Originally Posted by Globaliser

The former: a total of 4 qualifying flights, and a total of 600 TPs - all within the same membership year.

The reason that the graph shows promotion to Silver after flight 5 is that all of the flights in the illustration are standard CW flights at 140 TPs each. After flight 4, that person has enough qualifying flights but still only has 560 TPs. It's the fifth flight that takes them over the 600 TP mark.