A solar-powered plane on the riskiest leg of a round-the-globe bid is closing in on Hawaii after a record-breaking flight which has tested its exhausted pilot to the limit in "difficult" conditions.
Veteran Swiss aviator Andre Borschberg, who has spent more than four days flying from Japan in the Solar Impulse 2, is expected to land on the Pacific US island state early Friday if all goes well.
"After the longest and most tiring night of this flight, bringing the pilot and aircraft to the limits, Andre is now back under the oceanic sunlight," the mission organizers said in the latest update.
By 0200 GMT the plane had travelled 91 percent of the way to the tropical US state, having flown 7,471 kilometres (4,642 miles), with just 700 kilometres to go -- barely 12 hours of flying time, in theory.
More importantly, it had crossed a last cold weather front before Hawaii, which organizers described as "jumping over the wall" before the final stretch towards the Pacific archipelago.
Before that hurdle organizers had tweeted "@andreborschberg is tired. W/ turbulence at 8'000 feet & a cold front close, SITUATION IS DIFFICULT."
But later came the celebratory tweet, saying the plane had "successfully crossed the second & last front separating him from Hawaii! Everybody clap your hands!"
The pioneering plane is due to land Friday morning local time at Kalaeloa Airport on the main Hawaiian island of Oahu.
The organizers' latest estimate for arrival is 1600 GMT, although that could change depending on conditions.
So far,Borschberg has flown more than 104 hours -- easily beating the previous longest solo endurance flight by Steve Fossett, who flew for 76 hours and 45 minutes in 2006.
The whole trip from Japan to Hawaii was expected to take 120 hours.
The Swiss aviator is napping for only 20 minutes at a time to maintain control of the pioneering plane. He is equipped with a parachute and life raft, in case he needs to ditch in the Pacific.
- Thumbs up! -
The experimental solar-powered aircraft left Japan around 1800 GMT Sunday -- the early hours of Monday local time -- after spending a month in the central city of Nagoya.
The propeller-driven plane was originally scheduled to fly directly from Nanjing in China to Hawaii, but bad weather along the way forced a diversion to Japan that stretched to a month.
Borschberg is alone and entirely self-reliant in the unpressurized cockpit.
Traveling at altitudes of more than 9,000 meters (29,500 feet), he has to use oxygen tanks to breathe and experiences huge swings in temperature throughout the day.
Solar Impulse 2 set off from Abu Dhabi earlier this year in a multi-leg attempt to fly around the world without a single drop of fuel.
It has 17,000 solar cells and on-board rechargeable lithium batteries, allowing it to fly through the night.
Its wingspan is longer than that of a jumbo jet but it weighs only 2.3 tonnes -- about the same as a car.
As he approached Hawaii overnight Thursday into Friday, Borschberg flashed a thumbs up for the camera, in clearly optimistic mood.
"At the moment the airplane's critical components are working well! It's important to continue this way until the end."