Crankcase Breather Tube

If you fly a piston engine aircraft, and you fly in the winter, you should be aware that there is a tube running from the crankcase to the outside of the airplane, generally (but not always) running out the bottom of the cowling.

This tube is very important. The piston rings don't seal perfectly, and there is a certain amount of "blow-by" gases from the combustion that makes it past the rings. An older engine with worn rings will have more blow-by.

So, the crankcase is pressurized, and the older and more worn the rings, the more pressure there will be in the crankcase.

To vent the pressure in the crankcase, where the oil is kept in a wet sump engine, most aircraft manufacturers run a simple tube from the top of the crankcase to the bottom of the engine. And that simple arrangement works pretty well in the summer - unless you put too much oil in the crankcase, and it barfs the extra oil out onto the belly as the crankcase whips up the oil in the sump.

However, water is a byproduct of combustion, and in the winter, an emulsification of oil and water is present at the end of the breather tube, and it can freeze and plug the tube shut.

When this occurs, very bad things happen. The crankcase continues to be pressurized by blow-by, and the next weakest link is the crankshaft seal at the front of the engine, where the prop is bolted on.

The seal blows, and the engine oil gets blown out, covering your windscreen. You will notice when this happens.

To avoid this, with a solid breather tube, you need to drill a hole or two farther up the breather tube, inside the engine. If it’s a flexible tube, partially cut a slot with a sharp knife.

Some people like to wrap the holes with one wrap of electrical tape and hope it blows before the crank seal, but that makes me nervous. I don't cover the vent holes, and if a little engine oil makes it into the engine compartment, oh well. That's why God invented mineral spirits and rags.

Last year a friend of mine landed his Robinson helicopter for gas on a very cold day (for us) here in Eastern Ontario. I noticed what looked like brown snot hanging down from the breather tube. I broke it off, and realized that it was plugged solid.

I got my knife out, and dug out 4 inches of solid ice that had formed, plugging the end of the breather tube. Fortunately the manufacturer had drilled 2 tiny holes further up the tube, and they were venting the crankcase gases.

Recently, we took up the three Pitts. It was below freezing, but not terribly cold. Two vents had no ice, and the third was almost totally plugged up! I have no idea why that one tube plugged up as the airplanes are almost completely identical.

Anyways. You're Canadian. You fly when it's cold. You should know about this potential problem, and please check to make sure that you have a small hole (or two) drilled up in your crankcase tube, if you're going to fly in the winter!

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acboyd@gmail.com Jan 2012