Last month we reviewed the operation of the airspeed indicator, so this month let's review the information it gives us. To all of those who have done a cross country stage check with me, this should be familiar, I always ask and expect the aspiring pilot to be able to explain IAS (indicated airspeed), CAS (calibrated airspeed), TAS (true airspeed) and GS (ground speed). As I tend to be a bit long winded in my explanations on each of these, I will split it into 2 and this month we will just discuss IAS and CAS. Next month will be TAS and GS. So here goes...
IAS - Just as the name suggests, this is simply the airspeed read right off the airspeed indicator. Whatever the needle points to, that is your indicated airspeed. Note that IAS is the airspeed used in all pilot-ATC communications while airborne. When ATC wants to know your airspeed, it is IAS. When there are speed restrictions due to airspace, altitude or ATC requirements, this is always IAS. As we discussed last month, the airspeed indicator operates with the air pressure received by the pitot tube and static port. Aircraft at the same altitude and same local vicinity will be operating in the same air mass so there is a direct correlation between the IAS of aircraft A and aircraft B.
CAS - Again, the clue is in the name; Calibrated Airspeed. Calibrated to what? Well every aircraft has the pitot tube and static source located by the aircraft design engineers at the best possible location to get the most accurate measurement but...the aircraft moves about 3 axis and also we throw out disturbances to the airflow in the form of flaps, moving up and down, landing gear, leading edge devices etc. Additionally, the airflow from the pitot tube and static source has to travel via tubes that turn and bend around aircraft structure before it even get's to the airspeed indicator. These all set up errors in the system. During the flight-testing of each aircraft, a "boom" containing a pitot tube and static port is mounted to the nose of the aircraft and protrudes far forward into undisturbed air. The pitot and static ports on this boom are connected to an accurate airspeed indicator. The aircraft is flown throughout its envelope and in all configurations and at all points the airspeed read off the aircraft panel mounted indicator is compared to the "boom" mounted system. Errors are noted and this appears as a correction table in the AFM. Each aircraft has an airspeed correction table in the AFM and this will list the configuration of the aircraft and the corrected airspeed to use for this condition of flight.
So, calibrated airspeed is simply IAS corrected for errors in the pitot static system and can be found as a correction table in the AFM.
To review:
IAS is Indicated Airspeed and is the airspeed read right off the airspeed indicator. This is also the airspeed used in all pilot to ATC communications when airborne.
CAS is Calibrated Airspeed and is IAS corrected for errors in the pitot static system. It can be found as a look up table in the AFM.
Blue Skies,
Peter Swift, Chief Instructor
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