There are also a great many variables and factors that you need to understand and consider, to ensure that an aircraft's fuel doesn't become contaminated–negatively affecting how those thousands of kilograms of metal aircraft operate.
This guide is your comprehensive resource for all things related to aviation fuel, contamination and fuel testing, aiming to remove some of the confusion and ensure that you obtain the knowledge you need to access your fuel testing needs.
In this field guide, we’ll show you why fuel testing is so essential for maintaining effective aircraft operations. We’ll look into where fuel contamination begins. We’ll explain who is at the greatest risk. We’ll take a deep dive into the operational costs of discovering contaminated fuel. We’ll address when fuel testing should be done. And finally, we will explore the various options available for how you can protect your aircraft and your investments.
Throughout the guide, you will find links that will help you navigate throughout the specific fuel testing topics, as well as a table of contents that will provide you with a quick overview of this Field Guide.
In an industry where every nut, bolt, spring and gear must undergo rigorous testing and certification, it shouldn’t be necessary to point out the importance of testing aircraft fuel for contaminants that can compromise the integrity of an aircraft. Surprisingly, many airlines don’t have a stringent testing process.
Even though contaminated fuel within an aircraft can lead to detrimental effects - not only the aircraft but also overall operations - there is unfortunately still a widespread reactionary culture around how, and how often, an aircraft should be tested for contaminated fuel.
Prevention, rather than reaction, is the key to success in this area. Optimising fuel testing regimes can have real cost benefits, when compared to the loss of revenue that can occur by letting fuel contamination get out of control.
Although aviation fuel is a highly volatile substance, burning through an aircraft engine at 825˚C (1517˚F), it is still quite susceptible to contamination from particulates, water, microbial growth or other petroleum-based products. Any one of the contaminants mentioned earlier can cause havoc on an aircraft fuel system, if left unchecked, and depending on the type of fuel that is being used, some aircraft might be at a higher risk than others.
Judging by the age of some of the material that you can find online regarding the subject, fuel contamination has been a widespread problem for many years. And while many innovations have led to the various types of aviation fuel used today, each is still susceptible to fuel contamination in its own ways.
Arguably one of the worst states that a functional aircraft can be in is on the ground and unable to fly. For large-scale operations, such as commercial airlines and cargo, aircraft MRO is often calculated down to the hour.
For this reason, when fuel contamination is discovered within aircraft, it can lead to unanticipated time needed to resolve the problem, leading to extended AOG (aircraft on ground) times.
Contamination and particulates present in an aircraft's fuel can cause minor to considerable damage to the aircraft.
On the minor end, microbial contamination and particulates in the fuel can lead to incorrect fuel quality readings by the aircraft's fuel indication system. These indicators rely on measuring fuel levels by measuring the fuel's capacitance–the property of how two conductive objects, with a space between them, respond to a voltage difference applied to them. Both microbes and particulates have a different capacitance than the fuel, which could cause the indication system to signal more fuel in the tank than there really is.
On the extreme end, if an aircraft is left untested and untreated, it could require an extended AOG time for an engine and fuel system overhaul, fuel removal, and fuel tank cleaning. This situation is unlikely, unless the aircraft fuel is not tested at all.
In any situation, it is essential to test aircraft regularly. Particulates, microbial growth or excess water can create a cascading series of effects that will threaten the integrity of an aircraft's fuel system and its ability to fly safely.
Four main culprits are to blame, when looking into the causes of aircraft fuel contamination: Particulates, water, microbial growth, and other petroleum products. Each of these has its own situational and causational factors, which could introduce them to an aircraft’s fuel system.
Particulates pose a particularly tricky risk to aviation fuel systems because they can be introduced to the fuel anywhere during its journey from storage tank to aircraft. Despite the many contingencies that are in place during the fueling process, airborne particulates like dust and pollen can still slip through the seals of external floating roof tank or through the aircraft’s tank vents. While more solid particulates, such as rubber, plastics and fibres, can come from damage from equipment during the transport process.
In a study issued this year, looking into how Particulate matter from aircraft engines affects airways, it shows that even particulate matter that is too small to cause damage to the aircraft fuel systems may still have a negative effect. The study indicated that “primary soot particles from kerosene combustion in aircraft turbine engines also cause direct damage to lung cells and can trigger an inflammatory reaction if the solid particles,” ultimately concluding, “toxicity depends on the operating conditions of the turbines and the purity of the fuel.”
