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.

While the HB-SIA prototype continues its mission in the Kingdom of Morocco, its big brother is beginning to take shape as the Solar Impulse team in Dübendorf slowly brings to life this next generation aircraft.

In the construction of our second aircraft, the spar is the structural backbone of the airplane. Earlier this week we undertook the second structural testing of the spar, the 72 meter long carbon structure that will be the wing’s internal frame. This testing is a key milestone and one of the most crucial moments as we developed a new set of special technologies. It represents full validation of the design and construction technology employed.

A first attempt was made a week ago but, to the horror of the team of engineers, an unsettling cracking sound was heard triggering them to immediately stop the testing activity and verify the problem.

Just like during the testing of the motor gondola, the structural loadings are initially simulated in a computer modeling program before they’re done in reality. The program is used to study the resistance of the wings to the most extreme atmospheric and meteorological conditions the airplane might experience in flight. Although such conditions might not ever take place, it’s a way to ensure the wings will be prepared to resist in any given scenario. In fact, this is very similar to other transportation aircrafts which are also tested before put into operation.

The wing spar is set upside down, as can be seen in the photograph, and lead weights are distributed along the wooden frames, simulating aerodynamic forces. For the spar to be safe for flying, it needs to survive the structural test without any consequences, but as soon as the cracking sound was heard, the team turned pale-white fearful that irreparable damage was done to the spar. Although I am not an engineer, I can imagine how terrible it must be to think that months of hard work could be thrown down the drain.

Thankfully in this case what will most likely come out of the workshop won’t be a damaged spar but a new set of jokes. In fact, it turned out that the terrible screeching wasn’t from the spar or the testing equipment, but from the hangar’s roof! The weather never gets tired of playing bad jokes on Solar Impulse, does it? During the first test, the thermal conditions were so extreme, including a mix of heavy rain, strong winds and intermittent sunshine, eventually leading the tin roof to cry out for help and resulting in great agitation and cold sweats within the Dübendorf team. But all is well that ends well: during the second attempt, the structural testing unfolded smoothly to the joy and excitement of Solar Impulse’s engineers.

So when will we hear a new series of jokes? I guess we should ask the Dübendorf boys…

Having known André for a long time, I am always admiring of his performance, but never surprised by it.

I have in fact found it rather amusing to watch the excitement, even nervousness at times, of the medical teams over the past few days. They lay in wait for the slightest sign of failing in order to respond immediately, but I knew only too well there would be none!

With the yoga exercises in the simulator, André was as much a fakir as a pilot, and a fakir never cuts himself with his sword...!

The other aim of the exercise was to train the task team in situations of fatigue and stress. Once again, total success was achieved. Proof of this is the atmosphere of solidarity and concentration that brought all the participants together over the 72 hour period, which is already fostering feelings of nostalgia in them all.

We will see over the next few days whether some of them come to the task centre for a few hours a day to avoid too abrupt a withdrawal! You too should manage the withdrawal ahead of the flying tasks this Spring over the Mediterranean.

Great team spirit happening! The only causality thus far is the coffee machine, which has been working non-stop and could really use a 100'000 km check up!

A last burst of energy and inspiration to share with you the last subject on my list I absolutely need to learn a little "endurance" from André.

One of all the many aspects in keeping André sustainable for so long is the cockpit. This week we have been testing a prototype cockpit for the 2^nd airplane. Compared to the 1^st cockpit in HB SIA, this is definitely upgrade from economy to business class!

In collaboration with Lantal, a partner company, a special seat was developed and tested during the simulation. This seat is built with an inflatable pneumatic technology offers several positions (flying, relaxation and resting) and has an integrated toilet. Wow, no morewaiting in line to go to the bathroom!

As to avoid any health issues that can arrive from immobility, André has been pursuing a series of exercises developed by Doctors and Osteopath from Hirslanden. Watching these exercises on our screens from André's on-board cameras, these look really strange and one may wonder what he is really up to ! Actually my theory is that he is using some of those techniques he picked up in India.

Landing expected at 08h00 (UTC+1). Go for it André, less than 90 minutes left.

And for Bertrand, we will see you later this year here in the simulator. But in the meantime we have a few real flightscoming up soon