Viscosity of Aviation Fuels

Jet fuels and aviation gasolines used in civil and military aircrafts.

Aviation fuels in general are fuels suitable for use in aircrafts powered by turbojet, turboprop or piston engines. They consist mainly of hydrocarbons (paraffins, cycloparaffins, naphthenes, aromatics and olefins, whereby paraffins and cycloparaffins are the main components. Depending on their specific use they additionally contain additives.

The following aviation fuel types are available:

• Kerosene - Jet A-1 is the most common type in civil aviation, JP-8 is the military equivalent, TS-1 has improved low-temperature properties. It must fulfill viscosity specifications.
• Wide-cut fuels: These are a mixture of kerosene and gasoline and can be used in an extended temperature range. Wide-cut fuels must fulfill a density specification, but not all of them must fulfill viscosity specifications.
• High flash point kerosene: Provides more fire security because of a min. flash point of 60 °C. Must fulfill viscosity specifications.
• Low volatility kerosene: Gives good thermal stability and contains only a low amount of highly volatile components like toluene or benzene. Must fulfill viscosity specifications.
• Aviation gasoline: Also known as AVGAS, only for use with piston or Wankel engines. No viscosity specifications.

Low Temperature Scans on Jet Fuels

In order for jet fuel to endure long cold soak periods at temperatures believed to be -40°C or lower, knowing the viscosity of jet fuel at such low temperatures becomes increasingly important. This is especially relevant for so-called Auxiliary Power Units (APUs), which are small gas turbines that need to start up on command. These APUs can be inactive for long periods (15 hours or more) at flying height. During these periods, the fuel's temperature drops down to ambient temperature. The APUs' ability to start depends on the viscosity of the jet fuel at such low temperatures. Too high viscosity prevents fine enough atomization of the jet fuel - consequently, droplet evaporation is not possible.

Data are available for two different jet fuels and a jet fuel reference standard. While fuel JF1107 is type Jet A, fuel JF1411 is type Jet A1. The fuels differ in their freeze point specifications (-40 °C for JET A and -47 °C for Jet A1). Therefore typically viscosity of JET A is higher and JET A also shows a steeper increase of viscosity with decreasing temperatures.

• Jet A JF1107 was tested as a range scan from -20°C to -55°C with 1°C steps.
  Test duration: 2h 15 min.
• Jet A1 JF1411 and reference standard Jet N2B were tested as tables scan with variable steps from -20°C to -56°C.
  Test duration: 1h 40 min.

All samples were filled at +20°C. With filling, cleaning and drying at -20°C, which requires a supply of dry air, approximately 15 minutes can be saved.

ASTM D1655, ASTM D7566, GOST 10227-86, MIL-DTL-5624U, MIL-DTL-83133E, MIL-DTL-38219D, GB 6537-2006, DEF STAN 91-91 (UK), Measurement SVM™.