The Airspeed Indicator ASI

The Airspeed indicator ASI

Pictured above are two conventional airspeed indicators found in most light aircraft. The ASI like the Altimeter and VSI are what we call pressure instruments.
Pressure instruments measure the atmospheric pressure by using a pitot-static system. The pitot static system detects:

• Total Pressure - Static + Dynamic pressure which is also called pitot pressure, this is measured by a pitot probe pictured below from a cessna.
  
  

• Static Pressure - The static pressure gets measured individually by either a static vent/port usually located in the undisturbed air on the airframe, or a combined static port on the pitot probe itself. Pictured Below

So what happens now we have a measurment of Total pressure and Static Pressure?

What you see in knots on the ASI is a measurement of dynamic pressure.
Dynamic pressure is just the difference between Total pressure and Static pressure:

• Dynamic Pressure = Total Pressure - Static Pressure
  
  *Example - total pressure has a value of 10 and static pressure has a value of 6 then dynamic pressure would be 4
  
  

Now we have an understanding how it works lets look at the parts that make up the pitot static system and the ASI.

• Pitot System

As you can see with the above diagram the airflow enters the probe, from there it gets distributed to the ASI, VSI and Altimeter.

Things i didnt mention above is:

• Pitot Heat - This heats the probe in icing conditions, and prevents blockage due to ice buildup.
  
  
• Drain opening - This does exactly as it says on the tin, drain any unwanted water retained in the probe. Drainers on the PA-28 warrior are located bottom left just below the left hand seat, but this may vary on different aircraft.
  
  
• Alternate Static Source - If the main static port/vent gets blocked then this is here for backup.
  

some more.