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Is measuring jet fuel Viscosity more accurate than Freezing Point?

September 3, 2025

The freezing point test has been the primary method for evaluating jet fuel performance in cold conditions for a long time. The logic was simple: if the fuel doesn’t freeze, it should flow. In practice, this isn’t always true. Recent studies show that viscosity – the measure of a fuel’s resistance to flow – is a much stronger predictor of whether jet fuel will reliably move through pumps and lines at low temperatures.

Freezing Point is losing dominance

According to a Coordinating Research Council (CRC) analysis, engine original equipment manufacturers (OEMs) show greater concern for how well fuel flows through the aircraft system, especially at low temperatures – not just whether it has solidified in the tank. The study concluded that “freezing point test is not an effective test for predicting fuel flow behavior in the aircraft at low temperatures.” In other words, just because a fuel sample hasn’t frozen doesn’t mean it will flow easily enough to support engines, pumps, and auxiliary systems.

Viscosity, on the other hand, directly measures how easily fuel can be pumped, atomized, and delivered where it’s needed. High viscosity can limit fuel movement, which leads to reliability risks, especially during long-haul flights where fuel temperatures drop significantly.

Recommendations from the CRC Report

  • Develop or validate a low-temperature scanning viscometer capable of accurately predicting pumpability.
  • Establish a viscosity-equivalent limit that can be used in fuel specifications.
  • Investigate chemical changes affecting viscosity at low temps.
  • Assess OEM support for replacing freezing-point methods with viscosity-based criteria.

Based on these recommendations, the aviation industry is shifting from theoretical freeze thresholds to flow-based diagnostics.

Enter the JFA-70Xi – A Modern Three-in-One Analyzer

Responding to this industry need, Phase Technology (now part of PAC) introduced the JFA-70Xi, the first and only certified analyzer to integrate:

  • Freezing Point (ASTM D5972/IP-435)
  • Viscosity (ASTM D7945) at –20 °C and –40 °C
  • Density (ASTM D4052)

All three tests are performed in less than 15 minutes, without chillers, solvents, or manual pipetting. It features automatic vial injection, self-cleaning and automatically calculates the 12 cSt viscosity limit.

Plus, the JFA-70Xi’s ASTM D7945 viscosity method provides significantly better repeatability and reproducibility than traditional standards like D445.

This innovation directly aligns with the CRC’s push for low-temperature viscosity as a key spec – and even earned inclusion in ASTM D1655, the official specification for jet fuel certification.

Read the full press article here