Contrary to how it might sound, there is always some level of water in an aircraft fuel tank. Most aircraft have a fuel tank sump built into the fuel tank, where excess water can safely accumulate, before being drained. The density of both Avgas (0.72 kg/l) and Jet fuel (0.85 kg/l) is less than that of water (1 kg/l), which allows the water to settle underneath the fuel gradually.
Water contamination can be diagnosed is a few different ways:
Underground Storage Tanks
Underground tanks are vulnerable to leakage if seals are not inspected thoroughly and replaced.
Aboveground Storage Tanks
Aboveground tanks can accumulate dissolved water, through condensation, as the outside temperature fluctuates.
Low spots in a pipeline can allow for the accumulation of free water.
Most fuel transport vehicles are built with protective valves, dams or poppets over their vents and manways–referred to as "roll-over protection systems"–in case the vehicle rolls upside down. While this is a necessary safety feature to protect against fuel getting out during an accident, these systems can provide an ingress point for water, if they are not kept free from debris or blockage.
Aircraft Fuel Tanks
Due to the molecular composition of aviation fuels, water can accumulate within an aircraft’s fuel tank at a molecular level. As the outside temperature of the aircraft fuel tank changes during flights and landings, the fuel absorbs atmospheric moisture.
Although aviation fuels are sterilised upon production, they all ultimately become contaminated with microorganisms over time. Tetraethyllead Avgas being the least susceptible, due to the toxicity of lead content.
Microbial growth occurs at the convergence of free water and fuel and can propagate at an alarming rate. These organisms live in the water while using the hydrocarbons of the fuel as sustenance to grow and multiply. If left unchecked, microbial colonies can propagate throughout an aircraft's fuel system, threatening system blockage and corrosion of metals within the tanks and pipelines.
According to an article by aviation pros, which investigated the factors that lead to jet fuel contamination, some microbial contaminants are far more harmful than others.
According to an article by aviation pros, which investigated the factors that lead to jet fuel contamination, some microbial contaminants are far more harmful than others.
“The most destructive of the microbes that grow in the aircraft fuel environment is the fungus Hormoconis resinae. One reason is because of its size. Compared to single-cell yeasts and moulds, it produces far more biomass. Secondly, it is the most common cause of microbial corrosion in aircraft fuel tanks.”
Microbial growth poses one of the most substantial risks to the integrity of an aircraft’s fuel system. While the potential danger it can pose to an aircraft can certainly be mitigated, it does require diligent and rigorous testing protocols. For if left unchecked, microbial contamination can cause lasting damage.
It is possible, although the likelihood is low, that other petroleum-based products, or the wrong additives, can contaminate the fuel, at some point along the supply chain to the aircraft. In a recent story, five aircraft in Miami-Opa Locka Executive Airport were given fuel that was mistakenly contaminated with an emissions-control substance, believed to have been confused for ice-inhibiting additive.
Typically, aviation fuel that is contaminated beyond use is returned to the refinery for reprocessing. Even after this process, though, small traces of the contaminant may remain unnoticed, which could threaten the integrity of the fuel. This is often the case if the contaminant is a surfactant, as it diminishes the ability for water to separate from the fuel.
It’s recently been found that aircraft which use biofuel may have a heightened risk for a contaminant called Fatty Acid Methyl Ester (FAME), which accumulates in the fuel and passes through a standard unsegregated fuel distribution system.
It is safe to say that every aircraft has some level of risk for fuel contamination, but airlines or aircraft owners who understand the urgency of the issue and take measures to test their aircraft regularly won’t find the problem too vexing.
That being said, there are situations where an aircraft is at a higher risk of contamination occurring. Specific variables, for example, change the level of risk. It is essential to be aware of what those variables are.
Each of the commonly used aviation fuels–Avgas, Jet fuel, and Biofuel–come with their own levels of risk.
Traditional leaded Avgas is lowest on the risk scale for microbial contamination, due to its inherent toxicity level. There aren’t many reports of microbial contamination, but there have been cases of foreign particulates and contaminants being introduced during the refining and transport process.
