The Swiss sun-powered aircraft Solar Impulse has been delayed by strong winds during a stop-off on its first planned intercontinental flight, organisers said on Monday.
The plane landed in Madrid early Friday at the end of the first leg of its attempt to reach Morocco without using a drop of fuel.
After technical checks and a pilot change it was hoped Solar Impulse would leave for Rabat on Monday.
"Today was the earliest possible departure date but we are waiting for the best weather window," project spokeswoman Alexandra Gindroz told AFP.
"We have too much south-east wind."
The departure is not likely to be before Thursday, when the forecast is for calmer weather, she said.
Pilot Andre Borschberg took off from the plane's home base of Payerne in western Switzerland last Thursday.
After the weekend in Madrid it is ready to begin its second leg piloted by Bertrand Piccard, when it will leave Europe for the first time.
If successful the 2,500-kilometre (1,550-mile) journey will be the longest to date for the craft after a flight to Paris and Brussels last year.
The trip is intended as a rehearsal in the run-up to the plane's round-the-world flight planned for 2014.
The high-tech aircraft, which has the wingspan of a large airliner but weighs no more than a saloon car, is fitted with 12,000 solar cells feeding four electric motors driving propellors.
Explore further: Pollution top concern for U.S. and Canadian citizens around Great Lakes