Oil Levels: The Black Art

Mike Busch’s AVweb article “Checking the Dipstick” got me wondering how much oil is truly required for safe operation.  He writes:

The engines on my Cessna 310 have 12-quart sumps — 13 quarts if you include the quart in the spin-on oil filter. When I first acquired the airplane, my mechanic would fill the sump to its maximum capacity at each oil change. It didn’t take me long to discover that the engines didn’t like that, and promptly tossed several quarts out the engine breathers.

My POH states that the “minimum for flight” oil level is 9 quarts. So I asked my mechanic to service the sump to 10 quarts (instead of 12), and I’d add a quart of make-up oil when the level got down to 9 quarts. That worked better, but I was still seeing a fair amount of oil on the underside of the engine nacelles and the outer gear doors.

He goes on to say that after experimenting, he found that running at 9 quarts and not adding oil until reaching 7.5 worked best.

By comparison, an SR22 has a 310 horsepower TCM IO-550-N.  That’s a big engine — bigger than in Mike’s C310.  Yet the sump has an eight quart capacity.  And I’ve found that if you fill it much above 5-6 quarts, it throws the excess overboard.

Yes, operating a 310 hp, 550 cubic inch engine at 80% power for hours on end with  only 5 quarts of oil.  It sounds wrong, doesn’t it?

In lieu of 14 CFR 33.39, Continental must have demonstrated proper operation of that engine with only 4 quarts of oil.  It’s interesting that the FAR is entitled “Lubrication System”.  My understanding of oil’s function in the engine is that as little as one quart is needed for lubrication.  The balance of the sump capacity is primarily for engine cooling.

I’m not suggesting that anyone go out and run a $60,000 aircraft powerplant with one quart of oil in it, but this might explain the lower capacity on the IO-550-N.  If air flow and oil cooling are improved, perhaps a lower quantity might be acceptable.  They’re referred to as “air cooled” powerplants, but that’s a huge misnomer.  If you took away all the oil that wasn’t required for lubrication, you’d end up flying a glider, especially on a hot day.

The IO-550-N is also standard on the Columbia, Legacy, and other fast airplanes, and they all seem to have an eight quart capacity.

I want to suggest that the cowling is efficient enough and the aircraft’s cruise speeds high enough that air takes on a greater role in keeping the engine cool, but the Columbia 400 sort of blows that theory out of the water.  The CL400 is twin-turbocharged and it operates as high as FL250, where there’s little air to cool the engine, yet power output remains very high.  All on an eight quart capacity.

I’m not a powerplant engineer, so I could be way off base with the rationale.  But there’s no denying that the SR22 operates with 50% less oil capacity than other airplanes equipped with basically the same engine.

For a long time, I used to get very uneasy operating an engine of that size with so little oil.  The O-470-S in my previous aircraft had a 9 quart capacity.  That’s a 13% greater oil capacity for an engine with 15% lower displacement and 25% lower horsepower.

Another data point:  I fly an Extra 300 with an AEIO-540 and it has a 14 quart capacity.  And let me tell you, you’d better not take off with less than 12 quarts in that sucker or you’ll be looking at higher engine temps.  Sure, I’m doing inverted flat spins and other things that frequently toss as much as 2-3 quarts of oil out the breather and onto the tail.  In fact, it’s quite common to come back with the empennage completely coated in oil.  But Decathlons have a similar setup — an inverted oil system — and their oil capacity is no higher than what you’d find in a non-aerobatic airplane with the same engine.

Consider also that the limited fuel capacity of aerobatic airplanes means they won’t be in the air for more than an hour or so.  A normal airplane with an IO-550-N will typically have a 4-5 hour range.  Longer flight times, lower oil capacity?

Anyway, back to the Cirrus.  My unease was abated somewhat by talking (repeatedly) to the factory.  When I was in Duluth, the demo pilots and instructors said they never ran more than 5 quarts in any of the SR22s or it’d be expelled by the engine.  This isn’t just what they do, it’s what they teach in the standardized training program.  The SR22 fleet has already surpassed one million hours of operation, and with the advanced engine monitoring capabilities in these airplanes, it’s a sure bet that operating at this “low” oil level is not harmful.

It does take some getting used to, though.

My final thought on oil levels is that most pilots probably never know what the engine is actually using versus what it’s venting, because most aircraft have a dirty underside, and unless it’s dripping with oil, you’d be unlikely to notice it.

In fact, depending on the location of the breather in relation to the exhaust pipe and the rest of the airframe, vented oil might never show up at all.  This is especially true if they’re flying something like a Pitts, Extra, Decathlon, or other such plane because the breather doesn’t exit under the cowling.  To keep oil off the belly, the tube runs down the longitudinal axis of the airframe and ends next to the tailwheel.  It could vent oil all day long and you might not see any sign of it on the airframe.

Mike mentions another skewing phenomenon.  When it comes to checked the oil level, is what you read on the dipstick an accurate representation of what’s in the engine?

If you check the oil level shortly after the engine has been run for awhile, the dipstick reading will be noticeably lower because a significant quantity of oil remains adhered to various engine components. Another reading taken 24 hours later will often show an oil level that is 0.5- to 1-quart higher.

Tailwheel aircraft, especially those that have seen significant maneuvering during the last flight, can have a fair amount of oil sitting in the long breather vent tube I mentioned earlier.  Put an oil bottle on the breather, and within 24 hours you can have a quarter of a quart in there.

With all these factors, how does one really know what’s causing oil consumption?  Simple:  get to know your airplane.  Listen to what it’s telling you.  Look at usage patterns and note when oil is added.  Check the underside of the airplane during the pre- and post-flight inspections (you do perform a post-flight inspection, right?).  These hand-built contraptions have individual personalities.  Pay attention to them and they’ll tell you exactly what’s going on.