The Answer Book

Oil analyses can help you discover engine problems before they turn into major failures.  But the analysis information gained is only as good as the sampling procedure.  Also, a single test is not enough to reveal trends and significant changes and can only tell you if there is already a serious problem like a scuffed piston.  Take oil samples properly. For best results, take the sample about midway through the draining of hot oil from your sump.  A sample pulled off at the beginning or end of the oil change may appear dirtier than it really is.  Sample the oil the same way every time.  An improperly taken sample can lead to some seriously inaccurate conclusions about engine malfunctions.  Rely on a series of consistent tests over time.  You’re looking for significant changes or trends over time, not absolute values.

Both Lycoming and Continental recommend oils qualified under the following specifications for use in their engines:

– SAE J-1899 former MIL-L-22851 (for ashless dispersant oils)

– SAE J-1966 former MIL-L-6082 (for straight mineral oils, intended primarily for break-in)

Military and SAE specifications are the same except for some additional packaging requirements for the military. In the future, the military specification may be dropped, although oil containers will still probably refer to the former military specification. AeroShell® straight mineral oils, AeroShell Oil W single grades and AeroShell Oil W multigrade oils all qualify under their respective specifications. The oil requirements for other aircraft engines such as Pratt & Whitney are less defined. All AeroShell and AeroShell Oil W oils are qualified for use in P&W radial piston engines. The oils for engines no longer in production may be listed by military specification or by product name. For more information, talk to an overhaul or repair shop that specializes in a particular engine, or call the Shell Technical Information Center at 1-800-231-6950.

The selection of a proper grease is clearly defined. For each grease point on a certified aircraft, the military specification or the qualified product is listed. AeroShell® greases are qualified under the following specifications:

AeroShell GREASE 5 former MIL-G-3545-C

AeroShell GREASE 6 MIL-G-24139A, former MIL-G-7711A

AeroShell GREASE 7 MIL-PRF-23827C, TYPE II

AeroShell GREASE 14 MIL-G-25537C

AeroShell GREASE 16 former MIL-G-25760A, BMS-3-24A

AeroShell GREASE 17 MIL-G-21164D

AeroShell GREASE 22 MIL-PRF-81322F, Grade 2 DOD-G-24508A

Yes, low oil temperature can lead to excessive rusting and corrosion of critical engine parts.  When an aircraft sits on the ramp or in a hangar, the engine heats up during the day and cools again at night.  While the engine is cooling, some of the moisture in the air condenses on the engine walls and drops into the oil.  This can form rust on internal engine components.  The moisture can also react with by-products of combustion in the oil,forming acids which can lead to corrosion.  The best way to remove this water is for the engine to boil it off during flight.  Studies have shown that the temperature of your engine oil increases about 50°F as it circulates through the engine.

Therefore, unless the oil temperature reaches 170°F to 180°F during flight, the engine will not boil off the water that has accumulated in the crankcase.  The result: rust and corrosion.  Note that an excessively high oil temperature will also cause problems.  Here are some tips to help avoid oil temperature problems:

Check your oil temperature gauge for accuracy. It should read about 212°F when the sensor is placed in boiling water.

Monitor the oil temperature during flight. It should be about 180°F even in winter. If it is lower, you may need a winterization kit. Otherwise, check with your mechanic to see what is causing the excessively low oil temperature.

A number of pilots have asked this question.  The answer is a definite no.  When Shell first started evaluating multigrade aviation piston-engine oils over 25 years ago, testing proved that multigrades formulated only with mineral base oils did not have adequate base oil viscosity (thickness) to properly lubricate all high load points in the engine.  Then we tested and flight evaluated a formulation made with all-synthetic base oils.  This formulation had excellent antiwear characteristics in all tests run.  However, in the flight evaluations, some engines would reach 600 to 900 hours, then lose oil consumption control and/or compression.  When the engines were disassembled, we found that the piston rings were covered with a gray tacky substance that was primarily made up of the lead by-products of combustion (from the use of leaded aviation gasoline).  Although synthetics are excellent lubricants with good high temperature stability and very good low temperature flow characteristics, they are relatively poor solvents.

