Or just use letters in addition to numbers in the flight number. It woudn't take...

SoftTalker 4 months ago | parent | context | favorite | on: Airlines are running out of 4-digit flight numbers

Or just use letters in addition to numbers in the flight number. It woudn't take any more space in the data field, but chances are that a lot of them are coded as PIC 9(4) instead of PIC X(4).