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Date: Mon, 30 Aug 2010 10:42:47 -0400
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From: "Bill Wade" <super_chipmunk@roadrunner.com>
Subject: Static system
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  Bugs are not the only cause of static system blockage. One time during a 
transponder check the shop discovered that the encoder in my Chipmunk had 
moisture in it and had to be replaced. The plane is a taildragger and on the 
ground the static port is below cockpit level. It's hangared and isn't 
exposed to rain except on trips. However, humidity levels in the hangar can 
get quite high at times. I had installed the encoder at the low point of the 
system. Live and learn.

  If moisture could make its way up >ten feet of inclined tubing consider 
that an aircraft climbing from warm ground conditions to above freezing 
level has the potential for internal ice blockage. Any section of the system 
running through an unheated area could be affected. If IMC then there would 
be inaccurate altitude squawk, airspeed, altimeter, VSI, [AOA]. Autopilot 
altitude hold might also be affected. Not good. A WAAS GPS might supply 
altitude info depending on the display.

 North American designed the Navion static system with moisture traps at 
several points- TEE's with a short capped tube. They also built in dual 
static ports. One benefit is to average the pressure between both sides of 
the empennage and provide a consistent source. Another is to lessen the 
possibility of blockage. Cheap insurance, as is an alternate source that's 
kept closed until needed.  -Bill Wade