Originally Posted by Blip
My experience with the B737 has convinced me that the FMC's in that aircraft indicate the wind accurately and with no delay.
The FMC simply calculates the wind by comparing the two vectors Heading/TAS and Track/GS.
I am not a professional pilot - but a simple controller. It seems to me, though, that your assertion is slightly flawed with respect to the 'with no delay' bit. I fully agree that the wind velocity can be determined by comparing the HDG/TAS and TRK/GS vectors but these vectors are historical and the resulting WV will reflect the average over which the vectors are determined thus introducing some hysteresis into the measuring system.
As has been pointed out, comparing the aircraft calculated WV with a surface wind will almost inevitably produce differences, particularly if the aircraft has been descending toward the runway during the period considered for the calculation. Bear in mind, also, that many surface wind reports are generated by equipment that averages the surface wind measurements (usually over 2 minute period). Even an 'instant' wind will be averaged over a short period, commonly 5 seconds.