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Viscosity measurement to check the purity of aviation fuel

Aviation fuel is a mixture of different hydrocarbons. The longer the hydrocarbons are and the higher the molecular weight of the compounds, the more the chemical parameters such as melting point or smoke point differ. These chemical parameters have a considerable influence on the quality of the aviation fuel. An important parameter for quality control used by aviation fuel manufacturers is the viscosity.

The purity of the aviation fuel in particular is of vital importance: If the fuel is contaminated with water, ice may form in the fuel tanks and fuel lines at high flying altitudes – a dangerous situation. These ice crystals would disturb or interrupt the flow of fuel to the turbines. At worst this could lead to a failure of the turbines during the flight.

Standards specifying how to determine the purity of aviation fuel

The purity of the aviation fuel can be determined via a viscosity measurement. On this subject, standards have been developed and introduced which are recognized around the world. These standards are published and managed by the American Society for Testing and Materials (abbreviation: ASTM)[1]. The viscosity measurement of aviation fuel is covered by both ASTM D1655 (Standard Specification for Aviation Turbine Fuels)[2] and ASTM D7566 (Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons)[3].

What happens to aviation fuel when the viscosity is too high

If the viscosity of the aviation fuel is too high, this is a problem for the injection nozzles of the turbines. During fuel injection, the nozzles then have to use more energy to spray the fuel. This shortens the working life of the injection nozzles, which leads to greater maintenance costs and causes re-ignition of the engine in case of a failure during flight. The viscosity also affects the pressure drop in the fuel lines. The higher the viscosity, the higher the pressure drop. As a consequence the fuel pump must work harder to guarantee a constant flow rate so that the turbines can continue working as required.

What happens to aviation fuel when the viscosity is too low

If the viscosity of the aviation fuel is too low, the lack of lubrication in the system can lead to a total failure and cause a crash. The quality of aviation fuel is therefore an essential factor for people and machines in the aviation industry.

Measuring the viscosity of aviation fuel

Meeting the demands of ASTM D1655 and ASTM D7566 for the certification of aviation fuel requires a viscometer.