Originally Posted by
Tunapalooza
Experience tells us that pushing back is not the same as departure even if that is the metric that is used. Recently I flew SNA-SFO (commuter jet). We pushed back, taxied a few feet, and then shut down for 45 minutes. The UA app credited the flight with a departure and predicted an arrival time based on the supposed departure, but we sat on the apron instead. There was an issue of flow control into SFO, so the delay wasn't UA's fault. But this illustrates that "on time departures" ain't necessarily so.
This reinforces why they should push back and get numbers on taxi. No point in sitting there for numbers when they're going to have to wait after push anyway. Might as well free up the gate for the next inbound (also a problem for post-merger UA), and have a shot at going sooner if the flow issue clears up.
As for united.com showing an arrival time based on push, that's really a function of what's feeding into it. I usually use FlightAware when attempting to predict UA inbound arrivals. It updates much more frequently, and has ATC in there, so that if there's a wheels up time, it's reflected, and arrival time is updated as well. I take the FlightAware info post-departure, pad a few minutes for taxi to the gate, and it's usually much more accurate that united.com's estimate, which usually doesn't adjust nearly as frequently as other sources.