Expert: Paul Soderman - 12/21/2009

Question
how the speed of the aircraft measured

Answer
The standard way to measure aircraft speed is by finding the dynamic pressure from pitot-static probes near the nose of the aircraft.  The difference between ram and static pressures is the dynamic pressure: rho*v^2/2 from Bernoulli's law.  Solving for v gives indicated airspeed that must be corrected for several things to get true airspeed.  Density (rho) changes with altitude and at high speeds compressibility effects enter.  Since each aircraft has a unique flow field (pressure distribution) that depends on flight condition and configuration, pressure calibration charts must be consulted or are calculated by the aircraft computer if there is one.  Finally, the wind speed must be estimated to get true speed relative to the ground.  

If an aircraft has GPS capability, airspeed can be found much easier and more accurately from satellite signals.  I assume most large aircraft have both pitot-static and GPS systems.
Paul