As I was plumbing my pitot and static line through the fuselage it occurred to me," Why bother to connect the static line if I already have an open static line under the panel?"  The answer is I don't know and would appreciate enlightenment.

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Interior cabin is generally not at ambient atmospheric pressure (e.g. venturi of fuselage, open window, leaky door/window seal,etc.) The result is an erroneous reading on altimeter and airspeed indicator, (VVI as well, though relative) If alternate static ports are installed on production aircraft (generally opens to the cabin area) there will be a placard correction to altimeter readings when using the alternate static port.

Thanks for your input.  I will go ahead and connect my static line.

The airflow around the fuselage tends to pull a slight negative pressure on the air inside the cockpit via leakage through minute gaps, holes, and imperfect seals.  Same reason why carbon monoxide seeps into cockpits - it's being pulled in by negative pressure.

Sometimes, the reverse happens - the cockpit is pressurized positively by ventilation air scoops and ducts.  

Bottom line, the cockpit pressure can vary relative to the outside pressure.  The differences are small and probably not that significant for "low and slow bottom dwellers" like we fly, but there is a difference!

John

N750A

Thanks for your input.  I will go ahead and connect my static line.

All,

I am one of those folks who could not see a need for an external port. Something to do with a shortage of tube and connectors I suspect. Terminated line in rear fuselage. The differences between iPad and Panel altimeter have in general, been no big deal...except once. Flying near a class charley airspace a week ago the difference was 500 feet. Which one to believe? Which one to report to tower? I set altimeter to AWOS reported barametric and flew around the airspace without traffic conflict. I suspect that one of my winter projects will be to connect my static line to the external port. This may not resolve all differences between GPS and panel, but will reduce one possible source of error.

Ralph

N750RP

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