Originally Posted by
flyinbob
So 2 747s left LAX last night for SYD. 839 and 1748 (apparently this happens so often the replacement flight has its own flight number now!).
Replacement flights always have to have a different flight number - not only so that passengers know which flight they are on, but also so that ATC can tell them apart.
UA1748 is NOT assigned to LAX-SYD, it's a floating flight number that is used for rescheduled/positioning/etc flights. eg, it last flew JFK-SFO, and before that OGG-LAX. Recent delayed flights between Australia and the US have used other numbers in this range, such as 1750, 1752 and 1753.