During a case in 1999, an investigation followed the grounding of thousands of piston engine aircraft across eastern Australia, when black gunk was found in fuel systems, caused by what was later found to be a chemical contaminant, now known to have been ethylenediamine.
“The investigation found that a very small amount of an anti-corrosion chemical that was not removed in Mobil's Avgas refining process in late 1999, and not detected by the usual tests, led to the safety problem.”
Due to the chemical composition of Jet fuel, it becomes highly susceptible to microbial contamination, as a result of water accumulation. The molecular structure of Jet fuel absorbs atmospheric moisture during high-altitude flight, which then settles as free water in the tank. This free water becomes a breeding ground for microbial growth.
Although not widely used today, there are many airlines and non-commercial aircraft that have been exploring the use of aviation biofuel. Since few aircraft currently make use of biofuel, though, there are not many documented cases of contamination with it.
According to a recent report issued by the International Renewable Energy Agency (IRENA), the most significant risk of contamination to biofuels takes place during the catalyst process, when the biomass is being converted into fuel.
Aircraft that operate in warmer or tropical climates are at a higher risk for microbial contamination. More significant temperature fluctuations between higher altitudes and the ground, in hot and moist geographical regions, create prime conditions for water accumulation within tanks and the propagation time of microbial growth.
Every aircraft is at risk of fuel contamination from either water, microbes, or foreign particulates. What determines the level of risk for each type of aircraft comes down to the level of awareness and resources that can be dedicated to the process of mitigating the risk.
Most commercial airlines are at least aware of particular risks to their fuel supply, and have protocols and contingencies in place for dealing with fuel contamination. Although, it could be argued that with the millions of flights that happen each year - and the catastrophic events that can occur, due to improper fuel system management - fuel testing should become an industry mandated process.
Non-commercial and privately owned aircraft are at a higher risk of fuel contamination, because of the general lack of awareness, regarding the severity of the issue.
Regardless of whether the aircraft is an Airbus A320 or a Cessna 172, if the owner or airline does not employ some level of oversight, regarding the integrity of the aircraft’s fuel system, then fuel contamination is not a matter of “if,” but rather a case of “when.”
While larger airlines and airports have some level of control over the fuel supply chain, before it reaches an aircraft, there is also industry regulation that locks protocols down even tighter. But this cannot always be said about private owners of smaller aircraft, who may not have the time, money, or resources to allocate.
The first - and best - solution is to be aware of the risks and the specific circumstances of the aircraft and to employ some level of routine fuel testing.
For commercial or cargo airlines, whose operations are fixed, down to the minute, the cost of having an aircraft on ground (AOG) for a longer-than-scheduled-for time can have truly detrimental effects on overall revenue.
In regard to the process of fuel testing, if an airline is not resorting to new fuel testing methods that reduce the testing time down to minutes, then the AOG time is typically determined by the amount of time needed for a laboratory to report the results of the fuel test. This is anywhere from 3-5 days, depending on whether the laboratory is on or offsite.
In extreme examples, where fuel testing uncovers high enough levels of contamination, indicating that the aircraft has suffered damage, the cost becomes exorbitant. The aircraft’s fuel could have to be removed and either treated or repurposed. Specialists might need to be employed to physically enter the aircraft’s fuel tank to scrub and remove any contamination. Any compromised equipment, within the fuel system, may have to be replaced. During this process, there would assuredly be accumulated loss of revenue, caused by an extended AOG time.
In cases where an issue of contamination might cause a flight to be cancelled or delayed, there are also the rights of the passengers to consider. For airlines that operate within the EU, the European Parliament enacted considerable legislation, which states that passengers that are denied boarding or are delayed must be provided with assistance, accommodations, a choice of reimbursement options, and an immediate compensation payout, depending on the length of their delay.
Although fuel contamination is an inevitable risk, there is no real consensus amongst the industry on just how, and how often, an airline or an aircraft owner should test their fuel for contamination. It’s safe to say that more often is better, but variables such as flight frequency, geography, and fuel type should all be considered.
Beyond the implementation of a regular testing regimen, there might be some tell-tale signs that might indicate a fuel contamination problem.
The available solutions for aviation fuel testing have come a long way in recent years. With the appropriation of technologies from other scientific testing fields, the options for implementing a more rigorous fuel testing regimen are now better than ever. Whether you are the single owner of an aircraft, or you represent an airline with a complex MRO process, fuel testing no longer has to be complex and time-consuming.
