I am wondering that why do some major US airlines switches their flight number so occasionally. (like switching flight numbers every 3-6 months; especially their Domestic flight, although some international flights are also involved)
For example: NW's NRT-TPE flt in 1998 is NW011/012. But in 2001 (currently) the flt becomes NW007/008.
Another example: TW's YVR-STL flt in 2000 are 10 and 58x series. In 2001, it becomes 590/640
I also noticed that some airlines switches flight numbers involving code-share flights.
Any one know why airlines switches their flight number so occasionally? Don't airline usually switches flight number after a fatal crash?