What is Fuel Viscosity?

Viscosity is the measure of a fluid’s ability to transmit momentum resulting from cohesive forces among molecules. These forces appear as shear stresses between fluid layers moving at different velocities.

Viscosity is a commonly measured property of jet fuels. Viscosity is a necessary factor to the aircraft designer in specifying line sizes, pumps and related items. However, viscosity data at low near-freezing temperatures are limited. Low temperatures can be experienced on long, high altitude or polar flights, where fuel flowability in the wing tank itself, independent of the fuel forwarding system, becomes a concern.

At high temperatures, the viscosity/temperature slope is low. At low temperature, where wax precipitation is significant, the slope is higher (viscosity increasing rapidly with temperature). The breakpoint between temperature regions is the filter flow temperature, a fuel characteristic approximated by the freezing point. A generalization of the representation for eight experimental fuels provided a predictive correlation for low temperature viscosity.

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.

An example of establishing an LOUT follows:

*Consider a Type I fluid that has met the aerodynamics acceptance

*test down to -45°C. The reported freezing point of the fluid (as measured by the service provider) is -43°C. The OAT is -39°C.

The LOUT for a given fluid is the higher of:

• The lowest temperature at which the fluid meets the

aerodynamic acceptance test for a given aircraft type, in this

case -45°C.

• The actual freezing point of the fluid plus a freezing point buffer

of 10°C, in this case -43°C + 10°C = -33°C.

34. Non-Newtonian fluids have characteristics that are dependent upon an applied force. In this instance it is the viscosity of Type II, III and IV fluids which reduces with increasing shear force. The viscosity of Newtonian fluids depends on temperature only.

*On a practical point of view, in order to determine the fuel freezing point, apply the following:

• When the mixture contains less than 10% JET A, the fuel is considered as JET A1.

• When the mixture contains more than 10% JET A, the fuel is considered as JET A.

*Mixing all the residual JET A with all the refuel JET A1 to achieve maximum dilution is not considered practical.

*To practically achieve the best dilution, all the JET A should be placed in the inner wing tanks as these have the largest volume (by transfer of outer tanks JET A fuel into the inner tanks either during the previous flight or on ground before refueling).

*Depending on the aircraft model, inner tanks will receive fuel from the center tank early in the flight, further diluting the JET A.

*Note

The poor dilution of the JET A in the inner wing tank and its concentration near the inboard end of the tank has a potentially positive consequence. This is because the fuel near the inboard end of the inner wing tank tends to be consumed first by the engines. Thus, the concentration of the remaining JET A fuel on-board, later in-flight, when low fuel temperatures might be encountered in the case of low OATs, will be less than at take-off.

This gives a higher confidence margin that low concentrations of JET A in JET A1 will have a freeze point similar to JET A1 and can thus be treated as JET A1 with respect to the cold fuel alert.