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.

Dübendorf, 8 May 2012 – If one was to look at this quiet industrial suburb of Zürich, one would never think that something revolutionary was going on. In fact, not even within the military compound would someone imagine that spectacularly innovative activities were taking place. 

Yet, behind the silence of the large military hangars, innovative minds are at work developing what will be Solar Impulse’s second generation aircraft: the HB-SIB.

Squeezing through the sliding doors of the hangar, one can see individual aircraft parts lying here and there, scattered work stations in various corners of the loft-like space and silently efficient engineers circulating around the premises, each absorbed in their own individual tasks. On Tuesday 8th of May, however, the four teams of Solar Impulse’s Dübendorf site all came together to witness the first structural integrity test for what will become HB-SIA’s big brother.

The first of a series of tests was done on the gondola - the part which carries the motor, the batteries and supports the airplane’s propeller. In fact, every piece that is designed and built needs to be analyzed and tested for two things:

• Limit load: the force that a given part is expected to bear during flights. Each flying part of the aircraft’s structure has to be tested to its limit load.
• Ultimate load: meant to push the limits of a given part’s resistance. During the ultimate load test, 50% more weight is added to verify a given part’s reaction to extreme conditions. Such conditions, however, are never expected to occur but strictly serve as a safety margin. Ultimate load tested parts are not to be used on the aircraft, strictly acting as a test part. The airplane itself, and therefore each new part, must be certified by the Swiss Federal Office of Civil Aviation (FOCA). Ultimate load tests are the key to achieving certification for production and the eventual certification for the aircraft to fly.

To undertake the test, the gondola was hung on a metal frame and, little by little, Solar Impulse’s structural testing team added weight. The suspense in the room was great and, with each additional weight, the tension in the hangar grew exponentially. As the last weight was placed, silence reigned. Should the gondola not support the final objective of 1.3 tons, it would result in months of added work.

Seconds later, the relief and satisfaction of the on-looking engineers could be felt in their collective sigh of relief: the test was a success!

Compared to the first gondola of the HB-SIA prototype, whose main structure consists of carbon fiber tubes and connectors covered by a foamy eggshell, the new casing is an integral part of the gondola, making it more resistant to light rain and humid climates; a requisite for the tour around the world. 

Steady state is for a human being what sustainability is for humankind. It means that the physiological parameters of the human body have reached an equilibrium that allow them to continue to work the same way for a long period. When you run, a sprint is not a steady state, a jogging is.

In our case, it would be possible to make a sprint where the pilot doesn't eat neither sleeps for 72 hours, but it would be useless as this situation could never be sustained if the flight was to last longer for a reason or another. The pilot would be so tired that he would never be able to land the airplane.

So the goal here is to push the limits for the lack of sleep, the food patterns, the hygiene in the cockpit, the use of the toilets, the shifts of the mission team, but without going too far. Until now, not too far is already quite impressive!

Bertrand (remembering that my first profession was medical doctor)