Fuel Contamination

Contaminated fuel still remains one of the primary causes of fuel starvation and engine failure in light piston engine aircraft. Most of these aircraft use 100LL Avgas (the LL stands for low lead), which has all but replaced the old 100/130 Avgas, which contains a higher amount of tetraethyl lead. Two of the primary forms of contamination are water and cross contamination with jet fuel, the most common commercial jet fuel being Jet A-1.

Early in our flying careers we are taught how to check for fuel contamination during our pre-flight checks. The usual method is to drain a quantity of fuel from the aircraft’s low point sump drains into a fuel testing sampler or pipette like the one shown below.

Once the sample is taken, the pilot checks first the colour of the fuel. 100LL Avgas has a light blue colour, while 100/130 Avgas is green. Second, we are also taught to check for particles of dirt or rust in the sample. Third, the pilot should check for water contamination. Water has a specific gravity of 1.0, whereas 100LL Avgas has a specific gravity of 0.72. This means that water is more dense than Avgas, so it will sink to the bottom of the sample, often appearing as a globule or ball.

However, these simple checks are not always adequate. For instance, did you know that it takes water approximately a quarter of an hour to precipitate out of 1 inch of Avgas? So completing these checks immediately after refuelling is not advised. Instead, leave enough time for the water to settle out before taking the sample. If the water has not completely precipitated out, the fuel may be emulsified, giving it a milky appearance.

If the aircraft is not on level ground when the fuel drains are checked, it is possible that any water present in the tank may be lower than the fuel drain valves, therefore the water would not be able to be removed by the sampler or pipette.

I have heard of a case where there was so much water present in the tank that the entire sample was water. In this case the pilot misidentified the water for clean clear fuel.

To check for this, or cross contamination with Jet fuel, use your senses of smell and touch. Water of course has no smell, whereas Avgas and Jet A-1 have their own unique odour. (Use caution when smelling fuels – the vapours are toxic and it doesn’t take much to identify the smell.) As for feel: Avgas evaporates very quickly, causing your fingers to cool; water evaporates less quickly; and jet fuel hardly evaporates at all, it is colourless, but it will leave an oily residue. (Once again there are health risks associated with handling fuels, so if you get fuel on your skin wash it off thoroughly as soon as possible).

These extra checks may take a little more time, but they can help protect you and your passengers and are worthwhile doing.

Until next time, fly safe.

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