Air/oil separators have been around a long time. They are tapped into the crankcase breather line and, using centrifugal force, return the oil being vented back to the crankcase. The result is a cleaner aircraft and reduced oil consumption. The newest entrant in this market is M-20 Turbos, whose recent STC from the FAA covers some 200 aircraft, including the Piper Comanche owned by AVweb's Dave Higdon. Dave recently installed one of the M-20 units; here's his report.
July 6, 2000
|Journalism school educators,
real-world experienced though they were, couldn't hold a candle to the
mercilessness of a crusty, curmudgeonly desk editor dryly intoning:
"Where's the story? I'm through the second graph and don't know
why I should read this." So here it is: Bill Sandman and his gang
at M-20 Turbos LLC have a winner in their new Model 300 air/oil
Separator. This $259 unit has been doing its thing for more than 45
hours on our hard-working PA-24-180 Piper Comanche. So far, our
1961-model bird has needed three fewer quarts of oil than usual this
far downstream of an oil change. To date, the oil residue that
collects on our Comanche's belly feels far dryer and touching it
leaves only a sticky smudge on the skin, not the dripping wet oil
sometimes spread beyond the low point of the fuselage and to the tail
skid, where it would dampen tie-down ropes and hands alike.
My conclusion: The little thing works, and there have been no
squawks so far. From my experience with the unit's installation and
performance, installing an M-20 Turbos air/oil separator belongs on my
list of experiences worth repeating.
Unlike the positive crankcase ventilation found on
modern automobile engines, aircraft engines don't recycle crankcase gases
through the engine intake. Instead, aircraft piston engines typically are
vented to the atmosphere, carrying overboard with them whatever liquids and
other substances have vaporized in the crankcase air. Often, that can result
in quite a lot of the engine's lubricating oil going overboard, too.
Enter the air/oil separator. In their modern configuration, these devices
dry the hot, oil-laden gases escaping through the crankcase breather tube by
creating a trap that separates and collects liquids from the vented vapor,
drains the collected condensate back to the sump and simultaneously allows the
drier blow-by gases to continue their trip to the atmosphere. Otherwise,
crankcase gases laden with oil vapor and other combustion by-products vent to
the atmosphere where they can condense and collect on the cooler aircraft
skin. The oil builds up, can ooze between skin joints and find its way into
hidden internal crevasses of an airframe until cleaning inside and out is a
necessity from several perspectives, aesthetic and airworthiness included.
Increasingly, planemakers include a separator on new-production aircraft.
For pre-owned planes, aftermarket options abound. Separators always seemed to
me to be a smart, responsible alternative to the equivalent of pouring about a
gallon of oil overboard between oil changes. Couldn't imagine why everyone
didn't want one and, in fact, it surprised me that not everybody saw
separators from my perspective. Shame on me for expecting unanimity on
anything in aviation even something that saves money, work, and the
environment all at once!
You Want To Do What?
Many long-time A&Ps and IAs discouraged my interest in adding a
separator to the Cherokee we used to own and to the Comanche we now fly. The
reasons? Maintenance burden, weight, cosmetics (the separators themselves got
dirty) and expense prompted their attitudes. Sometimes the devices actually
malfunctioned despite a lack of moving parts and created problems
otherwise not possible.
Basically constructed as a canister with a tall tube welded in the middle
inside, the units need little but good hoses, a solid mounting spot and a
basic cleaning once a year. Disassembly for cleaning allows an inspection for
corrosion or stress damage and to make sure nothing obstructs the vent inlet
tube, the stand-pipe outlet tube where the gases exit or the drain tube that
carries collected liquids back to the engine sump.
Eliminating Grunt Work
Those arguing against installing a separator didn't have to break out the
mechanic's creeper and roll around under my airplane to restore clean and
shine to its belly two or three times a year. Likewise, there was all that
perfectly good oil going out the vent tube at the rate of a quart about every
nine hours more on hot days and when flying above 10,000 feet; less in the
winter or when flying at lower altitudes.
Their admonitions basically went like this: Think of how many quarts of oil
you can buy with the (fill in an amount here) dollars it takes to buy, install
and inspect an air/oil separator each year. Sound advice, but considering the
cost of this unit, with purchase and installation running about $400 total, my
figures show that lower oil consumption should cover the costs in about 1,000
hours. That's worth a lot to me, since at our current annual rate of use,
we're talking about the unit and its installation paying for itself in fewer
than six years and eliminating one or two belly cleanings each year. And
for me the prospect of six or eight hours upside-down on a creeper each year
is worth the costs, and dealing with the annual cleaning and inspection
requirements is a worthwhile trade. From this perspective the 60, maybe 80
bucks or so saved in oil every twelve months is all gravy. From here on, about
every fourth oil change could be called a freebie, courtesy of the M-20 Model
Model 300 Installation Hardly Brain Surgery
As added accessories go, the M-20 Turbos Model 300 air/oil separator hardly
stands out as anything particularly challenging when installation is
considered. That more or less fits, since the design itself is fairly simple.
