How does the indicator work?
A basic airspeed indicator is a mechanical device that compares pressure from the pitot tube to pressure from the static port. The static port is mounted sideways with a hole the doesn’t face directly into the oncoming air. This way it gets a “static” measure of the air pressure. The pitot tube has a hole that does face into the airstream, so it has oncoming air forced directly into it.
The air from the pitot tube fills a diaphragm (like an accordion) and makes it expand. The air from the static port fills the gauge around the diaphragm and pushes it to contract. As the diaphragm expands and contracts it pushes a needle that we see on the instrument.
The airspeed indicator is very reliable but there are a few things that can go wrong.
Reading the airspeed indicator
The airspeed indicator is fairly self-explanatory to read. The most common mistake is not paying attention to units. Sometimes the instrument will measure miles per hour instead of knots. Always make sure you know which one you are looking at.
The indicator can also have some error, especially at high angles of attack. The manufacturer will often publish a calibrated airspeed table to help you determine the difference.