The Cessna 120-140 Association Forum

My recently acquired 1946 120 with an O-200 has a cruise speed that is 15mph lower than my previous 140 that had same engine and same 6950 prop. This 120 has the static air connected to the opening on the left side of the fuselage. The Encoder, Altimeter, ROC and Airspeed are all connected to this static air line. I reviewed the article in the archived section of the web site about the static system which recommends disconnecting the Altimeter and ROC from this line and use the cabin air as the source of static air, but leave the static line connected for the airspeed. The author implied that you should disconnect the encoder as well, but reconnect it when the encoder needs recertification every two years?? Not sure why. I also do not understand how this change makes the airspeed more accurate.

Any help with this would be appreciated.

Hmmmm,

I assume you have a GPS and can fly two legs, 180 degrees apart and confirm airspeed is indicating well off of true? (more or less ...pretty close?)

If so, then disconnect all the static lines and just leave them open behind the panel and try a flight and see if it makes any difference.

This comes up irregularly (on a regular basis) so I suspect having a static port on one side of the airplane only............affects certain aircraft.

It makes no sense to have some devices connected to one source and others connected to a different one!

Mine are connected to the port on the pilot's side boot cowl. If it were grossly inaccurate I'd put a second port on the opposite side and connect the two together. Also check the rigging to see if the plane is in a constant slip.

Another thing that can affect the static source is a venturi mounted near the port.