Interestingly enough, the T-15 minute rule is there despite tight turnaround time for aircraft, particularly UAX. At my home airport of CVG, which is as of earlier this month, all express, there are only a couple of CR7s (usually ORD x 2 and sometimes the DEN flight) - all others are 50 seaters. Boarding is listed as T-30. However, I have rarely seen the aircraft scheduled turnaround time above 25 minutes. Yes, not a one-off, constantly (The CR7's are usually about 40-50, IME). That means boarding doesn't start until at least T-15 (can't start at T-30, since the aircraft isn't supposed to be there by then), and these tight aircraft turnarounds don't bode well for on-time departures. I know last summer UA said they were increasing these turnarounds to be better on time, and for the most part, I've seen most at ORD at an hour or longer for mainline in recent months. But these changes don't seem to have occurred for express. My favorite was when we were an hour late leaving DEN on Trans States. Inbound arrived on time - just a half hour after our scheduled departure time. This was not a last minute switch - this was how the aircraft was scheduled for at least a day. Not smart.