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…