The good, and sometimes bad, news is that the operation and maintenance of aircraft piston engines is very lightly regulated. Unlike for turbine engines, which have extremely detailed operation, maintenance and overhaul procedures as part of their certification, the FAA leaves much of the operation and maintenance details of piston engines to pilots and certified mechanics.
It may seem like the FAA has tons of rules governing piston engines, but the regulations are quite broad. If you don't believe that, just take your airplane to a series of shops and see how different mechanics -- all licensed and experienced -- treat the engine differently. And the same is true for pilots, who all form their own opinions about the optimum way to operate the engine for maximum performance, efficiency and durability.
The varied opinions about how to treat piston engines exist because every method is going to be right sometimes but wrong others. I have more than 10,000 hours flying behind a variety of Lycomings and Continentals and have experienced about every kind of problem except a total loss of power. But then I have had engines fly on flawlessly way beyond TBO. There must be an element of luck involved, along with sound operating procedures.
However, there is one thing the long-lived engines all had in common -- I piled on the hours quickly. For many years I flew 400 or more hours per year covering the far-flung aviation industry, and the engines in my various airplanes loved that kind of use. So if there is one solid piece of advice anybody can give to get the best from a piston engine, it is to fly often and on trips. Engines like nothing better than droning along in cruise with temperatures stable and mixture constant. That's what they are built for.
Any type of engine is designed and built to perform a specific duty cycle. The duty cycle is the engine's typical expected operating range of rpm, power output and so on. For example, a desirable automotive engine must have good acceleration, smooth idle and a broad operating rpm range, and be able to cruise efficiently at relatively small percentages of full power potential. On the other hand, an engine designed for stationary power, such as a generator or pump, doesn't need acceleration, and smooth idle is not important, but it must produce a big chunk of its rated power continuously for long periods.
An airplane piston engine is designed for a duty cycle much more like stationary power than an automotive engine. The aircraft engine must produce large percentages of maximum-rated power for hours on end during cruise flight, while the car engine cruises at a small fraction of maximum power after using higher rpm and torque to accelerate from a stop. Running an automotive engine at 70 to 80 percent of its maximum power continuously would destroy it before long, but that's exactly what the airplane engine is designed to do. And by operating the aircraft engine as designed, you extend its life.
An infrequently used engine suffers from corrosion. As the engine sits, the oil drains off of key steel parts such as camshafts, cylinder walls and valve stems. With the oil gone, it doesn't take long for rust to form. The rust, of course, attacks the ferrous metal parts, and then when the engine is eventually started, the corroded metal surface polishes off as the parts rub against each other, and the debris travels through the engine, causing even more damage.
In addition to flying as frequently and as long as you can, what can a pilot do to extend the life of a piston engine? The most important engine-life component under a pilot's control is the oil.
The oil in an air-cooled aircraft engine is under a great deal more stress than in a modern automotive engine. First, there is the wide operating temperature range of the aircraft engine. While both the car and airplane engine start at ambient temperature, the auto engine temperature tops out not far above 200° F, while the aircraft engine temperature closes in on 500° F. The high temperature in the air-cooled engine shortens the useful life of lubricating oil compared to the relatively sedate conditions in the car engine.
Other design aspects of the aircraft engine also attack the oil. Because the airplane engine functions over such a large range of temperatures, the tolerances must be very loose compared to the car engine to allow for expansion. Those loose tolerances, and high pressures in the cylinders, actually shear, or cut up, the molecules in the oil. Also, the tolerances of the aircraft engine allow combustion byproducts such as carbon to get past the piston rings and into the oil, which eventually contaminates it and reduces its lubrication properties.
Because aircraft engines are so hot and their crankcases are vented to the atmosphere, condensation often forms inside the engine as it cools after shutdown. This moisture ends up in the oil. The moisture gets burned off when the engine is started the next time, but it is there to promote corrosion between flights, another reason to fly frequently.
Oil is also an essential part of the cooling process in an airplane engine. The cooling air flows over the cylinder heads and barrels and does a good job of carrying away heat in those areas. But air can't reach the heart of the engine, where heat forms at bearings, on lifters and in pistons, so it's the function of oil to carry that heat to an external oil cooler, where the heat is transferred to the atmosphere.
So you can see that oil is performing a heroic job of both lubricating and cooling the airplane engine. And under such demands oil doesn't last long, and that's why it is so important to change the oil frequently. Not only is fresh oil more effective at lubricating, the draining of the old oil carries away debris and combustion byproducts it has suspended. When oil darkens with use, you are seeing carbon and other materials it is holding in solution. If not drained out at the proper interval, the bad stuff can fall out of solution and settle in the engine, where it will eventually block oil flow and shorten engine life.
Fifty hours is a commonly recommended oil-change interval, but that must be balanced against calendar time. A conservative routine is to change the oil every four months or 50 hours, whichever comes first. Remember that the oil is being contaminated at least a little by sitting, so you need to get the bad stuff out on a regular schedule.
Which oil to use in your engine is the stuff of endless debate. Some airplane owners and engine shops swear by multiweight oil, while others swear at it. A multiweight oil -- which is the universal oil in the automotive world -- behaves as a lightweight at cold temperatures but retains viscosity when heated to operating temperatures. For example, AeroShell's multiweight is a 15W-50, which means it equals 15-weight oil at cold temperatures but equals 50-weight performance at high temperatures. Phillips X/C and Exxon Elite multiweights are 20W-50.
Phillips was first to develop a multiweight, sometimes called multigrade, oil for aircraft piston engines. The molecules that make a multiweight function are called viscosity improvers (VI), and the molecules are long. Because of their length, the VI molecules are susceptible to shearing under the high loads found in an aircraft engine or a diesel. Phillips chemists worked hard to develop a VI molecule that would withstand the pounding of big diesels, and they succeeded in the late 1970s. At about the same time, Cessna was building piston-powered airplanes in huge numbers, and often an airplane was finished and test-flown in one season of the year but not delivered until the next. What Cessna asked Phillips for was an all-season oil to put in new airplanes so that no matter when the airplane was delivered the oil weight would be correct for the season. That's how X/C oil was born. I was using it with great success in a new Mooney I owned in 1980, when Phillips was still getting its marketing and packaging going, so they supplied it to me in unmarked cans. I have been using multiweight oil ever since.
