Originally Posted by
avidflyer
I think it can also be measured as Gate pulled back. I have been on many flights where they rush to pull the gate back so they "left" on time but will sit and wait for an ATC delay at the gate or in the penalty box.
Guess what the pilot often does once the jetway is pulled back... Turn off the brake.
Pilots and FAs are usually paid from brakes off to brakes on. So that's part of the reason for the rush to get ready even if a delay is inevitable. The other reason is that delays can change quickly and they want to be ready to go if things open up unexpectedly.