Air-Oil Separator Time?

[Dan O'Brien]

I have a 10:1 compression IO550N. My dipstick is about 1 quart low when I put in 7 quarts, meaning it reads 6 (actually a bit under) after putting 7 in the sump. It reads about 5 when I put 6 in, just over 4 when I put in 5, etc. This is following an oil change. I've measured this several times.

While flying, I can't keep 5 quarts on the dip stick, 6 qts actual in the sump. Some dumps on the belly, and it seems to settle in at about 4 on the dipstick, or just under 5 actual quarts in the sump. It has always struck me that 5 actual in the sump is seems low, but my oil temps and pressure are fine. Whatever the case, it seems that anything over 5 actual in the sump dumps.

Reading the COPA list, many Cirrus pilots report the same thing. There's a thread about "dipstick calibration" (LOL), which I took to heart, and sure enough it seems the dipstick measures too low, at least in my plane. Nonetheless, the calibrated amount of 5 quarts has always seemed low to me. The engine holds 10 total, 8 in the sump.. So it's short-changed by about 3 in the sump (assuming as much as 2 [???] is elsewhere, e.g., the prop, oil filter, etc.)

Reading the BeechTalk list, there was a lengthy debate about the amount of oil the IO550 spews out the breather onto the belly. Seems not to be an issue for everyone, but seems to be an issue for many. Some like air-oil separators to mitigate this, and I note that some say that they can keep more oil in the engine that way (someone on LancairTalk also said this). Mike Busch and some others say don't do it, as (1) they don't like returning what they believe might be bad stuff to the engine, and (2) they believe doing so hides potential problems reflected by an over-pressurized case. I have no idea whether this is a big deal; in my case, with 10:1 compression, people have said that alone increases the pressure and may lead to more oil spewed out the breather. But we all probably agree that Mike Busch knows quite a lot, so its hard to dismiss his opinion. That said, in that discussion on the BT list, a representative from Andair (one of the air-oil separators out there, the more expensive one) chimed in and said he disagreed with the idea that bad stuff is going back in the engine -- said his company has researched it extensively and doesn't see an issue. He also said that IO550s (and many other airplane engines, he mighta said all airplane engines) have a flawed design that causes them to dump oil on the belly, especially when in a steep climb. He observed that autos have returned breather oil to the engine for years without bad effects. Of course, Mr. Busch had a response, which was that airplane engines are different than car engines, yada yada yada, and you can't make the analogy. But I found the notion that "airplanes that climb steep dump lotsa oil out the breather" interesting, and it would seem to at least weaken the argument against putting the oil back, because a lot of what is being dumped is just oil, not byproducts of combustion.

I like to climb somewhat steeply because when its cold enough, elevation is not too high, and the runway is long enough, I know I can out-climb my glide and return if the engine quits (...another big debate, how high and far before turning back if the engine quits ... I've researched this pretty extensively, and a well executed steep turn at a speed with a limited margin over stall gives the smallest radius and least altitude loss during turnback, but it's not for the faint of heart and requires planning to establish when to do it and when not to even try it and practice ... a whole 'nother discussion). A few weeks ago, after some discussion with folks on the "possible" impossible turn, I climbed out of Truckee (5900 ft MSL, 7000 ft runway) at between Vx and Vy (rather steeply, but not crazy) to confirm how high I was at the end of the and the feasibility of returning, and I noticed on landing that there was a bigger stream of oil along the belly than usual. Hmm, that's what the Andair rep said would happen. Some say that there is no reason to dump this oil on the belly, and that Air-Oil separators are an effective way to avoid it. As a closet but not well-informed engineer, I sit here wondering why would folks design an engine---even 50 years ago--that dumped oil on the belly during climb? Seems dumb to me, but what do I know. Lotsa folks say that oil gets dumped faster when climbing fast, and why not return that oil back to the engine? This seems to be another one of those debates for the ages.

But enough of all this motivation. The purpose of the post to get thoughts/views on the merits of an air-oil separator. I am getting sick of wiping oil off the belly, but I don't want to give up my strategy of out-climbing my glide in a mountain airport, and climbing on the faster end spews more oil. What do you all think? Live with the oil? Send it back?

Sorry for the length.

[Dan O'Brien]

BTW, where I said “Andair” above, I meant”Airwolf,” the larger and most expensive (and widely certified) Air-oil separator.

Happy and safe flying,
Dan