Fuel in Oil Analysis

Hi Michael,

Do you get your oil to 100C or above, for a sustained period, at least once in a days flying?

This recommendation, from Rotax, is intended to drive off volatile fractions/contaminents in the oil.

Volatile fractions include fuel residues and others.

😈

During normal flight it goes over 100 C.

But for maintenance I just heat it up a little and don't go flying before I take the oil sample. I will do that the next time. I read about that in the manuals too, but still surprised that fuel can get into the oil. 

I've seen diesel fuel contamination in regular 91 oct. direct from the gas pumps and this has seemed to cause this issue.

"During normal flight it goes over 100 C."

Hmm! - What do you mean by "normal flight" ?

In my world there are basically three stages of flight; Take-Off & Climb - Cruise - Descend/Slow to Landing.

My engine oil gets to Maximum temperature during Climb (which can happen again after TO/Climb eg Climb to new Cruising Altitude above Cloud).  The temp reached, depends on duration of Climb & ambient temperature. I usually see 100-110C.

In Cruise I would expect your temperature to stabilize somewher between 90-100C.

In Descent, your oil temperature may drop to somewhere between 85-95C

"....surprised that fuel can get into the oil. "

Internal Combustion Engines are not very efficient. A 100% fuel burn doesn't happen.

Traces of unburnt fuel and byproducts of combustion get past the piston rings, enter the crankcase & contaminate the oil.

I would be surprised if the engine oil analysis did not show traces of unburnt fuel, as well as a range of other contaminants.

It's the amount of fuel (as a %) that is important AND if this is changing (trend) over consecutive analysis ie a one off test, is almost useless.

You should also know that for the analysis to have validity, the number of variables should be kept as low as possible ie always use the same crankcase oil, take the oil sample at the same Hobbs hours each time. Always sample after a flight where engine & sustained oil temperatures, have been achieved (I suggest a minimum of 1 hr, involving at least one Climb to get the engine oil to 100+C).

If taking the sample after a sustained flight does not work for you, better to take the sample cold, rather than a short ground run, which almost certainly will deliver an atypical sample (fuel contamination?)

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