While some solutions for fuel testing seem antiquated in comparison to newer options that are breaking onto the market, it is still essential to understand what each of these processes are.
For many years, the most common - if not the only - option for fuel testing was to embrace laboratory testing. This process would involve fuel being extracted, either from the aircraft itself or its storage container, and tested by using specific testing methods and standards set by ASTM International, IATA and the Energy Institute, respectively.
There are a number of different standardised testing methods that can be conducted on fuel, depending on what an aircraft owner, or airline, estimates they might need to test the fuel for. IP 385 is an Energy Institute standard method of testing for microbial contamination, which is common amongst the aviation industry, whereas ASTM D4176 is used for testing for free water and distillate fuels.
In an article on how fuel testing can be optimised, Jeppe Damkjaer Nielsen, Product Manager at Satair spoke with regard to these older antiquated laboratory processes for fuel testing–and what kind of challenges they pose in today’s fast-paced operations.
»The conventional form of testing is based on physically counting colonies of microbes, colony-forming units (CFUs). It's a bit of an old technique. The testing can be done either by a laboratory, in line with what’s known as the IP385 standard or by using a commercial product,« Nielsen states and continues:
»Both options require a sample of the fuel to be put into a petri dish or growth bottle, which will then need to be incubated in a temperature-controlled manner. The test can take 3 to 5 days to allow the colonies to form, and then you’ll need to count the number of colonies manually.
There is also the transport of the fuel sample leading up to this process to consider. Since the sample contains live organisms, it should be transported under temperature control, typically on ice–and since it’s a fuel sample, it also counts as hazardous goods. All of this adds up to more costs.«
D975 Specification for Diesel Fuel Oils
D1655 Specification for Aviation Turbine Fuels
D4176 Test Method for Free Water and Particulate Contamination in Distillate Fuels
D6469 Guide for Microbial Contamination in Fuels and Fuel Systems
IP 385 Procedures for the determination of the viable microbial content of fuels and fuel components
IP613 Thixotropic Gel Culture Method for determining of the viable aerobic microbial content of fuels and associated water
The natural course of fuel testing evolution would be, of course, to remove the arduous laboratory processes from the equation and to keep the testing within the aircraft hanger. These under-the-wing solutions remove the risk of cross-contamination, false-positives and negatives and reduce the testing time considerably.
ATP is a standard test method that measures the Adenosine Triphosphate (ATP) content of microorganisms found in water–and in the case of fuel testing–the water found within the fuel. During this process, a testing apparatus, referred to as a pen stick, measures the contents of a fuel sample. The pen-stick is then inserted into a luminometer (light reader) to obtain a relative light unit (RLU) reading.
While there are solutions on the market that provide this process under-the-wing, there are a few issues. The ATP method is unable to discriminate between the type of microorganisms that are harmful to aircraft fuel systems and those that are not. Therefore, it requires high levels of sterility, due to the risk of cross-contamination. This method also requires specialised reader equipment, which needs to be maintained by a skilled operator and is usually only available from a fixed base and not in an under-the-wing environment.
While the laboratory is removed from the equation when resorting to the ATP method, it is still not without its failings.
Immunoassay tests are designed to detect specific chemicals by measuring the chemicals' response to particular antibodies. These antibodies are explicitly developed to bind with organic compounds, such as microbes that might be present in aviation fuel. These antibodies do not respond to different substances, which allows for highly-targeted testing for specific microbial contaminants.
Immunoassay testing was developed - and is widely used - within the medical field, most commonly in the form of pregnancy tests. This same quick-test technology is now being used in newer aviation testing solutions, such as Conidia FUELSTAT©.
Along with the evolution of many technologies, fortunately, fuel testing is also moving into the digital age. The benefits of this are in the ability to verify results in the field, along with the possibility of cross-referencing results with a large amount of digital data. Digitising the process also means results can be recorded instantly, in the case that further analysis is needed offsite–thus removing the need for sample transport.
Conidia’s FUELSTAT© tests offer the option of linking their immunoassay testing process with a smartphone. An app is available as a part of the testing process, which can help with the onsite verification of the testing results.