The performance and safety of your aircraft depend on this procedure.
At my firm, Aviation Marketing Services, we’re often asked about soak testing, which is used to verify the purity of aviation fuel.
If you’re responsible for—or depend on—fuel in your work, you likely know how important it is to perform a soak test after completing new construction or major repairs to tanks or piping. This applies to both fuel storage systems and servicing vehicles. Afterward, a laboratory evaluation of the fuel samples used in the soak test can detect any potential contaminants—from solvents used in coatings and linings, welding flux, preservative oils (corrosion inhibitors), valve grease, and other debris—that could compromise the performance and safety of the fuel.
Because soak testing is such an important step in purchasing fuel, any acquisition or modification contracts for new fuel systems or servicing vehicles should include a clause that requires the manufacturer or contractor to provide evidence that a proper soak test has been performed. The clause should also require that the test results verify the fuel meets the appropriate ASTM International specifications.
Let’s review how to conduct a soak test and the various lab tests involved.
Fuel Systems, Storage Tanks, and Related Equipment
A soak test consists of filling a fuel system (stainless steel, aluminum, epoxy lined, or rubber bladder) with an adequate volume of the appropriate-grade fuel and, after following the recommended recirculation procedures, allowing it to soak for a period of time recommended by ASTM or the specific fuel supplier. Before putting the fuel in the system, be sure to retain a sample to serve as a control batch should testing reveal off-specification product.
By following the stringent requirements of Energy Institute (EI) Standard 1541, Requirements for Internal Protective Coating Systems Used in Aviation Fuel Handling Systems, you’ll dramatically reduce the risk of fuel contamination. Adherence to this industry standard ensures that the proper coating materials were correctly applied and allowed to fully cure as recommended by the manufacturer, and that storage tanks (including piping, pumps, valves, meters, filter vessels, and so on) are filled to the normal level and the fuel recirculated completely at least once and allowed to soak for a minimum of four days and a maximum of seven.
At the end of the designated soak period, obtain a 1 gallon sample from the new or repaired system and send it off for laboratory evaluation. The best location from which to obtain a sample is the low-point drain. Remember to displace an adequate volume in the sampling piping to ensure a truly representative sample of the tank bottom.
Fuel-Servicing Vehicles and Hoses
All fuel-servicing vehicles with tanks and piping made of aluminum or stainless steel should have the appropriate fuel circulated throughout the system. Fueling vehicles (whether new, repaired, or those that have undergone an extended period out of service) should be filled to the normal level and the fuel recirculated completely at least once and allowed to soak for at least an hour. You may obtain 1 gallon representative samples of fuel from any combination of multiple low-point drains and combine them into a single sample.
For proper soak testing, every fueler loading hose and every aircraft fueling hose must meet industry standard EI 1529/IOS (International Organization for Standardization) 1825 for hoses and assemblies. The hose must initially be filled completely with the appropriate fuel and allowed to soak for at least eight hours. The fuel in the hose must then be disposed of properly and the hose refilled.
To verify the absence of any manufacturing residue, you must perform an appearance check of the fuel for discoloration. The fuel should then be recirculated in an amount equaling at least twice the volume of the hose, back into storage, upstream of filtration. Follow up with a hose-end nozzle strainer inspection to confirm the absence of any particulate contamination.
Lab Testing of Avgas and Jet Fuels
In the case of avgas 100LL (aviation gasoline 100 low lead), the critical aspects of contamination are interfacial tension (how well water separates out from the fuel) and gum contamination, which leads to engine anomalies. The tests that should be performed on avgas 100LL are:
- ASTM D4176, appearance
- ASTM D381, gum content
- ASTM D1094, water reaction
- ASTM D2887, simulated distillation (this test is more sensitive to residue and chemical contamination than the standard test for distillation, ASTM D86).
The tests for aviation turbine fuels (also known as jet fuels) are the same as those for avgas 100LL, with the addition of the following:
- ASTM D156, Saybolt color test
- ASTM D3948, MSEP (microseparometer analysis, for water separation)
- ASTM D2624, electrical conductivity
- ASTM D3241, jet fuel thermal oxidation test (JFTOT)
- ASTM D56, flash point.
The JFTOT is notable because it reveals any change in volatility along with oxidation characteristics and evaluates insoluble and soluble materials that form deposits in the engine.
Fuel sample preparation, handling, and follow-through are all key to successfully testing aviation fuel. If a jet fuel sample is drawn through sample points that incorporate metals such as cadmium, brass, or copper, the JFTOT results may fail. Similarly, using galvanized piping (zinc) in avgas 100LL could alter the lab results.
Finally, make sure the sampling point is clean and flushed before taking a sample. Accumulated solid particulate matter or any free water should be removed, and final fuel samples should be clear and bright. Use a 1 gallon, approved epoxy-lined sampling container, and flush and triple-rinse it with the fuel to be sampled and tested.