United Airlines passengers spent 28 hours traveling across the Pacific Ocean as their flight made two unscheduled landings.

Flight 870, from Sydney to San Francisco, typically takes around 13 hours.

The Boeing 777 departed Australia on Sunday at around 11:30 a.m. local time, an hour later than scheduled, according to data from Flightradar24.

If passengers were irritated by the delayed takeoff, they had a lot more awaiting them.

Four-and-a-half hours into the journey, the plane turned southeast toward the Pacific island nation of Samoa, per Flightradar24. It landed in the capital, Apia, two hours later.

An airline spokesperson told BI the flight "diverted to Apia to address a potential medical concern with a customer."

The plane spent more than three hours on the ground in Samoa before taking off again, but it only traveled as far as Hawaii.

The six-hour flight to Honolulu also saw passengers cross the International Date Line. So when it landed in the state capital around 7:30 a.m. local time on Sunday, this was technically earlier than when the passengers left Australia.

However, they then had to wait another seven hours before they could depart again for the final leg of their journey to San Francisco.

Flight 870 ultimately concluded there at around 10:30 p.m. PT, some 15 hours later than expected.

The United spokesperson said the flight "made an additional stop in Honolulu to swap crew members."

It looks like this was because the emergency meant the crew would have exceeded their maximum working hours.

As the carrier doesn't typically fly to Samoa, it wouldn't have been able to find a new flight crew there without disrupting other journeys. By contrast, United serves several cities from Honolulu.

This tactic for minimising disruption is not uncommon.

Last month, a British Airways flight from the Bahamas to London also made two stops after a medical emergency.

It first diverted to Canada's remote Gander International Airport, before stopping again in Iceland.

British Airways arranged for the second stop because it was easier to send a replacement crew there than to Canada.