Arrivals, Departures and their Numbers

Hello all -

 This is my first post. Quite exciting.

I took my wife to the airport this morning. In order to waste some time, I read the flight numbers on the big blue screen. I just couldn’t find and pattern or reason for them. Some had three numbers - some four. The bigger planes seemed to have only three. (My big discovery)

 So - is there any reason, other than random numbers for tracking, for choosing what flight gets what number?

 Joe

I found this at:
http://olias.arc.nasa.gov/ASRS/dl8_callsign.htm

If it is indeed left to airline marketing departments, there will absolutely be NO rhyme or reason to flight numbers!