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Why Is Oil Streaking Down Your Engine Cowling?

Boldmethod

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 they prevent pressure from building up in the crankcase.

When your engine begins to heat up, the oil reaches extremely hot temperatures - hot enough for small quantities to vaporize into the air. Oil breathers are part of the accessory case, and they're installed toward the top of the engine 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 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 relatively 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.

SuperCub.org / Flickr

But many aircraft, especially older aircraft, aren't equipped with an air/oil separator. And when airplanes don't have an air/oil separator, 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 almost always 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

Oil weight also makes a difference. The thinner the oil, the more likely it is for small quantities to vaporize and vent out the bottom of your plane.

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

Checking For Problems

It's important to check the oil breather tube for blockages, because excessive crankcase pressure can lead to blowing out the front seal of your crankcase. If that happens, it 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 in flight.

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.

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