Passengers to lose a day as fourth longest flight in the world takes off​​​​​​​​​​​​​​

Inaugural flight took off from Houston on January 18 and will land in Sydney on January 20

A United Airlines 787 Dreamliner operates the world's fourth longest flight. Nam Y Huh / AP

Most long haul routes leave air passengers feeling fatigued or jet lagged, but a flight between the United States and Australia will leave body clocks spinning.

Passengers on board the inaugural United Airlines flight 101 from Houston to Sydney will be left wondering what day it is when they land in the Australian city on Saturday.

Flight UA101 took off from Houston at 8pm Central Standard Time on January 18 and is scheduled to land in Australia at 6.30am local time on January 20.

It is the fourth longest non-stop flight in the world, with a duration of 17 hours 30 minutes. Factor in that the Australian time zone is 17 hours ahead of Houston, will mean the passengers have "time travelled" a day.

The flight is operated by a Boeing 787-9 Dreamliner, which covers 13,827km across vast stretches of the North and South Pacific Ocean and crosses the International Date Line.

Airline tracking monitor Flightradar24 showed at 1pm Arabian Standard time that UA101 had barely covered half of the flight distance and had yet to reach the International Date Line.

Sometimes flights in the other direction earns travellers a day.

On New Year's Day, passengers on board a delayed Hawaiian Airlines flight that took off from Auckland found themselves back in 2017, when they landed in Honolulu.

The five longest flights in the world

1. Auckland to Doha

2. Dubai to Auckland

3. Los Angeles to Singapore

4. Houston to Sydney

5. Dallas to Sydney

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