Breather Tube Whistle Slot

Hi All - I understand it's mandatory to to put a whistle slot in your crankcase breather tube even if the tube's exiting directly onto the exhaust, is that correct? If so, where should it go and how big?

It's not mandatory (at least in the U.S.), but certainly recommended.

Anywhere within the cowling where warm air will be blown into it. Drill a 3/16" hole, insert a punch, and bend upwards.

Why?

Why is that recommended Mel ?

Also, is it still recommended with an inverted oil system?

Thanks,

If the breather simply hangs out the bottom; Sub-freezing temps (winter, or high alt) will freeze water comming out the breather. This blockage will result in the front seal blowing out due to crankcase pressure build up. Susequently all the oil will be lost and the engine will fail within a few minutes of operation at zero oil pressure.

The slot prevents this sequence of events.

If the breather exit is inside the cowl above an exhaust pipe (as many RVs have installed), the ice will not form.

Mel,

Why do you state it is still recommended in this case?

Thanks!

Mike, thanks for the answer. That makes sense.

Temperature Data

I have a cowl exit air temp probe mounted through the cockpit floor on the aircraft centre line just aft of the firewall to hopefully detect an engine compartment fire (EGT probe to a stand alone instrument with an over temp alarm).

Typically during cruise I see cowl exit air temperatures about 40 to 50 degrees C greater than the OAT. Presumably, temperatures right next to the exhaust would be even higher so the OAT would have to be mighty cold for a breather exit in the cowl and near the exhaust to freeze.

Fin
9A

Learn something every day. Thanks guys.

Thanks for the replies all, and thanks to Mel for the location and size. I plan to put one in mine as I know of a pilot who owns a Piper Archer and had his breather freeze up. Very costly boom.

I thought I had read somewhere that it was mandatory in Canada to put a whistle slot in the breather tube. Kind of like we have to put a gascolator in regardless of whether or not we're carburated or fuel injected (I'm injected). I'm always worried that I'm going to miss something that's required to be done in Canada.

A relatively unrelated example about unknowns is my Precision Airmotive fuel servo. I was setting everything up based on the documentation from the website, and then found out in an e-mail to Precision that my fuel servo was "reversed". Reversed? Never even knew that existed. The e-mail was actually about exchanging the mixture arm, but something was mentioned and that's when it was discovered. If I had continued the path I was going, my mixture control would have been completely backwards. It's enough to drive you crazy sometimes.