After reading an article, I'm somewhat puzzled about the different units that describe airspeed. To be honest, I never thought about it before.My understanding is that -- I need to say that I fly airliners in the first place -- what the airspeed indicator shows is either IAS or Mach, where I can imagine that the Mach values respect altitude and thus air pressure somehow, so they're "calibrated" in some way.But when I read that many airspeed indicators indicate TAS, I'm no longer sure what to expect from the instruments.So, this goes to the experts: When do I need IAS, TAS, CAS etc. and what is used for the instruments of an airliner?Andreas

After reading an article, I'm somewhat puzzled about the different units that describe airspeed. To be honest, I never thought about it before.My understanding is that -- I need to say that I fly airliners in the first place -- what the airspeed indicator shows is either IAS or Mach, where I can imagine that the Mach values respect altitude and thus air pressure somehow, so they're "calibrated" in some way.But when I read that many airspeed indicators indicate TAS, I'm no longer sure what to expect from the instruments.So, this goes to the experts: When do I need IAS, TAS, CAS etc. and what is used for the instruments of an airliner?Andreas

I guess this could be what you're after?http://en.wikipedia.org/wiki/Indicated_airspeed