Aircraft Maintenance: Oil use and cylinder cracks

Aircraft engines are designed to use a certain amount of oil, and determining what is excessive is debatable. Lycoming uses the following formula to determine maximum oil consumption: (.006 x BHP x 4 )/ 7.4 = maximum quart/hour consumption. This means that, for a typical 180-horsepower engine, using one-half quart per hour is acceptable to the manufacturer. One quart of oil every two hours may be acceptable to Lycoming, but I prefer to have my aircraft’s range limited by fuel burn rather than oil usage, and flying around with two cases of oil in the baggage compartment can limit your useful load.

As with low compressions, it’s the source of the problem, in this case oil loss, that’s the key. Oil can leave an engine in a variety of ways including the breather tube and crankcase leaks. It’s the actual burning of oil that's most important. The best way to determine whether oil is leaking past the rings is to examine the spark plugs. Finding an oily bottom spark plug is not necessarily a problem, but oily top and bottom plugs typically indicate that excessive oil is leaking past the rings. Surprisingly, oil consumption and low compressions do not always go hand in hand. If a cylinder’s compression rings are OK, but the oil control ring has failed, you may have excellent compression readings, yet still have oil-soaked spark plugs.

The borescope inspection is useful in evaluating cases of oil usage because you can look for scratching up and down the sides of the cylinder wall indicating a broken ring. If there are no signs of ring damage, the compressions are within spec, and the oil usage is within the manufacturer’s spec, I would be hesitant to recommend pulling a cylinder based on oil consumption alone. It’s not ideal to have to keep adding oil, but it makes sense to monitor the situation to see if it’s progressing before jumping to cylinder replacement.

The last topic for us to cover in evaluating cylinders is cracking. Cylinder heads are made of aluminum and, like any metal, excessive heat, thermal cycling, and stress cause them to become more brittle over time. This often results in cracking at or near the exhaust port and/or spark plug hole. Cracks can also form around the fuel injector boss if the injectors have been over-torqued. These areas should be carefully checked any time that the cowl is off, especially if the exhaust has been removed for inspection. A simple check for cracks is to spray a very thin penetrating lubricant outside the cylinder around the exhaust port. If cracks exist, they can be seen from the inside of the cylinder as the lubricant penetrates the dry soot coating inside the head. Lycoming and Continental have published materials regarding cylinder head cracks, but they generally consider any crack cause for repair or replacement of the cylinder.