Airframe and Powerplant

Your Engine — Use it or Lose it

November 1, 2000

Engine components can stand almost anything — except corrosion

Our expensive aircraft engines, the trusty metal hearts that pull (or push) us airborne and transport us over the mountains, rivers, and weather to our destinations, don't like to be ignored. They like a lot of action. They like to fly. They're most likely to provide yeomanly service if you fly them every day. If your aircraft engine had a living personality, every time you came into view it surely would act like an enthusiastic canine, dancing on the doorstep with its leash in its mouth, begging to go for a flight.

More engines rust out than wear out, especially for general aviation pilot/owners who may only fly 100 hours or fewer a year. Unfortunately for their engines and wallets, the majority of private airplane owners can't fly every day; most are fortunate if they can fly once a week. This is too bad because the more these engines are flown, the better the chances are that they will go to TBO without costly interim repairs.

One engine manufacturer uses 40 hours a month for extended warranty coverage adjustment. When engines are operated at this frequency the engine life-shortening problems attributed to inactivity become a nonissue. Those engine life-shortening factors are acids, water, and lack of lubrication. By inference, it can be seen that 100 hours a year is far short of this manufacturer's definition of normal usage.

Owners who fly their airplanes for an hour each week have taken the most important step there is toward ensuring a long, dependable relationship with their airplane's engine. But it's not a perfect world, so weather, maintenance, and unavoidable and unforeseen changes in activities or priorities can, and do, upset flying plans. What constitutes a period of inactivity? When there are periods of inactivity, what should an owner do to protect his engine? What measures do manufacturers and users suggest when flying stops? Should airplane engines be pickled over the winter? Whatever the reason for the inactivity, there are steps that can be taken to ensure the continued health of your airplane's engine when you're not flying.

The basics

Keeping clean oil in your engine is always step one. Because of the makeup of modern ashless dispersant oils, contaminants are circulated with the oil during engine operation. An oil filter catches solid particles such as carbon, but acids continue to circulate. Water vapor and excess fuel that accumulate in the oil from operating an engine rich will evaporate out of the oil when the oil temperature is elevated often enough to at least 180 degrees Fahrenheit. According to a paper that the late Peter Tanis of Tanis Aircraft Services presented to the Society of Automotive Engineering, when oil is exposed to heat it oxidizes, with longer periods of time and more heat causing more oxidation. Moisture and oxidized oil form an acid that attacks engine parts. Regular oil changes are the only way to remove the acids. Metal surfaces that have been specially designed to withstand thousands of pounds of direct pressure during normal engine operation are surprisingly susceptible to the action of corrosion. The hardened surfaces of camshafts, wristpins, cylinder walls, and hydraulic lifter bodies can stand almost anything—except corrosion. So keeping clean oil in the engine, especially during periods of inactivity, is critical.

Most owners seem to know the intervals for oil changes—25 hours if the engine has a pressure screen and 50 hours if it has a filter—but how many are aware of the calendar requirement to change their oil every four months no matter how many hours have elapsed? Many owners, having learned about the potential for internal engine damage due to oil-borne acids, have decided it is prudent to change the oil every 25 hours, oil filter or not. Engine rebuilder Charles Melot of Zephyr Aircraft Engines in Zephyrhills, Florida, added emphasis to this decision, saying, "The engines that we see that come in at the end of TBO and don't need an overhaul have all had 25-hour oil changes." Melot says engine rust is pretty widespread, with one-third to one-half of the cylinders that come into his shop showing evidence of rust forming at some time. Nitrided steel cylinders are very susceptible to rust since a corrosion-resisting varnish layer formed during normal operation takes time to develop. For this reason it's especially important to fly a new, remanufactured, or rebuilt engine every week during the first 50 to 100 hours after installation. Continental started coating its nitrided cylinder barrels with a corrosion-resistant coating (manganese phosphate) in 1998 to prevent cylinder wall corrosion until a varnish layer could be built up.

The results of inactivity

Flying the engine and getting it up to normal operating temperature for an hour accomplishes two things that prevent premature engine overhaul—flying lubricates all the internal steel parts, and it causes any moisture that has accumulated in the oil to be cooked off. Failure to fly regularly lets oxidation have its way with your engine. It's hard to specify exactly how quickly inactivity will damage an engine, since local climates vary in moisture content, but Howard Fenton of Engine Oil Analysis in Tulsa, Oklahoma, says, "Nitrided cylinders can start to rust in a week to 10 days." An indicator of internal rust from inactivity shows up as a higher-than-expected iron (Fe) reading in the first oil analysis sample of spring. Melot says that nitrided cylinders that have had the varnish layer removed "will rust a little overnight and get pretty bad over the weekend."

Both Lycoming and Continental publish service information for Flyable Storage, which is aimed at engines that are inactive for as short a period as from seven to 30 days. Airframe manufacturers also specify storage procedures in the service manual and, in some cases, the pilot's operating handbook (POH).

