Rick et al:

Like the rest, I suspect your static port is in error.
Leakage to cabin could cause artificially low pressures,
or it could be a placement error.

The problem is not a restriction in the line if your VSI
is working correctly.


The easy test for static errors is a pair of tower flybys.
You want to fly at some location where you can visually
guarantee that the two passes are at the same altitude.
Make two passes at substantially different speeds. If the
altitudes differ, the static port is producing an error.
By using higher speeds, this will be much more dramatic
than the errors seen during a high speed taxi test.
Also, the presence of the ground alters the flow pattern
around the airframe in a way that may alter the static
indication.

So.... if you're looking for an excuse to call the tower and
request a high speed pass, this is it. Visually line up the
top of the tower with the horizon and check the altimeter.
Depending on your local environment, and hand steadiness,
it may be preferable to have a copilot take the data.
Make one pass at 100 kias, and one at 200 kias, and you should
see a significant deviation if the static source is the problem.