All About Aircraft Oil :For Piston Engines

All About Aircraft Oil For Piston Engines By Stephen M. Sunseri, ExxonMobil March 2000 A great deal is published in aviation magazines about "best maintenance practices." And there is no doubt that preventive maintenance is the...


In turbo-charged engines, high temperatures are often a concern. Peak temperatures can often be 70 F higher than oil sump temperatures (gauge temperatures). So if engine oil temperature is significantly above 180 F, check baffles to make sure there is good airflow to cool the cylinders. Tell the pilot to keep an eye on CHT, EGT, and lean the engine appropriately.

Oil Consumption
Some oil consumption is good. It indicates that the oil is providing a seal at the compression ring. Low oil consumption may indicate that sealing is inadequate, leading to blow-by, power loss, and increased cylinder bore wear.

So what's the right amount? Well, if engine oil consumption exceeds the oil consumption test limits as defined by the OEM (this is the number that helps determine if the rings have seated during run-in), then it's too high and there may be a problem with an unseated or broken ring.

The maximum limit for all Textron Lycoming engines can be calculated using the formula: .006 X BHP X 4 ΒΈ 7.4 = Quarts per hour

Again, this is the maximum rate of oil consumption allowable during run-in, and averages about one quart per hour. Following break-in, oil consumption should stabilize at some lower rate.

The lower limit is more difficult to establish because there are many variables affecting oil consumption. In the early 1980's, the University of Illinois followed 12 engines through TBO and established average rates of .081 quarts per hour and .095 quarts per hour using multi-grade and single-grade oils, respectively.

Unless there is a problem, engine oil consumption should stabilize and remain somewhere in the range of between 0.08 quarts per hour on the low end, and the oil consumption test limit should be as defined by the OEM on the high end.

Oil Analysis

If changing oil on a timely basis is the most cost-effective insurance for an engine, then oil analysis is certainly the most cost-effective rider you can add to this policy.

As an aircraft engine oil does its job, its composition changes. It accumulates contaminants, such as combustion byproducts, dirt, corrosion particles, and metallic wear particles, which can impair lubrication and accelerate wear. Analyzing the content of this used oil can help you discover engine problems before they become failures. The best part is that oil analysis costs only pennies per flight hour.

There are several things you can do to enhance the accuracy of this analysis. First, take oil samples properly.

Filling the sample container mid-way through your drain will ensure you get a representative sample and NOT all the dirt on the bottom of the pan. And, as stated previously, always drain engine oil when it is hot. Additionally, change the oil and take samples at consistent intervals.

Finally, don't rely on the results of a single sample. Unless something is drastically wrong, trends take time to develop. Trends established over five to ten drains are much more representative of what may be going on in a given engine than any single sample.

Additives
Supplementary additives - even those sold under popular brand names - are expenses your customer can live without. If you've purchased top quality engine oil (SAE J1899), it's unnecessary for other products to be added to the crankcase.

The manufacturer of the engine oil has done all the work necessary to ensure that the oil will perform all the necessary functions the engine requires it to perform. The notable exception, of course, is the Lycoming additive LW-16702, an anti-scuffing/anti-wear additive required for some Lycoming engines (O-320-H, O-360-E, LO-360-E, TO-360-E, LTO-360-E, TIO and TIGO-541 engines).

Frequently Asked Questions About Oil...

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