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In planes using avgas, sampling fuel from the tanks is generally done before every flight, or at least before the first flight of the day and then after any refueling. This is done because water contamination is fairly high risk. Gasoline does not mix with water, so it collects at the bottom of the tank, and because it does not burn, when it gets to the engine, the engine will quit.

Water does mix with kerosene though so this isn't as big risk and it can't be checked this way anyway because it won't collect in the sump. It could detect other contamination though. For example in this incident hugely excessive addition of biocide caused damage to the engines, and in this case the report says the biocide did form a separate layer below the fuel.

So how often is fuel in turbine (turboprop and jet) aircraft usually sampled? And is it subjected to any additional tests beyond visual inspection?

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    $\begingroup$ Water in fuel accident: British Airways B772 at London on Jan 17th 2008 avherald.com/h?article=4270d893 $\endgroup$ – D Duck May 17 at 16:10
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    $\begingroup$ @DDuck not really. That accident was caused by problems in the FOHE (Fuel Oil Heat Exchanger) design causing it to ice up under certain conditions. The fuel was not contaminated with water: "Fuel samples were analysed and showed [...] a natural allowable water content between 35 and 40 parts per million. There was no contamination of the fuel." (from the site you linked) $\endgroup$ – Bianfable May 17 at 17:46
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Water and microbiological contamination are a threat to turbine aircraft even if water mixes with kerosene based fuels:

Industry-wide, the major threat to FQIS [Fuel Quantity Indication System] availability is microbiological contamination of the fuel (see FAST 38) and water in the fuel tanks. Settled water in the fuel tanks may affect FQIS indications, typically driving FQIS probe readings out of limits and leading to aircraft delays for tank draining. Therefore, a regular water drain task is essential for smooth aircraft operations.

(Airbus FAST 42)

The reason is that both water and microbiological contamination in the fuel lead to a higher dielectric capacitance, which is used for fuel quantity sensing:

The FQIS system is calibrated for use with jet fuel and the probes measure from 'unusable' to full tank capacity. However, the dielectric of water is approximately eight times higher than the jet fuel dielectric. Therefore, when there is water in the proximity of the probes, the fuel measured capacitance changes. Typically, the probes will read a higher capacitance level than the actual fuel level.

(Airbus FAST 42)

If only a small amount of water is suspended in the jet fuel, it can be safely fed to the engines without causing a flameout. However, water collecting at the bottom of the tanks must be drained:

All modern civil aircraft incorporate water drainage and/or scavenge systems to help achieve this. The Airbus wing design ensures water is maintained in suspension and fed to the engines; any remaining water is then drained at regular intervals via the water drain valves.

(Airbus FAST 38)

For an Airbus A320, this fuel tank draining must be performed at least once every 36 hours:

Airbus A320 MPD

The amount of water drained is about 200 ml per week and per tank on an A340 after improvements done in 2007 (Airbus FAST 42).

Airbus recommends to visually inspect the drained water and to perform a fuel analysis at least once a year to detect contamination:

A second key element of prevention is monitoring. Often overlooked is the need to visually inspect any water resulting from the MPD water drain task. Drained water and fuel will separate within the container with the water settling to the bottom. Both should be clear with no particulates evident to the naked eye. Any 'cloudiness' in either fluid indicates possible contamination and action is required.

Currently, Airbus recommends an annual analysis of fuel from each aircraft to test for fuel/water contamination, which should be considered a minimum for all operators.

(Airbus FAST 38)

Should the visual inspection show any contamination, or should the pilots notice discrepancies in their fuel analysis (see e.g. here for example calculations), or should the FQIS show fluctuations or degrading (fuel quantity indicates XXX), a detailed analysis should be performed according to this flow chart:

Fuel contamination flow chart

(Airbus FAST 38)

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  • $\begingroup$ My understanding of the last quote is that the drained water should be visually inspected every time, and the once a year thing is the more thorough analysis to determine the level of biological contamination. $\endgroup$ – Jan Hudec May 17 at 11:48
  • $\begingroup$ @JanHudec You're right, it sounds like the "action is required" would then be the full analysis of the fuel. "Often overlooked" sounds like this visual inspection isn't done every time, but they recommend doing it. $\endgroup$ – Bianfable May 17 at 12:00
  • $\begingroup$ Indeed. This suggests in the incident I referenced they should have done the draining at least once (it was two days between the biocide treatment was completed and the incident flight), yet somehow they didn't notice or didn't follow up on the unusual brown color (mentioned forming in the samples taken after the incident) of the drain. $\endgroup$ – Jan Hudec May 17 at 13:39
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Flying single and twin engine turboprops, it is our practice to check sumps and drains in an identical manner as with gasoline fueled aircraft. Typically this is done at each leg that the engines are shutdown.

The manner for inspection is identical. Generally, fuel is collected, and not dumped on the ramp due to the less volatile fuel not evaporating as quickly (although collection of gasoline sump draining is implemented at many airports).

Operations are conducted under Part 91, and this comment is focused on general aviation fleet operations.

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