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Q-talk 101 - Whistle Vent and Breather Tube

There are two important features you should incorporate into your engine's crankcase breather tube. The first is called a "Whistle Vent." It is possible for the moisture in the breather tube to freeze, if the conditions are right and it can cause excessive pressure to build up inside the crank-case. This could compromise some seals. In years past, A&P's would poke an 1/8" hole with an awl in the aluminum tube that exited the breather tube port and then they would lean the awl toward the firewall. This resulted in a hole that resembled the one found on a tin whistle, hence the name.

The plastic pipes found in today's tightly cowled engines could still benefit from a hole, but you will not be able to reproduce the distinctive whistle shape. The plastic materials used in this situation, however, could present a temperature problem of their own. High heat could soften the plastic to the point that the walls may actually collapse upon themselves; especially in tight bends. This, too, could lead to a high-pressure situation in the crankcase. To remedy this, the second feature to incorporate into your engine's breather tube would be to place a door spring inside the plastic tube to support the radius of the bends.

(Ed Note: I talked with Terry about these suggestions and we created a way to incorporate both features at the same time on my plane. I plan to make the 1/8" hole in the breather tube as well as another smaller hole about an inch aft of the "whistle" vent. I am going to loop safety wire through the small hole, through the loop end of the door spring and then up through the 1/8" hole, twisting the safety wire loosely. 1 do not believe the safety wire has to be pulled tight as that may cause the safety wire to cut a slot between the two holes. My biggest concern is to retain the spring and prevent it from moving.

Another trick Terry shared with me regarded the breather tube outlet. He uses a short (3.0"-4.0") piece of tubing whose inner diameter (ID) is the same as the outer diameter (OD) of the breather tube. He attaches this short piece in a way that allows lowest point to be near the outside air at the bottom of the cowling. He crimps the lower portion of the tube, without closing it off, until he gets a flat slot for the breather air to exit. He then places the end of the breather tube into this outlet making it secure. Terry also said you could use a metal tube that fits inside the breather tube and is flattened for the end as well.)



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