Whenever fuel is poured, pumped or moved from one container to another, a static charge is generated.  The same principle is in effect when you walk across a carpet in the winter and get a shock from a doorknob.  The charge level and the distance that can be jumped or arced depends on several factors—pump rate, temperature, humidity and containers.  Static electricity is the reason why a ground wire is always connected to commercial airliners and transport trucks whenever fuel is being transferred.  When you transfer fuel into your car or light aircraft, the hose has a built-in ground wire that acts as an electrical path to dissipate any static charge.  As an added precaution, there is usually an excessively rich air/fuel ratio in the fill pipe which will not burn.  There are two primary areas where a pilot should exercise caution when transferring fuel.  First is draining an aircraft tank.  For example, if you’re draining a wing tank, you should always connect a jumper cable from the plane to the fuel container.  This will dissipate the charge and eliminate the chance of a spark jumping from plane to container, causing a fire.  Remember, when you’re draining fuel, there can be enough air circulation so that the air/fuel ratio is in the burnable range.

Pilots are always taught to “trust your gauges,” which is a critical lesson especially when flying IFR.  Most of us apply this lesson to our engine as well.  However, another part of this lesson should be to periodically check the calibration of all instruments, including oil temp, tach and oil pressure gauges.  Let’s remember that quite a few general aviation aircraft are over 20 years old.  So it’s not surprising to hear numerous reports of tachs being off by several hundred RPM and temp gauges being off by 10, 20, even 30 degrees.  It’s important to have your gauges checked and calibrated periodically.  One method used is to put marks on the oil temp gauge so that the “preferred” range can be easily seen.  (The “green” band on many oil temp gauges starts at just over 100°F, which is okay for taking off, but too low for normal cruise.)  Remember, oil temperature is one of the most critical parameters to be measured and controlled.

A good rule of thumb for changing piston engine oil is to change it every four months.  Of course for every rule, there are at least two exceptions.

Exception #1: If you're able to fly frequently with proper oil temperature, you should adjust the four-month rule accordingly.  Change out your oil after 50 hours if you've flown the hours in less than four months.  If your engine doesn’t have an oil filter, change it after 25 hours.  Always remember: the four-month rule is the most critical.

Exception #2: In recent years, the annual flight hours of many private planes have decreased.

Preheating your engine makes a world of difference.  This procedure heats the oil so it’s thin enough to flow through the engine and properly lubricate all critical wear surfaces.  Preheating also heats the metal parts in the engine.  That’s important because aluminum crankcases have a higher coefficient of thermal expansion than iron crankshafts.  This means as your engine cools down, the clearance is reduced.  And as a result, you may not have sufficient oil film thickness for proper hydrodynamic lubrication at very cold temperatures.  In other words, the wear rate is going up.  If you’re using a pan type heater, make sure it’s a system that heats the whole engine, not just the oil.  Also, some “oil pan” heaters can raise the oil pan surface temperatures to over 300°F which, over time, can decrease the performance of the oil.  One final note of caution on heaters: Do not plug in a heater and leave it on for extended periods of time.  If you have moisture in your oil, the heater will increase vaporization, which will condense on the cool, nonheated engine parts and increase rusting.

Baffles and seals are critical to keeping an engine cool, yet they’re often overlooked.  When you’re flying, air enters the cowling and creates static pressure above the engine.  This pressure then forces cool air down through your cylinders and oil cooler to the lower pressure areas below and behind the engine.  From there, the air travels out through the flaps or other flaring openings.  What’s important to consider is that there is only a given amount of air coming in through the cowling at any given time.  If your baffles are broken or misshaped, the amount of air going past a particular cylinder or area will increase.  And if you increase airflow in one area, then airflow past other cylinders and the oil cooler will decrease, leading to higher temperatures in some parts of the engine than others.