Understanding Fuel Contamination in the PT6A

The customer had sent in an exchange set of fuel nozzles at his normal change interval of 300 hours. The nozzle set appeared to be reasonably typical in appearance; however, every one of the nozzles exhibited streaking during the spray check as received! Nine of the nozzles cleaned up during the cleaning process and spray checked ok. Five nozzle tips had to be replaced to be able to return the set to service. This was unusual for this operator, and he sought for ideas as to what might be the problem.

The following recommendations were offered:

You might “audit” your fuel filtering systems or perhaps the refueling technique used to see if there is any chance outside contamination could be a factor. One of our customers would begin refueling and conduct fertilizer loading at the same time. Granular fertilizer was hitting the top of the wing and drifting back over and into the fuel fill port. The load hand, while loading fertilizer, was not paying enough attention to the fueling operation to notice the fertilizer entering the fuel fill port.

Another operator had a rebuilt tanker truck that had been a chemical tank truck prior to his buying it. When it had been refurbished as a fuel tanker, all valves had been replaced except for two that appeared to be like new! There were some recesses in the valve bodies that had trapped solid materials. These materials apparently broke loose during fueling operations and contaminated his aircraft fuel system. The valves were changed out and the problem went away.

We recently received an engine for sudden stoppage inspection that had experienced a flame out near the end of the runway on a take-off run. The pace of the spraying operation was intense, and when a rain shower came the pilot and ground crew continued with the operation. What’s a little rain? The rain entered the fuel port during fueling operations. The resulting contamination of the fuel system was enough to fill the high-pressure fuel pump bowl on his PT6A engine 3/4 full of water. Water injection in aircraft engines is something the military has done for a very long time, but it generally doesn’t work well in Agricultural Aviation!

Ron Hollis and Students Swarm the -21, Engine Training 10-15-2009Another operator was complaining that his engine was becoming erratic in its response to power lever changes. It was starting ok but sometimes would idle at its normal 52% and other times would only idle at 48% or 50% Gas Generator speed. He was occasionally unable to reach take-off torque. Subsequent investigation revealed black, sludgy appearing contamination of the high-pressure fuel pump inlet screen and outlet filter. The firewall filter was investigated and found to be heavily contaminated with the same material. This type of material is generally bacterial growth in the fuel supply or in low spots in the aircraft’s fuel system, header tanks, etc. Once the material was removed and the filters replaced the performance returned to normal. Bacteria can become a problem very quickly when the conditions for growth in your fuel system get just right. The presence of moisture and warmer than normal temperatures can accelerate bacterial growth. The aircraft’s fuel system header tank is a notorious place for bacteria to congregate. When the amount of bacteria in a typical header tank gets past a certain point, it can show up in alarming quantities. Generally, if you deal with the moisture in your fuel supply, you get rid of the bacteria.

Fuel contamination can slip up on you! Like the proverbial frog being boiled to death one degree at a time, it can get you before you think it’s a problem.

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