It happens. Engines—new and used—end up sitting for longer periods than we want them to. That’s not a good plan for longevity, especially in harsh climates with wide swings in temperature and humidity. And so is the case with our still-in-the-crate Rotax 912 ULS—sleeping in an unheated hangar as the rest of the Van’s kit comes together. The concern isn’t so much with corrosion, but more the drying of seals, O-rings and gaskets.
That got me thinking about guidance from engine makers, and what the factory limited warranty prescribes.
BASIC PRESERVATION: HIT THE MANUALS
While not always practical, engine manufacturers say that units stored in excess of 30 days really need some special preservation methods and chemicals, more so if the engine is stored near saltwater or in a similar humid environment.
Tempted to pull the propeller through? There’s an interesting point in Lycoming’s SL L180B on doing so when the engine isn’t run or flown for over a week or so.
“Pulling the engine through by hand prior to start or to minimize rust and corrosion does more harm than good. The cylinder walls, piston, rings, cam and cam follower only receive splash and vapor lubrication. When the prop is pulled through by hand, the rings wipe oil from cylinder walls,” the bulletin says.
Additionally, Lycoming says the cam load created by the valve train wipes oil off the cam and followers. After two or three times of pulling the engine through by hand without engine starts, the cylinders, cam and followers are left without a proper oil film. Starting engines without proper lubrication can cause scuffing and scoring of parts, resulting in excessive wear.
Lycoming offers a few procedures for installing a preservative, including draining the engine oil and replacing it with a preservative oil mixture. It consists of one part by volume MIL-C-6529C Type I concentrated preservative compound added to three parts by volume of MIL-L-6082C (SAEJ1966), Grade 1100, mineral aircraft engine oil or oil conforming to MIL-C-6529C Type II.
Remove the top spark plugs and through the spark plug hole, spray the interior of each cylinder with approximately two ounces of the preservative oil mixture using an airless spray gun (one is the Spraying Systems Company Gunjet Model 24A-8395 or equivalent). In the event an airless spray gun is not available, a moisture trap may be installed in the air line of a conventional spray gun. Reinstall the spark plugs and don’t turn the crankshaft after the cylinders have been sprayed. And remember, of course, the preservation mixture won’t lubricate the engine, so don’t run it.
If the aircraft is stored in a region of high humidity, or near a sea- coast, it is better to use dehydrator plugs instead of merely replacing the spark plugs as prescribed. Cylinder dehydrator plugs, MS-27215-2 or equivalent, may be used.
Preferably before the engine has cooled, install small bags of desiccant in exhaust and intake ports and seal with moisture-impervious material and pressure-sensitive tape. Any other opening from the engine to the atmosphere, such as the breather, and any pad from which an accessory is removed should likewise be sealed.
Firmly attach red cloth streamers to any desiccant bags installed in the intake and exhaust passages to ensure material is removed when the engine is made ready to fly again. Streamers should be visible from outside the aircraft. The propeller should be tagged, “Engine preserved—do not turn the propeller.” This isn’t exactly a do-it-and-forget-it task.
At 15-day maximum intervals, a periodic check should be made of the cylinder dehydrator plugs and desiccant. When the color of the desiccant has turned from blue to pink the preservation procedure must be repeated.
Ready to fly again? Remove the seals, tape and desiccant bags. Use a solvent to remove the tape residue. Remove the spark plugs or dehydrator plugs. With the magnetos off, rotate the propeller by hand through sufficient rotation to remove excess preservative oil from the cylinders. Drain the remaining preservative from the engine through the sump.
Got a Continental? To prepare the engine for long-term storage per Continental’s SIL 99-1, drain the engine oil and remove and replace the oil filter with a new one and service the engine to the proper sump capacity with oil conforming to MIL-C-6529C Type II. Fly the aircraft for one hour at normal operating temperatures.
After flight, remove all the ignition leads and remove the top spark plugs. Spray atomized preservative oil that meets MIL-P-46002, Grade 1, at room temperature through the upper spark plug hole of each cylinder with the piston at bottom dead center position. Rotate the crankshaft as opposite cylinders are sprayed. Stop the crankshaft with none of the pistons at top dead center. Re-spray each cylinder and install the top spark plugs, but not the leads. For long-term storage, Continental says to install dehydrator plugs in each of the upper plug holes, making sure each plug is blue in color. Use the same care in labeling the engine as described in the Lycoming section here. Continental recommends the cylinder bores of all engines prepared for indefinite storage be re-sprayed with corrosion preventive mixture every 90 days.
FLY IT OR DON’T EVEN START IT
Tempted to make tracks to the airport and fire up a slumbering engine for a few minutes of ground running? It won’t get hot enough to do much good. It might do more damage than good. To quote Lycoming again: “Engine temperature and length of operating time are very important in controlling rust and corrosion. The desired flight time for air-cooled engines is at least one continuous hour at oil temperatures of 165 degrees F to 200 degrees F at intervals not to exceed 30 days, depending on location and storage conditions.” That one hour of operation does not include taxi, takeoff and landing time. According to Lycoming, if recommended oil temperatures are not obtainable, contact the aircraft manufacturer for oil cooler winterization plates.
More than one tech told us that engines that are not operated in flight to normal operating temperatures at least once a week should be managed under some preservation or storage program to reduce the effects of corrosion.
In our view, the ground running of an engine is not a substitute for flying it when it comes to dispelling moisture. Running it on the ground simply doesn’t get the engine hot enough, plus it tends to cause uneven heating at higher power, so you’re likely just wasting fuel doing so.
Remember, all preservation/storage methods are concerned with one goal—keeping moisture and other corrosive agents from contacting metal surfaces by placing some type of protective coating or barrier between the metal and the corrosive-causing environmental conditions.
ABOUT THAT MOISTURE
According to Lycoming, “Our experience has shown that in regions of high humidity, active corrosion can be found on cylinder walls of new engines inoperative for periods as brief as two days. In engines that have accumulated 50 hours or more time in service in a short period, the cylinder walls will have acquired a varnish that tends to protect them from corrosive action; such engines under favorable atmospheric conditions can remain inactive for several weeks without evidence of damage by corrosion.” Lycoming goes on to reiterate that aircraft operated close to oceans, lakes, rivers and in humid regions have a greater need for engine preservation than engines operated in arid regions.
Conversely, for those engines subject to moderate amounts of blowby and relatively high oil consumption, accumulating moisture and acid in the oil is a foregone conclusion, regardless of how often the aircraft flies.
In some respects, we can do only so much to keep the damaging impact of corrosion to a minimum, but some form of storage should be considered when limited operation and frequent periods of downtime are expected.
Corrosion is also supported from the byproducts of combustion, which include moisture that contaminates the engine’s lubricating system and helps to form acidic byproducts in the oil.
Also, think in terms of shelf life because this would also be applicable to engines removed from the aircraft awaiting reinstallation at a later date. We know plenty of new and newly overhauled engines sit for long periods of time in unheated hangars. Our spanking-new Rotax is one.
That’s why we’ll put it on the aircraft, ground run it for a bunch of hours, dump the oil, send some of it off for an oil analysis and hope for the best.