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3 June: Dallas-Saint Louis
"When you repeat an exploit several times, people think it has become almost a routine for you. But it’s not, especially in the case of Solar Impulse, as each flight is different because of weather conditions, terrain and requirements from air traffic control. So once more, we had to manage unexpected issues during the third flight of our “Across America” mission, between Dallas and Saint-Louis.
First of all, during the night before the flight, a strong storm damaged the hangar supposed to shelter the aircraft in Saint-Louis, pulling off a significant part of its roof. As the Solar Impulse needs to avoid the rain, our Mission Control Centre team had to make a quick decision: would it be a semi-rigid hangar built on purpose? Or our mobile hangar, lighter and more exposed to strong winds, but ready much faster? Finally, the mobile hangar was chosen in order to have enough time to build the semi-rigid one.
Taking off after the storm
So, Solar Impulse took off from Dallas Fort Worth Airport (KDFW) at 04.08 am local time (09.08 am in Payerne - Switzerland), with good conditions even if we could notice some crosswind at low altitude, a delicate situation which always requires a fine tuning from the pilot.
Two hours after sunrise (8.20 am local time, 01.20 pm in Payerne - Switzerland), the plane entered a large zone, partially covered with cirrus clouds. This type of clouds forms at high altitude (generally at least 20,000 ft.) and are made of ice crystals. They are easily recognisable, as they look like white feathers or long, semi-transparent cotton fibers… We had to pay much more attention as it was the first time we were actually flying under such an extended coverage of cirrus clouds. But it did confirm that, fortunately, the calibration of our cloud model for this specific case is right: both battery state of charge and energy collected from the sun were accurately estimated.
After the storm comes a calm
After this, we were not done with facing difficulties, mainly because of unfavourable weather at high altitude. So we kept a quite low altitude during ten hours, postponing the “mission climb”- it refers to the climb which leads the aircraft to reach its maximum altitude (~8,000m), at noon, solar time.- to the latest possible moment. Once again, we had to compute in details all the parameters of this climb in order to give the best flight strategy to Bertrand. And at noon by the sun (01.20 pm local time, 6.20 pm in Payerne - Switzerland), the aircraft was properly at the right altitude (27,000 ft.), with full batteries – as predicted by our computations. Bertrand could fly at this altitude during ninety minutes before sunset and then, he began the slow descent towards Saint-Louis.
But at this moment, we were already refining the details of the two last important phases of this flight: the holding in the vicinity of Saint-Louis, and the landing at Saint-Lambert Airport (KSTL). Processing the last weather update, available for 30 minutes, we tried to optimise three main parameters: the state of charge of batteries when arriving at Saint-Lambert, the delay before landing, and the speed of the low level jet Bertrand was going to encounter between 1,000 and 2,000 ft., during the last descent before landing. Finding the best strategy requires a lot of computations and verifications: each time, we analyse with the air traffic controllers the different possibilities we have and we cross-check with the meteorologists our computations and their own models.
Finding the best flight strategy
Finally, in the middle of the night (in Payerne - Switzerland), we could give Bertrand the optimised solution, with a reduced holding and a quite early arrival at Saint-Lambert Airport, allowing the best conditions to manage the low level jet at landing.
Following the aircraft on our computers, then on the television screens 10 minutes before landing, we saw once again the solar airplane, as silent as a night bird, spreading its long, illuminated wings above the runways, and slowly touching the ground, after 21 hours and 21 minutes of a great flight.
Well done, Bertrand, it was really perfect!"
Altran expert on Solar Impulse for Advanced Modelling and Simulation