Rick,

 

I have used Pitt tubes in the past but never troubleshooted an airspeed indicator, so the following is just the rambling of an engineer, said to like to argue the way a pig likes to wrestle in mud.

 

The airspeed indicator should be fairly simple to test for calibration. It simply reads the difference in static vs. dynamic pressure. The values of each are irrelevant, just the difference. To check zero calibration, just check the reading in the hanger. It should read zero. To check the span, simply apply compressed air through a rubber hose fitted snugly over the dynamic port. The gauge pressure of the compressed air should correspond to the indicated air speed as V = square root of (2 * pressure / density). This is pretty simple to do, so I would bet the instrument is working properly. (If you want to try this, work out the pressure for the speed you want to check in advance. Don't just arbitrarily apply 100 psi. You could break the instrument if you use too much pressure) 

 

Thus, I would expect the problem to lye in the positioning of the static and dynamic ports. If the pitot tube is not parallel with the free stream airflow, the result will be a low speed reading. You cannot make it read high with incorrect angle relative to the free stream.

 

I would therefore look at the location of the ports. You might want to try checking the static pressure during a high speed taxi test. It should remain the same whether stopped or moving at any speed. If there is an error, does it correspond to the indicated air speed error you are seeing? (use the equation above) If so, then you may want to move the static port to a better location, unless you want to impress your passengers. If this isn't the problem, then it must reside in the location of the dynamic port.

 

Rob