Pilots take pride in the health of their engines, hoping for near-perfect compression readings at the annual inspection and with barely a quart of oil added  between oil changes. True engine health, however, isn’t measured by the lack of oil used. It’s about using a reasonable amount of oil in the way it was designed to be consumed.

According to the technical support guidance from the major engine manufacturers, the loss of oil to these normal processes can be as high as one quart every two to three hours before it is considered worth investigating, and as much as one quart per hour before it becomes an airworthiness issue (the maximum oil usage for most engines is listed in the engine operator’s manual). That said, a healthy engine installation should generally use one quart of oil every five to 10 hours of operation. If your engine is using more than that, the most important thing is to know where it is going.

In a healthy engine, oil from the crankcase splashes and coats the walls of the cylinders from below the piston, providing a thin layer of oil for the rings to glide over. The combustion process inevitably burns up some of that oil before it is replenished from below. This normal process protects the metal parts from grinding against one another and results in only a very small amount of oil consumed over time.

In the crankcase, all of those moving pistons, shafts, rods, and gears get bathed in oil, and the air inside is thick with oil mist. The pressure changes caused by all those moving parts are vented to the oil breather tube, which also results in a loss of oil in the form of that mist.

When diagnosing excessive oil usage in an engine, the first step should be to look for leaks. Small drips or oil wetness is fairly common where the crankcase halves meet or where the oil pan meets the case. Because those locations are difficult to reseal without major engine disassembly, most shops recommend just monitoring the leak. Similarly, rocker cover gaskets are a common source of slow drips and can often be fixed by cleaning up the surfaces, ensuring everything is flat and true, switching to a  gasket provided by the original engine manufacturer, and properly torquing the covers. While thick, silicone aftermarket gaskets are marketed to stop leaks, they usually mask the underlying issue, and it can be difficult to maintain proper torque, which often leads to recurring leaks. Oil leaks from accessory mounting pads are usually caused by a failing Garlock seal, which can also be replaced. All of these leaks are generally considered minor and are rarely associated with high oil usage. Oil coming from cracks in the crankcase or cylinders, in contrast, are serious issues that should ground an aircraft until resolved.

The next step should be to pull the bottom spark plugs and check the inside of the cylinders. If a plug is wet with oil, that is an obvious sign that too much oil is getting into the combustion chamber. There are generally two causes of this: a leaking intake valve seal or a failure of the oil control ring to function properly.

Intake valves have seals around the stem that prevent oil from being sucked into the combustion chamber during the intake stroke of the engine. These seals can be easily replaced from inside the rocker cover without major cylinder disassembly. Oil control rings are another matter. They are installed just below the compression rings on the piston and cannot be reached without removal of the cylinder. When an oil control ring fails, it is often because it has become clogged with sludge and cannot scrape and drain properly. In this case, you can perform a procedure called “ring washing” to clear gummed up oil control rings. Oil is drained from the aircraft, and solvent is manually forced through the offending cylinder’s rings in an effort to free up the oil control ring. Savvy Aviation provides a detailed description of the procedure..

If the belly of the aircraft is frequently wet with oil, the oil breather should be investigated. As noted earlier, the breather system allows the crankcase to be vented to the outside, alleviating the pressure pulses from inside the engine. Because the air also contains oil mist, the path of the oil breather is designed to be routed upwards as the air cools, allowing the oil to condense out of the air and run back into the engine. Following that initial rise in the system, the breather then heads back down and exits the cowling at a neutral pressure point in the boundary layer of the outside airflow. The placement of the breather in a neutral pressure location is critical to prevent pressurizing the case (if placed in a high-pressure location), or putting a vacuum on the case (if placed in a low-pressure location). I have frequently seen oil breather tubes extending too far into the slipstream, resulting in siphoned oil mist and an excessively wet aircraft belly. A quick adjustment and relocation of the breather tube usually solves the problem.

If these steps fail to identify the issue, then the internal crankcase pressure may be excessive and should be investigated. High crankcase pressure can cause oil to be forced out of the breather during normal operation. The M-0 maintenance manual for Continental engines has a detailed procedure for measuring crankcase pressure that can be applied to Lycoming engines as well. The procedure involves plumbing a spare airspeed indicator into the crankcase using a modified oil filler cap with a fitting and hose. The engine is run up on the ground, and the resulting airspeed indication is noted. The maximum limit for smaller engines such as the A-65 through O-300 is 44 mph on the airspeed indicator. For larger engines such as the 360 to 550 engines, the maximum limit is 90 mph. Excessive crankcase pressure can be caused by blockages in the breather system or air/oil separator (if installed), by high-pressure air leaks into the crankcase from a leaking oil filler or crankshaft nose seal, or by excessive leakage past the rings (which can usually be identified during a compression check).

In all but the most extreme cases, excessive oil usage can usually be resolved without major engine work. It just takes a disciplined process to identify the means by which the oil is leaving the engine, and then work to resolve the issue. A healthy engine will always use some oil, but it shouldn’t get to the point that you’re carrying around a case of oil just to avoid visiting the maintenance shop. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.