Can biocide-treated fuel be treated the same as regular fuel, or are there any safety concerns to take into consideration?
The concept of the use of biocides in aircraft is that the fuel can then be used and burned in the engine. However, as we mentioned in the webcast, there needs to be an approval in place for this which is controlled by the airframe and engine manufactrers. Due to recent incidents, the approval to burn fuel containing Kathon FP1.5 has been removed by GE & Safran for engines for which they are the design authority. There is a potential for those that have mixed fleets to be able to take fuel off an aircraft with a a GE or CFM engine and put it onto an aeroplane fitted with different engines. This would mean a defuel & refuel procedure and this woud seem to be an operational a complication that would be worth avoiding where possible, especially if an alternative biocide can be used (such as Biobor JF). It may be that there are airlines who have already treated aircraft that have GE / Safran engines with Kathon FP1.5 and such aircraft will have to be de-fueled before returning to service. Should a customer need Shell to be involved with this, we would have to understand the demand for this as we do not have vehicles which are capable of defuel at every location at which we operate.
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Are there any long term R&D commitments made in the industry to add a permanent additive to fuel storages in order to support the prevention of microbial growth?
No. This would be an expensive option as it woud require all fuel to be treated with a new product.
There have been other options considered in the past, such as irradiating fuel with ultraviolet light to kill microbes, but these have all been discounted. The preferred strategy is to design fuel-supply systems to eliminate water and thereby remove one of the essential elements needed for viable microbial growth. -
Does Shell Aviation market any product solutions for long-term fuel preservation of parked aircraft?
Not for parked aircraft, or temporary inactivity such as might be the case for heavy maintenance inputs. What we see now with COVID-19 – many aircraft inactive for several months, but intending to go back into operation – is unprecedented. The industry has not anticipated the need to develop solutions for this situation. We do have some preservative fluids for engine inhibition, but these tend to be used for long-term storage, or shipment of engines when no longer on the wing.
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Has microbial testing been carried out on fuel samples downstream of storage tanks on hydrant systems, bowsers and hydrant carts?
Not typically. At Shell Operations, even during this COVID-19 period, we continue to manage tanks and filters in a way that should eliminate water, and we adequately drain low points as we would during normal operations. Therefore, from a supply side, we are not anticipating that microbial contamination will be an increased threat.
In the case that we aren’t able to drain our filters, due to pressure or flow issues, we have been isolating and draining the vessel and removing the elements, thereby removing another of the essential elements for microbial growth: the fuel. Any filters that have been drained and decommissioned will then be recommissioned once we go back into operation. This procedure eliminates the possibility of microbial growth by draining the vessel itself. We have concentrated on prevention rather than detection and therefore, we’re not actively testing for microbial contamination.
Supply systems that have microbial concerns can give clues. Certain tell-tale indications can reveal microbial issues within the supply chain, for example, “leopard spotting” of filter coalescer elements. Leopard spotting is the phenomenon in which we can recognise marks of microbal growth on filter elements. This, as well as indications in the tank sump sampling, are rare within the supply chain, but when they appear they are easily recognised and the source of the issue can quickly be eliminated so that problems are not passed on downstream.