As we have repeated so often, before every structural test : « If, at 100 % of the intended load, the part breaks, then it must be too fragile; but if it doesn’t break, there’s a good chance it’s too heavy ! ».

This is the philosophy that we have applied since 2007 in optimizing the design of each component. But once the design process was complete, no component has ever again broken … up to now at least… each part proving both strong enough and light enough in weight.

We are always trying to work at the limits of the technologies we are using, and so we have further optimised the design and construction of the airplane’s spinal column, the main wing-spar. A few hours ago, the central section of the main spar of HB-SIB, our second aircraft, failed a torsion test, breaking before it reached the calculated maximum load factor.

It was the very last test, and everyone was getting ready to celebrate passing an important milestone. But the day was to finish in tears and bitter disappointment. More even than for Bertrand and myself, these prototypes are the cherished offspring of the engineers who did the calculations, designed and built them.

This is a major set-back – in fact the first since the Solar Impulse program was launched in 2003. We don’t yet know exactly what happened, what the explanation for the failure is, nor whether it will cause delays to the future operational timetable. The specialists will now have to start examining the matter, but I wanted to make you aware of it immediately.

One week after the wing spars were delivered to our Dübendorf hangar, we have finished assembling the 3 parts and can now (for the first time) contemplate the wing with its full 61 metre span! With the initial shock now past, I have to admit that we are already accustomed to this carbon structure invading our hangar, and are nearly used to its extraordinary dimensions.

Within a few days we will have fixed this wing to the cockpit, itself already attached to the fuselage and rear stabilizer. We will then have before us the first skeleton aircraft and be able to start extensive vibration testing, which I will be telling you about shortly.

The heavier the aircraft and the pilot, the more energy will be needed to keep them flying. So are hunting down the very last gram.

An aircraft's wing loading is calculating by dividing its total weight by the wing surface. With the amount of solar energy captured by our 200 m² of solar cells we cannot afford a wing loading of more than 8 kg/m². Which is precisely that of a seagull.

You could say we are looking to fly on board a seagull with the wingspan of a Boeing 747. And yes, we're in our right minds.

As you can see from the video gallery, the HB SIA's carbon composite structure is slowly coming together, like a finely engineered Swiss watch, piece by piece, test by test. The fuselage has passed muster, but a few days ago the landing gear failed to make it.

The next big moment everyone is waiting for is in a week's time, when we receive and can finally test the main wing beams. Let's hope they carry no hidden kilos! A seagull's life is a hard one.