Upon landing after my last flight home from Michigan to Florida, I noticed I had gone through an inordinate amount of engine oil during the trip. The oil dipstick showed only 6 quarts remaining after starting out with 9½ quarts. But how much oil consumption is too much? Teledyne Continental, the engine manufacturer, says one quart of oil per hour is acceptable, provided compression checks are good and oil analysis shows no out of the ordinary wear items.
A lot of pilots brag their engine does not burn but a single quart of oil in 20 or 30 hours of operation, as though it is a sign of superior engine health. The truth is that optimal oil consumption is probably somewhere in the middle of these two ranges.
About 10 years back, TCM engineered for greater tolerances on the bottom oil scavenger ring on each piston. These looser oil rings allowed for greater top end lubrication and cylinder life, albeit at the expense of slightly greater oil consumption.
Until this last trip, the oil consumption on the TCM IO-550 in my aircraft was running at 8 hours per quart. On the trip home from Michigan, it jumped to a quart every three hours, so some investigation was in order.
First, there was a very slight oily film residue on the windshield. More like a light mist. Secondly, the air-conditioning evaporator on the belly of the plane had some oil residue on it. The evaporator is in the slipstream of the engine crankcase breather overflow. Finally, there was a very slight bit of oil around the crankshaft flange outlet on the front of the engine.
After doing some research, my mechanic and I determined it was time to replace the crankshaft seal on the front of the engine. All of the symptoms indicated it was leaking, allowing the crankcase to become internally pressurized by incoming airflow. This pressurization was causing oil to be blown out through the breather.
We pulled the prop to begin the replacement process. Sure enough, once the prop was off, the old seal showed signs of age. The glue that goes around the outside of the seal had broken down, allowing the seal to spin in its seat. Not good. Clearly the seal was worn and was ineffective in keeping the airflow out of the crankcase.
The new seal will be heated sufficient to allow it to be carefully stretched over the crankshaft flange. It will then be glued in place with some retainer rings and finally held in place with some C-clamps. After re-installing the propeller and doing an engine run-up, we are confident that the new seal will stop the crankcase pressurization and out boarding of engine oil.
Ah, the joys of aircraft ownership.