Fuel Contamination


When we think of fuel contamination we often think of the most common contamination – water. In this story, Marc Brogan tells us of discovering a different type of contamination.

Further Key Information

The pilots that collected this aircraft did not know enough to question the work-around, and then when they stopped for advice during the flight, the advice they got was also plausible – although incorrect.


Unfortunately, this incident was never reported to the CAA, either as a partial engine failure or as a defect.

Swiss Cheese Slices

As Marc said in the video, the primary lesson from this incident was not to ‘normalise’ things that are out of the ordinary. An engine that needs to be leaned at 1500 feet is not normal, and should have been investigated. As it was, the piece of clamp was found fairly easily once the issue had been clearly reported to the engineers.

Fault Recognised and Questioned

Was this fault ever properly recognised as a reportable fault? It seems not to be the case.


Plausible Work-Around

The ‘work-around’ seemed reasonable and plausible and was implemented without too many questions being raised. It also seemed to fix the problem.


We all use ‘work-arounds’, such as knowing the aerodrome gate sticks and you need to lift it when you open it, but when they affect the primary systems of your aircraft, they deserve more thoughtful questions not fewer. Why is a work-around needed? Who suggested the work-around? How long has it been in place? How long is it planned to be in place? What is the ‘work-around’ trying to fix? Are there any remaining risks which need attention?


Normalised the Abnormal

This fault became normal, it was simply accepted that in this aeroplane the pilot needed to lean the mixture at 1500 feet, and then everything was fine.


Tech Log

If this defect had been entered in the Tech Log, then the fault would have been investigated, isolated, and rectified – thereby removing the risk.


Causes of Contamination

Colin Alexander, owner of Solo Wings and experienced engineer, gives some advice on contamination in fuel tanks.



Rainwater frequently enters aircraft fuel tanks through defective seals, so if you notice the cap does not fit tightly, get the seal replaced. Postpone refuelling the aircraft when it is raining heavily, and if the wing is covered in water use a rag to remove it from around the cap and filler area.


Water can get into the tanks without the caps being removed. Water condenses out of the air in the tanks when they cool. So, the advice is to keep the tanks full whenever you can.


Draining Protocols

Complete a proper fuel drain and inspection after each refuelling and before every flight. After refuelling, allow the fuel to settle for as long as possible. A minimum of 10 minutes is recommended. This gives any impurities a chance to settle into the drain sump of each tank.


You should know how many drain points your aircraft has – just part of the systems knowledge you should have of each and every aircraft you fly. To help you with this we have developed an app which helps you to refine your fuel systems knowledge. This app can be downloaded from the Google Play Store or the App Store for free.


Just as it is important to dip the tanks on level ground, it is also important to drain them on level ground. When checking the fuel on anything other than a level surface, the drain point may not be the lowest point in the tank.


Confirm that each spring-loaded drain valve shuts securely after use, especially the hard to reach drain valves. Any leaks will result in higher than normal in-flight fuel consumption, and could even lead to an engine failure.


Some aircraft have long fuel lines, meaning that contaminants can take some time to reach the drain point.


In aircraft with little or no wing dihedral, contaminants will tend to spread more evenly across the bottom of the fuel tank.


The Flight Manual will provide any special procedures for fuel draining, and if not, talk to an engineer.


What if Nothing Comes Out?

Find out why. You should always be able to get fuel out of a drain point. If there is no fuel there, how can you be sure fuel in getting to its most important destination – the engine?


Fuel drains can become blocked, and this is an excellent indicator of what might be lurking in the bottom of the tank. Get it checked out by an engineer. Don’t move on to the next drain without finding out why fuel is not coming out of the drain.


In cold winter conditions, small amounts of water can freeze the drain plug. It will need to be warmed to drain any water, for example, by moving your aircraft into a hangar.


Fuel or Water?

Ensure your drain sample is actually fuel – and not pure water. Not as stupid or uncommon as you might think. The mild tint in avgas can be hard to see if you hold the sample to the light.


However, holding the sample to the light will allow you to detect small globules of water sitting on the bottom of the testing vessel. So, do both; hold it against something contrasting like a light coloured wing or fuselage to check the colour and for debris, and hold it to the light to check for water.


Then smell it – but be cautious; water can carry a fuel odour if the two have been in contact and plastic fuel testers can retain fuel odour.


If you do find water in the sample, empty the tester and continue draining until a pure sample is obtained. Be sure to empty the sample into a fuel disposal container. Don’t dispose of the sample fuel on the tarmac because it degrades the bitumen, and don’t dispose of it on grass, because it kills it off leaving unsightly ‘burnt’ patches.


Even more importantly, do not tip the sample back into the aircraft tank, even if it is ‘clean’. It is a bad habit to get into and if you keep it up one day you will tip a contaminated sample back into the tank.


Water often collects in wrinkles and low points within fuel bladders. If the aircraft is not on a level surface and contamination is suspected, move the aircraft to a level surface, and allow the fuel to settle. Then carry out another check, and keep checking until you get a clean sample.



Water often collects in wrinkles and low points within fuel bladders. You can also rock the wings to move any pooled water or contaminants.



If the aircraft is not on a level surface and contamination is suspected, move the aircraft to a level surface, and allow the fuel to settle. Then carry out another check, and keep checking until you get a clean sample.


Jet A-1

Detecting water in Jet A-1 can be even more difficult, because Jet A-1 isn’t tinted. When checking for the possibility of water in Jet A-1, use fuel testing capsules or paste, which are good for identifying water contamination. If the sample has a cloudy appearance, there could be a lot of water suspended in the fuel.


Caps and Dipsticks

Colin Alexander, owner of Solo Wings and experienced engineer, gives some advice on dipsticks.



One sure-fire way to introduce contamination into the fuel tanks is to contaminate the dipstick or the tank caps.


Always keep them with you and always replace them.


It’s also an expensive mistake, leaving your fuel cap off and then getting airborne.


Foreign Bodies

Foreign bodies in fuel are not particularly common, but nonetheless it is a potential problem.


The most common foreign-body contamination is sludge and gunk (a technical term) in the fuel system components, most commonly the pumps, filters and gascolator – anywhere there is a filter. This is obvious when you think about it because that is exactly what a filter is designed to do.


If you find evidence of ‘gunk’ in the fuel sample just discarding it is not enough. Have an engineer check it out, it is probably an indicator of other contamination in the system.

Gascolators and Pumps

Below is a finger filter from an electric fuel pump and lying next to it you can see it the foreign matter that came out of it. Clearly this is a problem for fuel flow and because it raises the question of where all this gunk came from?



At the top left of the picture below is a filter from a gascolator (strainer), and at the bottom the debris removed from it. As you can see, it is quite rusty, indicating the presence of water in the gascolator.



Looking into this gascolator below you can see the damage water has dome to it. It is hard to believe that this aircraft never showed any indications of water in the system. Was this system checked and sampled regularly?


Sloshing compounds

Some fuel tanks have sloshing compound added to seal small cracks. This liquid is added, swirled around, and left to dry. It adheres to the tank walls and seals it.


However, sometimes it does not stay on the walls and can peel off. Make sure you know if your aircraft has this compound, and keep a close eye on the tanks for deterioration.


Know Your Aircraft

As you can see, it is important to know what is normal for your aircraft – every one of them that you fly.


If you know your aircraft well you will not only be more comfortable flying it, but will be able to fault-find and resolve problems much more efficiently.


Even if you have built your own aircraft and think you know it well, it’s still worthwhile completing our app; Know Your Aircraft to cement your knowledge. You can download this app from the Google Play Store or the App Store for free.