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Date: Thu, 22 Mar 2012 13:25:56 -0400
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From: Brent Regan <brent@regandesigns.com>
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Subject: Re: Crankcase "venting" after flight
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Remember that if you burn a pound of avgas you get half a pound of 
water. We have all seen water dripping out of a car exhaust on a cold 
morning. Combustion products blowing by the rings enter the crankcase 
where they come into contact with relatively cool surfaces and condense. 
Early Porsche motors were famous for making "Motor Mayonnaise", an oil 
and water emulsion that would collect in the stem of the oil tank.

Letting the crankcase vent after a flight is a good idea. It is a better 
idea after a short flight where the oil has not been hot for long, after 
low altitude flights where the vapor pressure is low relative to the 
crankcase pressure or in humid climates.

Opening the oil cap will cause air to flow in the crankcase vent and out 
the filler due to convection. Most of the crankcase vapors will likely 
be displaced in the time it takes to secure the airplane. Secure the cap 
and close the door when ready to leave. If you have to walk away with 
the cap off, flag the oil door with a "Remove Before Flight" banner.

Brent Regan