The Model 300 consists of an aluminum tube two inches in diameter, with a cap
welded onto each end. Just below the top cap, a 3/8-inch inlet tube penetrates
the body at about a 45-degree angle. Exiting the bottom cap are two tubes. The
first, mounted at a slightly steeper angle than the inlet tube at the top is
the drain. The second, mounted in the bottom center is the vent tube connected
to the stand pipe that runs most of the separator's six inches of length. The
whole thing weighs less than six ounces. The hoses and clamps involved in
completing its installation didn't bring the total up to a pound.
The unit is small enough and light enough that it can be installed almost
anywhere you can make it work. Figuring out where to install the separator and
how to route the lines, plus disconnecting the existing vent hardware,
consumed the bulk of the first hour of work by Correy French, the head
A&P/IA for Guernsey Aviation at the Augusta (Kan.) Municipal Airport (3AU)
where we base our Comanche.
Picking the right spot was the key for Correy, since a lot more than just
hot air and vaporized oil pass through it. These substances include water
vapor and acids that form as by-products of combustion and become suspended in
the water vapor. To eliminate the prospect of the separator drain freezing and
to encourage the venting of lighter volatiles, like some of the acids, keeping
the separator warm is important.
Correy settled on the aft engine cooling baffle, where engine heat should
keep the drain free and the separator warm enough to prevent acids from
condensing and returning to the engine or lingering around the standpipe.
Using the baffle as the mounting location also allowed us some latitude in how
high above the drain pipe we attached the mount, so we could better route the
vent inlet line with no low points that might collect oil, water or the like.
Finally, the location also gave us an easy way to route hoses from the
separator's exit tube to the existing crankcase breather tube's exit point on
the right side of the firewall.
Once he settled on a location, Correy fitted the clamps, installed a new oil
drain connection into an existing drain line off the #3 cylinder, bolted it
all together and tested the installation. With a fresh oil and filter change,
the entire process consumed a total of less than three hours.
Had we not opted to tap into a cylinder-head oil-drain line one that
drains oil from the rocker box back to the crankcase we could have tapped a
drain line directly into the crankcase or a plug that opened to the engine
sump. Many options means finding one for any given airplane should be
At its light weight, the Model 300 separator also could have easily been
mounted on an engine-mount tube, on the firewall itself or even suspended
freely between the inlet and the exit hoses, as long as each one is anchored
to something solid. The only real restrictions are to keep the unit as close
to vertical as possible, and to not mount it in a way that the unit's drain
line is not the lowest point.
With a tee-fitting installed in the oil drain line, the hoses cut, routed
and secured into place, and a bracket to hold the separator, all it took was
fastening the clamp to the engine baffle, hooking up the vent and drain hoses,
tightening the clamps, and rolling the old bird out for a test run. The test
flight followed the next day.
Living With It
The unit's impact showed up before the end of the week, when the Comanche
carried me and a heap of camping and working gear to this year's Sun 'n Fun
fly-in at Lakeland,
Fla. For the past 400 hours on our Comanche starting about three hours
after a major overhaul the little 180-hp Lycoming O-360 needed only two or
three hours after an oil change to blow out the first quart; another 10 or so
hours usually followed before the oil level fell to six quarts and my first
added quart. By the time the next old change rolled around, a tab for five to
six quarts wasn't uncommon during the 50 hours.
On departing Augusta for Lakeland, the oil level showed right at 7.5
quarts. A full eight quarts had gone in at the oil change a few days and 1.5
hours earlier. On arrival at Lakeland, the dipstick still showed seven quarts on
board. Back home at Augusta, there were between 6.5 and seven quarts still in
the engine. A couple of weeks later, when preparing to depart on a five-hour
IFR flight to San Antonio, Texas, it seemed time to add a quart, the first,
after more than 20 hours. Fifteen hours later, preparing for a five-hour round
trip between Augusta and Dallas' Love Field, I weakened and put in a second
quart. This second quart was added at about 40 hours since the oil
change/M-300's installation, bringing the oil level back to about seven
quarts. If a third quart goes in, I'll be ahead about three quarts if no
third quart goes in before I change the oil at 50 hours, I'll be a full gallon
of oil ahead already, something I can only attribute to the M-300.
Also worth noting at this point is that there have been no leaks of any
kind showing up from the separator's plumbing neither from the drain tube
or any of the fittings between the Model 300 and the cylinder head drain.
Everything works flawlessly, so far. Considering that few things in aviation
work right the first time, that news is a real treat.
An Upgrade No-brainer
In talking to various folks about air/oil separators, it's become clear to
me that pilots have suffered enough problems with such systems to make them
reluctant or outright opposed to using another one. Among the most-serious
concerns were acid accumulation that damaged a separator and, by extension,
weren't doing the engine much good, either. But from my talks with sundry
mechanics and petroleum company experts, it's hard to pin down a consistent
line about the engines, oils and separators that suffered the problems.
Instead, this is a topic one to discuss with your particular engine
manufacturer or mechanic. For me, it's something I'll watch for during regular
engine-compartment inspections before and between flights.
Otherwise, my experience so far with the M20 Turbos' Model 300 air/oil
separator leaves me with no downsides. The little device was easy to install
and works as advertised. Whether your engine and belly would benefit as
much as mine have are questions only installing a separator would answer. But
for me, this is one upgrade that I'd repeat, for the oil saved and for the
cleaning work reduced.