Plane & Pilot
Tuesday, October 22, 2013

Issues With Oil

Oil may be more important than fuel

The more frequently an airplane runs, the longer it's liable to continue running. The late Lyle Shelton, primary builder, owner and pilot of the unlimited F8F Bearcat Rare Bear, rebuilt his warbird in the hangar next to mine in Compton, Calif., and Lyle kept a Cherokee 140 that his crew and I used for shagging parts or any other business on the Bear.

Lyle's Cherokee sometimes flew an hour or two a day, and he used to change the oil an average of about once every two weeks. The O-320 Lycoming ran perfectly for nearly 3,000 hours, or 1,000 hours past its TBO, flying 20- to 30-minute hops around the Los Angeles basin. Even so, when the crew finally overhauled the little engine, the cylinder walls, bearings and all the other greasy bits looked practically new.

That's partially because Lyle used the best grade of aviation oil he could find. As most aircraft owners are aware, automotive oil isn't appropriate for aircraft piston engines. Auto oil is designed for engines that are water-cooled and built to very tight tolerances. Air-cooled engines are constructed to looser tolerances and are specifically designed to burn oil. That means auto engines operate at much lower temperatures than do aircraft powerplants. Oils specifically intended for auto engines will break down in the high temperatures of aircraft engines. These oils leave metallic ash additives, which, when burned in a combustion chamber, leave deposits that could cause detonation.

If you need to add a quart or two of auto oil in an emergency, that's normally not a problem, especially in high-capacity sumps of 11 or 12 quarts, but a regular diet of motor oil as opposed to aviation oil is setting you up for a fall.

There are two types of aviation oil, mineral or ashless dispersant, the latter sometimes incorrectly referred to as detergent oil. Mineral oils are preferable for breaking in new or overhauled engines specifically because they have no additives and allow the engine to "set" or stabilize oil burn before switching to ashless dispersant oil. The rule is you can add a quart of mineral oil to a service of ashless dispersant, but don't try to go the other way.

Though it may sound like a cliché, oil is quite literally the lifeblood of any piston aircraft engine. Anything that interrupts the flow of oil is almost guaranteed to cause problems. For that reason, it's important that the paper filter be replaced or the metal filter cleaned every time you change oil. A proper oil service may therefore demand slightly more oil than what the book calls a full sump, because you'll need to refill the filter. An eight-quart engine may actually demand 8.5 or even nine quarts to bring the engine to full. Conversely, the engine may tend to blow out the top half-quart of oil. A little too much oil isn't necessarily a bad thing.


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