This was in 27 yrs of Rvator. Perhaps that was what you were looking for.
Don?t Blow the Breather Tube [2001 6th]
by Ken
One of the more overlooked installations on aircraft engines is the ?breather tube.? If most of us think about it at all, it?s just to cuss the oil mess on the belly of our airplanes. Otherwise, we rarely look at or think about it.
Why do we need a breather tube anyway? It?s inherent in piston engines. The seal between the piston ring and the cylinder wall is pretty good, but it isn?t perfect. The pressure in the chamber on top of the piston crown goes from ambient atmospheric to very, very high when the spark plug fires and the fuel/air mixture ignites. The great majority of the expanding gas shoves the piston down, but a small portion of it forces its way past the oil on the cylinder walls, past the three or four piston rings riding on the oil film, past the piston skirt and into the crankcase. Multiply this by all the cylinders and there?s a significant quantity of gas going into the crankcase, which raises the pressure. Obviously, this gas, which is doing no useful work, has to go somewhere. The usual solution is a simple vent to the atmosphere. On a typical 160-180 Lycoming, this vent is found at the top of the accessory case on the rear of the engine.
The gas escaping from the crankcase is not pure. It contains atomized oil, water vapor and combustion byproducts. Since having these contaminants sprayed liberally around the top of the engine compartment is not desirable, a tube run from the vent to the outside of the cowling dumps the whole mess overboard.
Maybe. There are several things that can go wrong, as noted by Kyle Boatright, an RV-6 builder/pilot from Kennesaw, Georgia:
After landing following a short flight on Sunday, I saw afew drops of oil on the ground under the aircraft. I rolled it into my hangar, took a look the belly, and found it COVERED in oil. I pulled the dipstick (which was sitting there loose, for some reason) and checked the oil level down 6 quarts to 4.5 quarts. This wasn?t good, even though there had been no signs of a problem from the engine instruments. Looking at the mess gave me that awful ?this could be really expensive? feeling in the pit of my stomach. I pulled the cowl and found no ruptured hoses or loose fittings, other than the dipstick. There was little oil inside the cowl or on the firewall. In fact, the pattern of oil down the belly began right behind the breather tube, so it seemed that all the oil was lost through the breather. I called Lycoming. They suggested I do a compression check and look for a broken piston ring. High pressure gasses
escaping past a broken ring might pressurize the crankcase and force oil out the breather. They also suggested checking the breather installation to make sure it wasn?t pulling a vacuum on the crankcase. The compression check showed 79/80 on all four, so there was no broken piston ring that was a big relief? However, when I check my breather tube exit, I found I had beveled it about 45 degrees, and installed it facing aft. When I went over this with my Al, we discussed the breather tube, and the fact that I?d found the oilflller cap loose. He said, ?That?s it. The air exiting the cowl rushes past the breather, and with that orientation it causes a low pressure area behind it. That pulls a slight vacuum on the crankcase, and with the oilflller cap loose, there?s a path for the vacuum to pull air and oil right through your engine.? I reversed the bevel of the breather, added a quart of oil, double-checked the filler cap, went flying, and returned with no oil on the belly. Lesson learned. I?m glad it was on a short flight, not on a cross-country.
Kyle?s problem, fortunately, was purely cosmetic. But, there are things that can go wrong even with something as simple as a breather tube and the consequences can be deadly. Several years ago, an RV-4 was lost to a plugged breather tube. The airplane was operating in a very cold climate and the water vapor contained in the breather gasses condensed and froze to the interior of the tube wall overnight. When the engine was started the next day, the airplane was able to take off and climb before the pressure in the crankcase built to the point where it pushed the front crankshaft seal out of its seat. When that happened, the engine pumped its oil overboard. Neither occupant survived the subsequent forced landing. It is common practice to put a ?whistle slot? in the tube, so if a plug forms, gasses can still escape and the crankcase will not be pressurized. It is also common practice to put the end of the breather tube in close proximity to an exhaust pipe. The idea is that the oily vent gasses will at least partially burn off and not deposit themselves on the belly. At least in my experience, the result has been a burned oily smear, instead of an unburned oily smear.
Another common installation is a breather separator a canister in the breather tube which allows the suspended oil, water, and waste droplets to condense out while the gasses escape. In some versions, the oil is returned to the engine,.on others it simply collects the waste and must be drained off at intervals. We carry a simple separator in our catalog (EA Air-Oil Separator, $130.)
Aerobatic airplanes use an inverted oil system the Christen system is popular that includes a canister in the breather line. When the engine is subjected to negative Gs, the oil floats around the sump and a good deal is flung out the breather. Without a way to recapture it and return it to the engine, the airplane would end up covered at best, and out of oil at worst. Although they work well, Christen systems are expensive and hard to fit in the compact RV cowls, so relatively few RVs use them.
Since we had an airplane in the engine installation stage, I trotted down to the prototype shop to see how our guys had installed the breather tube. I found a piece of 5/8? rigid aluminum tubing fastened to the firewall with an Adel clamp. It arcs over the engine mount on the top, where a piece of automotive heater hose connects it to the fitting on the accessory case. The bottom ends within a couple inches of the exhaust pipe. This installation has served us well on several airplanes and is now part of the Firewall Forward package.
