Swoop
Well into the 'who cares' category, are the terms IAS, CAS, and EAS. Briefly...
IAS - Indicated Airspeed. What we see on the airspeed indicator. May have minor errors due to mechanical errors in the actual instrument.
CAS - Calibrated Airspeed. IAS corrected for 'position errors'. Position errors occur when the aircraft flys through its speed envelope in varying configurations (gear, flaps, external stores, etc). These errors are usually very small, so many folks just round them off and refer to CAS and IAS as being the same. All of this depends on the specific aircraft type. More often than not, the position error value is subtracted from the IAS to get CAS.
EAS - Equivalent Airspeed. CAS corrected for compressibility. Compressibility becomes a factor as the aircraft speed increases. In a high speed jet, these correction values can be as much as 5-10%. Usually, the compressibility factor (CF) is subtracted from CAS, ie if CAS=300, CF=25, therefore EAS=275.
Then there is TAS (True Airspeed). TAS is the actual speed of the aircraft through the air without reference to the ground. TAS takes into account the altitude and ambient temperature of the air. Generally speaking, as the aircraft climbs, the TAS value becomes larger than IAS value.
Because of this, this one can be tricky for us since many references to max performance give the speed in TAS, not IAS. So when you see a max speed at a certain altitude as being 400 mph, that means 400mph in TAS...the IAS that the pilot is seeing on the cockpit airspeed indicator is usually somewhat less. Also, these max speed values usually mean the max speed that the aircraft can accelerate to in 1 G level flight. As such, the listed speed is really good only as a comparative reference, since it is not the speed that you see on the airspeed indicator, nor is it the overall max speed that the aircraft could reach in a dive.
Hope this helps!
Andy
