Simple speeds explained (without clockworks!)
Indicated airspeed (IAS) = what's on the dial.
Rectified Airspeed (RAS) = IAS corrected for instrument and pressure error.
Equivalent Airspeed (EAS) = RAS corrected for compressability error (an ideal reference for expalining what happens to an aircraft flying faster than Mach 0.4 hence DP Davies' use of this reference)
True Airspeed (TAS) = EAS corrected for temperature - the true speed of the aircraft.
and also;
Calibrated Airspeed (CAS) = IAS corrected for pressure and instrument system errors by an Air Data computer and what you see on the dial or screen on an aircraft with an ADC.
Hope this is simple enough for you Americans!
And 411A - I take it you mean early American jet aircraft. If the readers were lucky to have flown a well designed early jet transport such as the VC10 (the nicest big aircraft I've ever flown) they wouldn't have such a suprise!
Merry Christmas.