Flyable storage

In contrast to current thought by experienced in-the-field industry experts, there are reams of service information telling owners to pull their prop through by hand every five to seven days when not flying regularly. Service manuals from Cessna and Mooney and service information from Continental and Lycoming all recommend this practice. Current thought suggests that the prop should not be moved during periods of inactivity. The primary argument is that the oil scraper ring will scrape off any existing oil, thereby lessening the cylinder wall's protection from corrosion.

Also in opposition to current thought are service manuals that state, "To maintain an oil film on internal engine parts, frequently pull the engine through. Run engine for 15 minutes at 1,500 rpm (or until temperature stabilizes) once every seven days." Just like pulling the engine through every week, ground running an aircraft engine is a controversial issue. Continental Service Bulletin M91-5 specifically says that ground running is not a substitute for flying. Many argue that ground running does more harm than good. The most prevalent argument is that the oil temperature doesn't get high enough—or if it does get high enough, it doesn't stay high enough long enough—during the ground run to boil out the moisture in the oil. Therefore, the pilot who engages in ground running is not only increasing the formation of acids within the engine, but unwittingly creating more problems by circulating the acids.

According to Shell Oil, the oil temperature has to get to 180 degrees Fahrenheit for at least 30 minutes for all the moisture to be cooked out of the engine. Most airframe manufacturers suggest the installation of a winterization kit when ambient temperatures drop. For instance, Cessna recommends the installation of restrictor plates limiting the amount of air over the cylinders, oil cooler, and carburetor air inlet when temperatures go below 20 degrees Fahrenheit. The point is to maintain normal operating temperatures. Some engine installations just run cool. After I calibrated the oil temperature gauge on my 1966 Cessna 182, I was surprised to learn that the oil temperature would never get to 180 degrees Fahrenheit unless the cooler was partially blocked off. This was true during all except the warmest summer months.

Temporary storage

Temporary storage is defined as a maximum of 90 days of nonoperational status. Service information from Lycoming (Service Letter L180A) and Continental (Service Bulletin M91-5) says that the top spark plug of each cylinder should be removed, and preservative oil should be sprayed in each cylinder when the piston is at bottom dead center. After each cylinder is sprayed with the preservative oil, the prop should then be moved until no cylinder is at top dead center and each cylinder should be again sprayed before the spark plugs are reinstalled. All engine openings, such as the exhaust pipes and crankcase breathers, should be blocked off and a sign attached to the propeller saying, "Do not turn propeller—engine preserved." There is also a de-preserving procedure that has to be carried out before returning the engine to service.

Thanks to Shell Oil, there is another alternative to this process. Aeroshell introduced its 2F aviation oil in 1998. Contrary to other preservative oils that must be removed before flight, this oil can be used as lubricating oil for 50 hours during the TBO life cycle of an engine. Owners who have hesitated to go through the pickling process during winter months because they are sure that this is the year they're going to brave the weather and do more flying, yet find their logbooks are only showing a few flying hours during the winter, might find Aeroshell's 2F to be a perfect combination of lubrication and preservation. At the present time 2F is only available as a 50 weight, so it may not be suitable for winter operations in the northern climates.

Indefinite storage

If your airplane is laid up for three months for any reason, then you need to comply with the storage process detailed in the Lycoming and Continental service bulletins listed above. It doesn't matter if you've just won the lottery and are going on a six-month around-the-world cruise or you've acquired a spare engine that you believe you will use in the future; a little action before storage will pay big dividends in the long run. Gathering all of the listed supplies and oils may seem like a daunting task. But Tanis Aircraft Services Inc., a company well known for its knowledge of winter engine operations and preheating, sells winterization kits with all the supplies needed to completely pickle an engine in accordance with both the Continental and Lycoming service information.

The Tanis engine preservative kits include two types of preservative oil, desiccant plugs, the Lycoming and Continental service information, tape, and other supplies necessary to preserve an engine. These kits are surprisingly inexpensive, especially compared to the price of a cylinder overhaul. The four-cylinder kit sells for less than $100 and the six-cylinder kit is under $150. Contact Tanis at 800/443-2136, or visit the Web site ( www.tanair.com) for more information.

Flying every week, changing oil at four-month intervals, and especially changing oil before periods of limited activity will do a lot to keep the iron horse under your cowling stepping out strongly for a long time. If you want to try preserving your engine over the winter, both Aeroshell and Tanis have made it pretty easy to get the job done, depending on your needs.

Don't let inactivity spoil next spring. Take action now, and spend next year's flying money on gas rather than engine repairs.


Links to additional information about preserving your aircraft engine may be found on AOPA Online ( www.aopa.org/pilot/links/links0011.shtml). E-mail the author at steve.ells@aopa.org.