Longer than usual flight time across Pacific

I took UA872 across the Pacific today (2/21). Before departure, it projects on time departure, 12+ hrs flight time, and 1h15m late arrival. It turns out that it departed 6min earlier, flight time is 12h15m, and 1h4m late arrival.

If you look at the flight history (link below), all previous flights took at most 11h15m, so this one is particularly long.


My questions are, what could be the delay factor (headwind?), and how come they can predict this before departure?
Thanks in advance.


They have some kind of maps with weather forecast or something like it. I know a pilot from Australia and he posts some of them from time to time. And those 6 minutes of earlier departure are kind of dependent on the airport itself. If the plane manages to taxi and go faster to the runway and if the airport is not so busy at this particular moment, the plane departs earlier and etc.
Here is one such a map with the route from LAX to MEL:

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Looking at your flight’s profile versus the two previous ones, it shows consistently lower speed and altitude, so chances are the weather was less than favourable that day. A 10% reduction in average groundspeed for an 11 hour flight will result in it becoming a just over 12 hour flight.

On long haul flights flight planning will look at finding an optimum minimum time and fuel burn track, but that is based on forecasts and pilot reports, so that will change over time. Sometimes it doesn’t matter which way you route, the weather just doesn’t play along.