The first round-the-world solar-powered flight, known as the Solar Impulse Two, is hoping to make the next and most difficult part of its journey.
The plane is powered completely by the sun and has been in Nanjing in China waiting to leave for a month.
So far on its epic round-the-world, Solar Impulse has only had to do short journeys of about 20 hours'.
The plane is now trying to fly non-stop from Nanjing in China to Hawaii in the Central Pacific.
The carbon-fibre aircraft has a huge wingspan of 72m - that's wider than a Boeing 747 jet - but it weighs about the same as a large car.
For a passenger airliner, the 8,000km journey would take around 10 hours.
But for this solar-powered, prop-driven, experimental aircraft - it could take 5-6 days and nights of non-stop flight.
Completing this journey will involve smashing several aviation records - not least the longest-duration journey for a single-seater plane.
The Swiss engineer Andre Borschberg, who will be at the controls, has lots of confidence in the technology, but he knows how tough the next mission will be.
"It's more in the end about myself; it's going to be an inner-voyage," he told the BBC.
"It's going to be a discovery about how I feel and how I sustain myself during these 5-6 days in the air."
Solar Impulse: The journey so far
LEG 1: 9 March. Abu Dhabi (UAE) to Muscat (Oman) - 772km; in 13 hours and 1 minute
LEG 2: 10 March. Muscat (Oman) to Ahmedabad (India) - 1,593km; in 15 hours and 20 minutes
LEG 3: 18 March. Ahmedabad (India) to Varanasi (India) - 1,170km; in 13 hours and 15 minutes
LEG 4: 18 March. Varanasi (India) to Mandalay (Myanmar) - 1,536km; in 13 hours and 29 minutes
LEG 5: 29 March. Mandalay (Myanmar) to Chongqing (China) - 1,450km; in 20 hours and 29 minutes
LEG 6: 21 April. Chongqing (China) to Nanjing (China) - 1,241km; in 17 hours and 22 minutes