Same route, same day, same airline -- different flight times?
 

This is probably a newbie question, but it's been bothering me -- UAL operates several daily nonstop routes from RAP to DEN. On the day that I'm looking to fly, the 3pm flight is booked at 79 minutes, while the 8am flight is booked at 91 minutes. That seems like a pretty sizable difference for such a short flight.

So, same airline, same route, same day, short haul domestic, but one is more than 15% shorter than the preceding flight. Are they factoring in morning traffic at DEN? Or is there something else going on?

Traffic, routing, winds, gate availability, ...

Lots of historical data goes into determining the flight times.
Yes, the Block to Block (gate to gate) time can vary significantly, based upon the time of day. Weather can't be predicted months in advance, but scheduled arrivals into a hub, and departures outbound, and projected gate utilization can.

Certainly they're not taking winds into account six months in advance.

But, in the context of just overall historic data, I guess it makes sense. It just seems like such an outsized difference for such a short flight. By comparison, if you extend the same difference out to a longer transcon flight, it would be like one flight booked at 5 hours and the next at 5h45.

DOT asks airlines to adjust their published flight times if specific flights are consistently late. While all the above factors are considered to some extent, I think the airport tarmac traffic and takeoff/landing waits are the most significant (and the most variable at different times of day). So if 8am flights are padded an extra 15 minutes to account for increased tarmac traffic at that hour, a similar amount is added to all 8am departures. A transcon would also increase by 15 minutes, not 15 percent of its duration. Therefore, it is much more noticeable on short flights.

There is historical data pertaining to everything. Certain times of day are more conducive to flights in particular directions.

Much of difference may have nothing to do with time in the air but rather time on the ground --- taxi time, ground traffic congestion, ....that can add 15-30 minutes comparing different times of the day and the congestion level at the airport.

DEN can have significant differences in taxi time depending on the runway availability and winds. It's designed so that you're usually landing "into" the gate areas, so taxi time is relatively short.

But yesterday, for instance, we landed to the north using runway 34R instead of the normal 35L or 35R. This landed us about as far away from Concourse B as you can get, adding 10-15 minutes of taxi time.

Right. And UA actually does have historic information about the prevailing winds and strength on a seasonal basis. Winds are commonly stronger later in the afternoon after temperature gradients have formed, for example.

But, generally, it's just "we expect more congestion on the taxiway at a peak time."

34R is nearly parallel to 35L/R. I suspect this wasn't weather-related, but rather traffic. The point stands, though -- UA has to take taxi times into account when they plan the schedule.

Incidentally, this is why you can seemingly spend forever taxiing, only to end up arriving at your destination on time. The pilots didn't (usually) go faster than the originally-filed flight plan; it's just that the airline padded their schedule because they expected the long taxi.