AC 2 0-29C Appendix A A-1 Appendix A. Water in Aviation Fuel.
A.1 The tendency of water and fuel to mix, and a fuel’s particular saturation level, varies with its composition and temperature. The saturation level for jet fuel, in parts per million by volume (ppmv), is approximately equivalent to the temperature of the fuel in degrees Fahrenheit. For example, a jet fuel at 50°F may contain approximately 50 ppmv of dissolved water. However, this guidance pertains primarily to conventional petroleum-derived fuels and may vary for fuels that contain synthetically or alternatively derived compounds. Water that remains in solution as dissolved water will not have any adverse impacts on aircraft or engine operation. See AC 20-125 for additional guidance and information pertaining to water in aviation fuels.
A.2 Free water exists in fuel, either as a separate and distinct free water phase (or slug) or as entrained water. The free water phase is a separate layer or body characterized by a well-defined boundary with the fuel, such as the “water bottoms” in fuel tanks.
Entrained water exists as small droplets dispersed in the fuel. See ASTM D3240 for the test method used to measure free water.
A.3 Ground handling filter separators contain water coalescers and separators used to remove the free water phase from the fuel; however, they do not remove all of the entrained water particles because the droplet size is so small. These entrained water particles that remain after filtration will eventually settle out of the fuel. Experience has shown that jet fuels uploaded onto aircraft before flight contain 10 to 30 ppmv of entrained water but may increase to up to 70 ppmv in the aircraft fuel tank with aircraft flight operations.
A.4 As an aircraft climbs in altitude, the air temperature typically decreases. The reduced ambient temperature causes the fuel to cool and reduces the fuel’s water saturation level. This reduced fuel saturation level causes the dissolved water to fall out of the solution in the form of entrained water in minute droplets. Until this water can coalesce and migrate to the bottom of the tank, it will be carried in the fuel. At temperatures below the freezing point of water (0°C or 32°F), these minute droplets may undergo a phase change to produce ice particles, which can adhere to aircraft and engine fuel system surfaces, filters, and screens. This ice may accumulate and obstruct fuel flow leading to engine power loss or shutdown.
AC 20-29C Appendix A A-2 A.5 The concentration of free water in the fuel tanks may also increase during flight due to the condensation of moisture-laden air in the tank ullage space. A long-duration high altitude flight will result in tank surfaces and fuel colder than the air drawn into the tank during descent. When moisture-laden air enters the tank space, condensation may occur on the cold surfaces and mix with the fuel as entrained water. This water may exist in the fuel tank as entrained water for multiple flights before settling as free water if the fuel tank remains at a low temperature due to the higher viscosity of cold fuel. Under these conditions, the entrained water in the fuel may reach up to 130 ppmv throughout a flight. Some aircraft incorporate automatic water sumping systems, and others rely on manual activation of the sump drains to remove free water from the fuel tanks. Water sumping is recommended whenever practicable.
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