Aviation Safety Letter 3/2004
Jelly in the Fuel Filter Causes Engine Failures
The owner of a Challenger ultralight aircraft had used over 6 400 litres of a 50:1 mixture of Shell Gold Premium fuel and Quaker State TC W3 oil, without a problem. In January, he decided to use a new fuel mixture using synthetic instead of mineral oil in his Rotax 503 DCDI engine (dual carburetor and dual ignition). Immediately following this change, he experienced the first of several in-flight engine failures caused by fuel starvation. Inspection of the fuel filter revealed a gold coloured jelly-like substance blocking the screen of the Kimpex 07-245 filter. This prevented the fuel from reaching the carburetors. Fortunately, the engine failures did not result in a forced landing because the aircraft was equipped with a parallel electrical fuel delivery system that the pilot activated to restart his engine.
In subsequent flights, it was necessary to replace the filter four times because of repeated contamination by the jelly-like amber coloured substance. He noted that it was of a colour similar to the synthetic oil that he was using. In order to find the cause of this chemical substance, the pilot decided to do some experimenting on the ground. First, he had to determine that it would occur, so he set-up a 25 L Jerrycan of a 50:1 mixture of new fuel and oil identical to the one he used in flight. He installed a Kimpex filter and a length of clear 5/16-in. fuel line between another similar container. He transferred the mixture from one container to the other, using a 2-5 psi pump, in temperatures similar to those he encountered in flight; approximately -20°C. He obtained the same results, a significant presence of an amber-coloured goo or slime in the filter screen. He repeated the experiment several times. The manufacturer was asked to proceed with similar tests, but has been unable to reproduce the same results. The fuel filter blockage only occurred when the pilot switched from a mixture of fuel and mineral oil to one with synthetic oil.
Fuel suppliers modify the chemical constituents of their fuel from time to time to take into account the seasonal temperature fluctuations, regional use and the various octane-rating environmental factor requirements. Chemicals vary from one fuel and engine oil manufacturer to the next. The use of different fuel and oil may change the performance of your engine. In this case, the engine kept running until fuel flow to the carburetors stopped. The chemical reaction creating the substance seemed to have been occurring inside the fuel filter; more specifically within the fuel filter element walls. The filtering element itself seemed to have served as a catalyst. This is purely hypothetical, however, as we do not know the cause at this time.
As an aircraft owner, your responsibility is to ensure that the fuel and oil you use is of a type recommended and approved by your aircraft engine manufacturer. If any of you have experienced similar situations, we would like to hear from you. Please e-mail me at firstname.lastname@example.org. Thank you.
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