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Home> Maintaining
your Continental and Lycoming engine
Quick Tip For Pilots - The Dipstick Check
You've pulled the dipstick and checked the oil level - fine, but
there's more information contained on that dipstick than just the quantity
of oil in the sump. Each time you look at the oil on the dipstick notice
its color. After an oil change it should be clear with a tinge of brown.
As more hours accumulate on the oil it will turn a darker brown. On some
engine breather pipe.
A Dynamic Ring Leakage Test
The mechanic checks for ring sealing by performing a compression
check; this check is always done with the rings stationary in the cylinder.
Ring sealing can be good under these static conditions and not good during
actual engine operation
the rings are going up and down the cylinder wall, and in and out of the
piston ring grooves. Ring leakage occurs under these dynamic conditions
where it may not when the piston and rings are stationary and the engine operation. It's quicker
and you can spot ring leakage problems sooner. Some oils, such as Exxon Elite, may produce darker
color oil then say Aeroshell oil 15w50.
Ok, so now I have oil that's quickly turning black, so what?
Heat from combustion flows into the piston dome and out the piston
rings and into the cylinder wall and then into the atmosphere. This is
why the cylinder barrel has cooling fins--to cool the piston and rings.
Leakage of combustion gas past the piston ring belt prevents the piston
and rings from dissipating heat into the cylinder. The piston and rings
overheat usually causing the piston rings to stick from the burnt oil in
the ring grooves. When the rings stick several things can happen:
 The rings may stick in the groove
causing massive amounts of combustion gas to blow into the engine oil to blow out the engine oil sump.
Either way the problem is not self correcting and therefore needs
fixing.
When you see that nice tan oil on the dipstick you know that the
piston rings are doing their job of sealing the combustion gasses in the
cylinder head where they can do their job of producing power. With a couple
of more checks in future articles you will be able, as a pilot, to tell
the mechanic whether the compression in your airplane engine problems, especially those that involve bronze bushings,
start off by wearing microscopic pieces of metal off the part and depositing
them into the oil. Checking for these fine metal particles involves an
inspection of the oil screen / filter or sending the oil out to a lab for
spectroscopic oil analysis. There is another test that you as the pilot
can do each time you check the oil level. Although it does not replace
the traditional inspections described above, this test is quick, easy,
and can be done before each flight.
When you check the oil level on the dipstick let some sunlight
shine directly onto the oil on the dipstick. Fine shiny metal particles
(especially bronze) will sparkle in the sunlight. I have seen oil sparkle
from bronze particles that were so small you couldn't see them without
the sunlight shining off of them. Normal engine oil does not sparkle in
the sunlight.
This test doesn't detect many types of metal contamination in
the oil so it doesn't give you a clean bill of health if the oil doesn't
sparkle. Its value is in:
- Finding possible problems before you would using other techniques
- Finding possible problems that other techniques may miss.
Both of these checks use good pilot techniques of observation
and knowing whether something looks normal or not normal.
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Believe it or not your chute is now open. Parachute Sense
US Navy 1944
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