Why do some flight numbers have a letter at the end when they usually dont. For example, UAL611T and UAL611. It shows that UAL611 never took off from Chicago even though it was scheduled. UAL611T did take off and land when UAL611 was supposed to.
It’s so the same flight ID isn’t active at the same time or, in the case of UAL, it’s quite often due to a aircraft substitution.
UAL611 is scheduled to fly DCA-ORD-LAX. However, due to “aircraft service” (shown on the UAL flight status page), the ORD-LAX had a 767 substituted for the scheduled A320. So, the renumbered the flight by adding a “T”.