Steve Fossett's attempt to fly solo, non-stop around the globe without re-fuelling is in trouble.
The attempt is progressing well despite some hitches
His GlobalFlyer plane has reached China - half way around the world - but he has insufficient fuel to get him home to Kansas without favourable winds.
Mission controllers will have to decide whether to call off the attempt before Fossett heads out over the Pacific.
The adventurer left the US on Tuesday at 0047 GMT and was expected to return to the Salina airport on Thursday.
"I don't have a high level of confidence at the moment," Steve Fossett said from GlobalFlyer.
"This is a huge setback. Immediately I started thinking about what the alternatives were - whether a route could be taken through Mexico, and I began to think how far I could fly with the engine out [just gliding] just to make the landing."
There are two ways of calculating the fuel load on GlobalFlyer - fuel burn sensors and fuel probes in the tanks. The latter show the vehicle is missing 1,200kg (2,600lbs) of fuel.
Mission controllers are at a loss to explain the discrepancy - whether the plane was incorrectly filled on the runway or it burnt off too much fuel in the early stages of the flight - but they say they have to work on the basis that the probes' (lower) measurement is correct.
This means Fossett has 15% less fuel than he thought, and he will now struggle to complete the mission without good tail winds over the next day.
Fossett reached the half-way point in his flight at 0705 GMT on Wednesday.
As of 1440 GMT, just after briefing the media on his fuel worries, he was moving out over the East China Sea, just east of Shanghai, travelling at an altitude 13.7km (45,100ft) and a speed of 618km/h (384mph).
Mission controllers may decide to bring Fossett down in Japan or Hawaii rather than risk sending him right across the Pacific.
VIRGIN ATLANTIC GLOBAL FLYER - LONG-DISTANCE JET PLANE
(1) Fuel tanks - Gross weight is 10 tonnes; empty weight is 1.5t
(2) Engine - Williams FJ44-3 ATW (10,200 Newtons of thrust)
(3) Cockpit - Pressurised and large enough for pilot to lie down
Length - 11.7m; Height - 3.6m; Wingspan - 35m
Speed - in excess of 460km/h; 290mph; 250 knots