Solar Impulse 2 yesterday neared the end of its epic journey to become the first sun-powered airplane to circle the globe without a drop of fuel to promote renewable energy.
When the experimental aircraft touches down in Abu Dhabi it will cap a remarkable 42,000-kilometre journey across four continents, two oceans and three seas.
Solar Impulse 2 was expected to enter UAE airspace at around 1.30 am local time on Tuesday (2130 GMT Monday), and land in Abu Dhabi at around 4 am (0000 GMT).
With Swiss explorer and project director Bertrand Piccard in the cockpit, it is due to arrive at Al-Bateen Executive Airport in the UAE capital where it began its historic series of flights on March 9, 2015.
"My deepest admiration and respect for your courage," UN Secretary General Ban-Ki moon told Piccard in a live-streamed conversation. "This is a historic day not only for you but for humanity."
By 1900 GMT yesterday, Solar Impulse 2 had travelled more than 2,500 kilometres in nearly 44 hours on its final leg, flying over the Gulf after crossing the vast Saudi desert and Qatar's northern tip.
The aircraft was some 100 kilometres from touchdown at Abu Dhabi.
Monaco mission control said it was in a holding pattern at 11,000 feet awaiting ideal landing conditions.
Dubbed the "paper plane", Solar Impulse 2 has been circumnavigating the globe in stages, with 58-year-old Piccard and his compatriot Andre Borschberg taking turns at the controls of the single-seat aircraft.
It took off from Cairo on the final leg early on Sunday, having previously crossed Asia, North America, Europe and North Africa.
'Achieve the impossible'
Borschberg, 63, smashed the record for the longest uninterrupted journey in aviation history with the 8,924-kilometre flight between Nagoya, Japan and Hawaii that lasted nearly 118 hours.
No heavier than a car but with the wingspan of a Boeing 747, the four-engine battery-powered aircraft relies on around 17,000 solar cells embedded in its wings.
Its broad wings and light weight make it particularly sensitive to turbulence.
The plane has clocked an average speed of 80 kilometres an hour.
The pilots use oxygen tanks to breathe at high altitude and wear suits specially designed to cope with the extreme conditions.
They have had to withstand temperatures inside the tiny cockpit ranging from minus 20 degrees C (minus 4 degrees F) to plus 35 degrees C (plus 95 degrees F).
The plane, equipped with a parachute and life raft in case of accident, flew at an altitude exceeding 30,000 feet (9,144 metres) over Saudi Arabia on Sunday.
Piccard has said he launched the project in 2003 to show that renewable energy "can achieve the impossible".
His dream looks set to come true, but it has taken much longer than planned.
The attempt was initially expected to last five months, including 25 days of actual flying.
- 'For a better world' -
But the aircraft was grounded in July last year when its solar-powered batteries suffered problems halfway through the trip.
The project has also been beset by bad weather and illness which forced Piccard to delay the final leg.
While in the air, the pilot is constantly in contact with mission control in Monaco, where a team of weathermen, mathematicians and engineers monitors the route and prepares flight strategies.
"It's a project for energy, for a better world," Piccard told journalists in Cairo before taking off on the final leg.
A psychiatrist who made the first non-stop balloon flight around the world in 1999, Piccard had warned that the last leg would be difficult because of the high temperatures.
"It's been two hours now I'm flying into high up and down drafts. And I can't even drink. It's really exhausting..." he tweeted on Sunday.
Speaking to the BBC from the cockpit, Piccard described the final stage as a "fantastic moment" and likened the plane to a "flying laboratory".
"We are testing all these new, clean and modern technologies in order to fly with an endless endurance," he said.
While the pilots do not expect commercial solar-powered planes any time soon, they hope the project will help spur wider progress in clean energy.
"We have new insulation material, new LED lamps, we have new extremely light carbon fibre structures... All this can be used now on the ground," dividing "by two the energy consumption and therefore the CO2 emissions of the world", Piccard said.
"It's a complete revolution in the protection of the environment."