The ASI senses the static pressure of the ambient air via the static port and the total pressure of the passing air stream via the pitot tube. The instrument responds to the difference between the two to obtain a value called the dynamic pressure (total pressure - static pressure = dynamic pressure). Put simply, dynamic pressure is the pressure of the 'wind'.

Because the instrument does not measure dynamic pressure directly, any error in either the static pressure or total pressure measurements results in an error in the dynamic pressure and therefore the indicated airspeed. It is likely that the static pressure measurement is the biggest problem, since it is very difficult to accurately measure the static pressure of moving air and the exact position of the static vent is critical.

There will also be some errors caused by mechanical effects within the instrument itself. These errors are collectively called position and instrument error. When these errors are compensated for, the result is called calibrated airspeed CAS, (IAS with position and instrument error applied gives CAS). An airspeed calibration card is prepared to allow the pilot to convert IAS to CAS for a particular installation. The difference is usually very small and, for GA aircraft, it is usually ignored so that pilots assume that IAS and CAS are the same.

Wow! Thanks Bob!

Also just checking is there another word for Position that CASA could use?
I actually did the exam yesterday just missed out by 2 questions. One of it was this question which i did not understand, now i do.
But the options had no Position on it, if i could remember it was something like this:
What does an ASI Card do or measure or function as(around those words, can't remember exactly)
A) Instrument and Pressure
B) Compression and Pressure
C) Compression and Density
D) Instrument and something i forgot but was not Position

Thanks

Position error is sometimes called pressure error or installation error. So it could be your answer (a) depending on the actual wording in the question.

Thank you so much Bob!!

My resit of the exam is tomorrow will let you know the outcome !

Thanks