The Importance of Viscosity in Flight

It’s easy to fly. Buy your ticket, check in, drop off your bag, find your seat, and buckle up! A well-traveled passenger knows turbulence might come up unexpectedly or the landing might be the roughest part of the flight. However, no one considers the fuel viscosity in their travel plans. Fortunately, fuel labs take care of that for us!

Know the Flow

At every stage between refining and the aircraft tank, the quality of fuel is checked by sampling and laboratory analysis. These checks ensure that the fuel conforms to requirements specified for the grade when it is delivered to the aircraft. While several parameters of jet fuel are measured to define its properties, its kinematic viscosity is determined mainly for safety.

The operation of jet aircraft at high altitudes over long periods of time dictates good low- temperature flow behavior of jet fuel. If the fuel’s viscosity is too high during fuel injection, then the pilot may not be able to quickly relight the engine during flight. Not a good feeling at any altitude. On the other hand, if the viscosity is too low when involved with the fuel pump, then a lack of lubrication can lead to mechanical failure of the system.

ASTM Certification

The importance of measuring viscosity prompted the American Society for Testing and Materials (ASTM) in 1937 to publish precise standards titled, “Test Method for Kinematic Viscosity of Transparent and Opaque Liquids,” commonly known as ASTM D445. Only calibrated capillary viscometers, often referred to as Ostwald or Ubbelohde viscometers, have been certified by ASTM as meeting D445 criteria. Ostwald/Ubbelohde viscometers measure a liquid’s volumetric flow rate through multiple capillaries. Because change in rate per unit temperature is significantly higher for viscosity than for other properties, temperature control is the single most important parameter for obtaining accurate and precise kinematic viscosity measurements.

ASTM has released only one other test method for jet fuel viscosity: D7042 “Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer (and the Calculation of Kinematic Viscosity)”. In interlaboratory studies guided by ASTM, Anton Paar’s Stabinger Viscometer™ SVM 3000 has undergone the stringent testing required for aviation fuel certification for both the traditional petroleum-based jet fuels according to D1655 as well as for aviation turbine fuel containing synthesized hydrocarbons according to D7566.

Jet Fuel Grades

Depending on grade and usage, aviation fuels are measured at +20 °C, -20 °C, -30 °F (-34.5 °C) or -40 °C. The relevant specifications are stated in the respective standards and regulations. The table gives an overview about the viscosity specification of some jet fuel grades.

Jet fuel grade Kinematic viscosity [mm2/s] Specified at
Jet A/A-1, JP-7, JP-8, AVTUR Kerosene, Jet fuel according to ASTM D1655 blended with SPK max. 8.0 -20 °C
JP-5 High flash point jet fuel max. 8.5 -20 °C
JP-7 Low volatility jet fuel max. 8.0
max. 15
-20 °C
-34.5 °C
No. 3 Kerosene min. 1.25
max. 8.0
+20 °C
-20 °C
TS-1 Premium Kerosene min. 1.30
max.8.0
+20 °C
-40 °C
TS-1 Regular Kerosene min. 1.25
max. 8.0
+20 °C
-40 °C
T-1 Regular Kerosene,
T1-S Special Kerosene
min. 1.50
max. 16.0
+20 °C
-40 °C
T-2 Wide-cut Kerosene min. 1.05
max. 6
+20 °C
-40 °C
RT Premium Kerosene min. 1.25
max. 16
+20 °C
-40 °C
AVCAT/FSII Kerosene with high flash point max. 8.8 -20 °C

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Image ©iStock.com/Cameron Strathdee

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