UX has a severe problem with reporting accurate information.

My sources tell me that mainline equipment report their flight status via satellite by using the following measurements:

Parking Break Release = Departed
Landing Gear Raised = In flight
Reverse Thrust = On Ground
Parking Break Engaged = Arrived

It's been my experience that mainline flight status is pretty accurate. If you want to see a really bad flight status system, check out aa.com.