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Aircraft Oil Sampling - Dirt, Silica and Silicon

Dirt is by far the most typical cause of high wear metals in the aircraft oil sample. If you find the wear metal quantities are high, and the amount of dirt is high, its pretty evident the dirt is creating excessive amounts of wear metals. The silica (also known as quartz) component of dirt causes abrasive wear. Silica, combined with metallic oxides known as silicates, are the basis of sand and nearly all rocks. When silica combines with carbon, it forms an abrasive called "Carborundum" which has a hardness near that of diamond. Silica is an oxide of silicon called silicon dioxide (Si02). Oil analysis labs do not directly analyze for dirt. Most of the silicon the lab reports is from silicon dioxide. The lab readings therefore tell you the amount of silica in the oil. To make it easy, just replace the word dirt for silicon.

The maximum amount of silica (free silicon) in an aircraft oil sample should be a maximum of 5 ppm for a four cylinder engine and 10 ppm for a six cylinder engine for 50 hours of oil use. As a "ball park figure", this would be equivalent to 2.5 to 5 ppm for 25 hours or 10 to 20 ppm for 100 hour oil changes.

Dirt is not the only source of silicon in the oil. The additive package in ashless dispersant oils tends to cause the oil to foam. Silicone is used in the oil as an anti-foam agent. Silicone is a polymeric silicon compound based on the elements silicon and oxygen. Approximately 6 to 10 ppm of the silicon reading is from the silicone that was added to the oil. Different brands of oil differ on the amount of added silicone. Consequently, the amount of silicon the lab detects is somewhat dependent upon the brand of oil. Mineral oil does not contain silicone since there are no additives that induce foam. If you switch from a mineral oil to an additive oil, you may see an increase in silicon in the oil. Some labs subtract part of the silicon reading from the report so the reported reading is based more on the amount of dirt present in the oil. If you had an identical analysis sent to a lab which did not subtract out some of the silicon or which subtracted out a different amount, then the readings may vary widely.

The tech data for DC4, typically used to lubricate oil filter gaskets, shows that it contains silica as a thickening agent. If too much DC4 is used on the gasket some will wind up inside the filter and get into the oil, giving a high silica reading. In this case, wear metals will remain low while silica is high, probably due to particle size. You might also find some small deposits of DC4 on the filter element during inspection which could be interpreted as small globs of a smooth, soft sludge. To avoid this condition and possible confusion or misinterpretation, use only a very thin coating of DC4 on the gasket face only.

We know how much dirt is in the engine from aircraft oil analysis. We know how much dirt is bad; but we need to know how bad the dirt is! For this we need to consider particle size, shape and hardness. Dirt or any other particle do not cause wear unless the particle is larger than the oil film separating the parts. A rather low-tech method of determining whether the particles are of sufficient size, shape, and hardness to do damage, is to place a drop of contaminated oil between two glass microscope slides and rub the slides together. Then examine the slides with a microscope for scratches; if the glass scratches then so will your crankshaft journal. Super fine dirt causes damage if there's a sufficient quantity for the dirt to agglomerate. There should not be high levels of silicon in the oil. If there are high levels of silicon and wear metals, it indicates the oil should be changed and the induction system and intake filter inspected for leaks.

An easy check of the aircraft induction system is to remove the air filter and reach back into the air box and feel for grit. If it feels gritty, your engine is ingesting dirt. Inspect the condition of the air box. Possibly the shaft bushings on the alternate air door are worn, allowing the door to suck open during engine operation. After a sand storm, dirt may have collected on the alternate air door and in the engine cowling only to be sucked into the engine when you check carburetor heat during engine run-up. Another follow-up oil analysis after a short period should be made to see if silicon contaminate and wear metal levels have lowered to a normal level.

If wear metals are high but silicon is normal, then abnormal two-body (metal on metal) wear and part failure are occurring. In this case, an inspection of the engine is warranted. The proper method of using oil analysis is to base readings on a trend from one sample to another and use the same lab for each sample. This method eliminates variances in lab technique and the effect the oil additive package has on final readings. Take the oil sample shortly after engine operation so the oil has been circulated throughout the engine. Don't sample the first glob of oil that drains out; it's the part that contains any sludge that has accumulated on the bottom of the oil pan.

How do you determine silica content in the engine if the oil has been drained or flushed? Oil only tells you what the engine has ingested since the last oil change; but just as a geologist who can read the rocks, you can analyze carbon and lead deposits for dirt content. Lead and silica have a natural affinity to one another. Combining silica and lead oxide forms lead silicate. This process is slow, taking as much as twenty to fifty hours of engine operation. Spark plug fouling because of lead deposits increases as the engine ingests more silica (assuming your using a leaded gasoline). Lead deposits from the spark plug or carbon deposits from the piston dome contain a certain amount of silica. Geologically older deposits are found trapped in the recess between the barrel and head of the cylinder. Recent deposits can be found in the induction system trapped in the intake hoses. These deposits should be scraped off the parts with a non-metallic scraper and sent to a lab for analysis. The lab needs to report on the amount of silicon and aluminum oxide in the sample. The sample is very accurately weighed. Then the organic material is dissolved out of the sample. The remaining material, silica and aluminum oxide, is then weighed. The before and after weight of the sample gives you a ratio of dirt to total deposits. Silicon and aluminum oxide added together should represent no more than 4-1/2% of the total sample. Any amount of dirt above this is enough to wear out the piston rings. This type of analysis is used more as a failure analysis than as a preventive measure.

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Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.