But they admitted that veteran Swiss pilot Andre Borschberg is absolutely exhausted after nearly four days' continuous flying, making the final 24 hours or so of flight particularly challenging.
"UPDATE #PACIFIC: @andreborschberg is tired. W/ turbulence at 8'000 feet & a cold front close, SITUATION IS DIFFICULT," said the latest tweeted update from the mission control center (MCC) of the pioneering Solar Impulse 2 aircraft.
"#MCC #solarTEAM is working hard to assess the situation & help @andreborschberg during this stressful period."
So far Borschberg has flown more than 94 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.
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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.
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 3.8-cubic-meter (134-cubic-foot) 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.
Its wingspan is longer than that of a jumbo jet but it weighs only 2.3 tonnes -- about the same as a car.