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Why Is Oil Streaking Down Your Engine Cowling?


If you saw oil streaking down the side of your car, you'd probably be worried. So why are there oil stains and streaks under your airplane's engine?

There are a few reasons, but the primary one is your oil breather.

What Does An Oil Breather Do?

Oil breathers are installed on nearly every reciprocating engine, and prevent pressure from building up in the crankcase.

When an engine begins to heat up, the air expands and oil reaches extremely hot temperatures, hot enough for small quantities to vaporize into the air. Oil breathers are part of the accessory case, and installed towards the top to vent this hot, rising air. The breather ventilation tube is usually located on the bottom of your engine cowling.

Why The Mess?

Some airplanes, like piston-powered Cirrus aircraft, are equipped with an air/oil separator. The separator takes oil that's been evaporated into the air, condenses it, and returns it to the oil system. This is why engines manufactured with an air/oil separator lose very small amounts of oil.

Your car is probably equipped with an air/oil separator, which is why you shouldn't see much oil streaking from your engine. / Flickr

On the other hand, many aircraft aren't equipped with an air/oil separator. And when they aren't, air containing hot, vaporized oil condenses when it hits cooler air. This results in the watery, oily mixture that you've probably seen streaking down your cowling. You'll find more oil streaks on these engines without air/oil separators.

Conditions That Make It Worse

On days with high humidity or frost, you may see more of this oil streaking. These conditions provide the right environment for condensation to occur, resulting in a strong oil/water mix dripping from the breather tube and running down your cowling.

Corey Komarec

On older engines, with worn out rings and seals, excess pressure in the crankcase may cause excess oil to leak. Additionally, the thinner the oil, the more likely it is to leak.

If you overfill your engine's oil, you'll have the same leakage problems. Excess pressure will force an oil/air mixture from the breather tube.

Checking For Problems

It's important to check the oil breather tube for blockages, because you'll risk blowing out the front seal of your crankshaft if pressure builds to extreme levels. That could result in oil spraying all across your windscreen, which is a whole different problem.

During your preflight, you should also check that the breather tube is facing the correct direction. The opening should not be pointed into the wind. If that occurs, the crankcase could become pressurized by ram-air from the slipstream.

Making Sure You're Ready To Fly

Seeing small amounts of oil streaking around an oil breather tube is normal, especially when your aircraft doesn't have an air/oil separator installed. But if you notice large amounts of oil pooling or dripping, you probably should talk with a mechanic before you fly.

Swayne Martin

Swayne is an editor at Boldmethod, certified flight instructor, and commercial pilot for Mokulele Airlines. In addition to multi-engine and instrument ratings, he holds a PIC Type Rating for Cessna Citation Jets (CE-525). He graduated as an aviation major from the University of North Dakota in 2018. He's the author of the articles, quizzes and lists you love to read every week. You can reach Swayne at, and follow his flying adventures at

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