Is Measuring Jet Fuel Viscosity More Accurate Than Freezing Point?

The International Air Transport Association (IATA) had a meeting in 2008 where they decided they wished to determine if there is a potential replacement for the freezing point methods used to determine jet fuel viscosity. Their Coordinating Research Council (CRC) were tasked with this mission.

The request was the result of the IATA’s Freezing Point Harmonisation Group, who aimed to determine if it would be possible to use a single grade of jet fuel globally, with respect to the freezing point.1 The reasoning being that using just one grade globally would have significant benefits in terms of economics and fuel production.  

Viscosity Measurement

The IATA had research several different possibilities, with a focus on the freezing point around jet A and jet A-1 limits. The study identified several different candidates which warranted further research, the study also found that engine manufacturers were more interested in the viscosity of the fuel as it entered the engine as opposed to the freezing point of the fuel in the tank. The report concluded that “it is a generally accepted fact that the freezing point test is not an effective test for predicting fuel flow behavior in the aircraft at low temperatures”.2

From the need to develop a new testing method a CRC report was produced which analyzed the current state of manufacturer fuel systems, testing methods and research into the low temperature properties of aviation fuels.3 A key point from the report was that fuel viscosity could be a better indicator of fuel pumpability than fuel freezing point.

Four specific action points were agreed upon:

  • A low temperature scanning viscometer, which can determine the pumpability of fuel at low temperature, should be produced
  • A viscosity equivalent limit should be identified
  • Any chemistry changes to the viscosity should be identified
  • The support from manufacturers for replacing freezing point analysis with viscosity analysis should be determined

Phase JFA-70Xi System

The Phase JFA-70Xi is an encouraging new system for the measurement of viscosity, density and freezing point which is approved for aviation fuel certification. The JFA-70Xi uses conventional freezing point testing (ASTM D5972/IP435) and novel viscosity testing (ASTM D7945). The system can measure the kinematic viscosity of aviation fuel at -20°C and -40°C whilst also measuring fuel density.

ASTM D7945 is a method of freezing point testing which uses air pressure to drive a column of test sample through a capillary tube. The rate of motion across several calibrated timing segments are then correlated to the fuel viscosity by following the Hage-Poiseuille principle.

An ASTM study carried out in 2016 on conventional aviation fuels (including Jet A and Jet A1, which satisfies D7566, and a 50/50 mix) has shown that the new method, D7945, is more precise than the existing viscosity test methods, D445 and D7042.

The test was shown to have a reproducibility of 0.021 mm2/s and a repeatability of 0.011 mm2/s - more precise than D445 by a factor of four. The new D7945 method is unique in that it has approved precision at -40 °C, which is a more accurate emulation of the temperature jet fuel is pumped at.

The JFA-70Xi can also determine the temperature at which the fuel attains a viscosity of 12.0 mm2/s (following ASTM D341) using the measured viscosities at -20 °C and -40 °C. The 12.0 mm2/s viscosity is an essential figure for the use of auxiliary power units (APUs) during ETOPS (Extended Twin engine Operations). Evaluating the 12 mm2/s figure with the corresponding measured freezing point means the instrument can also determine if a temperature of -20 °C and -40 °C is best for flight operation.

An illustrative plot when the 12 mm2/s temperature is calculated to be colder than the freezing point.  In this instance, the calculated 12 mm2/s temperature will not be reported as the fuel will be in a two-phase regime, rendering the calculation rationale of D341 unsuitable.

By proving the measurement of both viscosity and the freezing point in one system (at the required temperature of -20 °C and the high potential -40 °C) the JFA-70Xi is a revolutionary instrument which can deliver a highly detailed measure of jet fuel pumpability at low temperature.  

Phase Technology was recently acquired by PAC. This makes PAC-Phase the number one provider of Jet Fuel analysis solutions. PAC's portfolio includes: the world renown JFTOT, OptiReader, OptiFZP and the Intelligent Heater Tubes used by labs worldwide.

References and Further Reading

  1. Fuel Freezing Point Harmonization, Final Report, February 2008, IATA Technical Fuel Group (TFG) Task Force.
  2. Ibid
  3. CRC Project AV-11-09, “Develop an Aviation Fuel Cold Flowability Test to Replace Freezing Point Measurement”, G. Richard, 2010.
  4. 2016 ASTM Research Report RR D02-1833, “Interlaboratory Study to establish statements for measuring the kinematic viscosity at -20C and -40C, and then calculating the temperature when the sample would reach 12 mm2/s using ASTM D341”

This information has been sourced, reviewed and adapted from materials provided by PAC L.P.

For more information on this source, please visit PAC L.P.


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