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22 May: Phoenix-Dallas

"For a few days, the weather has been improving, and today, 22nd of May, is the best opportunity to fly. Actually, we have this “weather slot” before once again it gets worse, according to Wim’s and Luc’s forecasts.

To be honest, on the basis of our studies performed between September 2012 and February 2013, we know that this flight is probably the most difficult of the “Across America” missions. And for two main reasons: first, it’s the longest distance to fly, more than 1,300 kilometers; and second, the weather in Texas is known to be difficult, windy and quickly changing, with a high risk of storms and tornadoes during the spring.

Our computations show that the flight is possible, but difficult, and as the MET team is sharing the same analysis, we give our take-off clearance and the Safety Board Review confirms the flight. At 11.47 GMT (4.47 local time), the Solar Impulse takes off from Phoenix, approximately one hour before sunset.

Following our flight plan, after a continuous climb during two hours, the Solar Impulse flies over the mountainous areas (2,000 - 2,500m), East from Phoenix, and then joins Roswell while performing what we call “the mission climb”. This expression refers to the climb which leads the aircraft to reach its maximum altitude (~8,000m), at noon, solar time. At this moment, both electrical energy in batteries and potential energy due to altitude are at their climax. The aircraft keeps this altitude till roughly ninety minutes before sunset.

As expected in our computations, this first half of the flight is not really complicated, although André encounters some “mountain waves”, a type of turbulences which are uncomfortable for the pilot (giving a “bumpy” flight). But what is now in front of us  is quite different: a strong tailwind during the descent, and a low-level jet in the vicinity of Dallas. In order to properly manage these difficult points, our computations help us define the best strategy: during the descent, André has from time to time to turn into the wind coming from West, in order not to arrive too early in Dallas. As the speed of wind is up to 35 knots above the speed of the plane, this means that the plane flies backwards, a really difficult way to fly a plane. Fortunately, the wind is decreasing as the aircraft is losing altitude. 

So, during two hours, at sunset in the sky of New Mexico and Texas, one can see an unknown aircraft flying in a very strange way. And I don’t know what people think about this, as we are not very far from Roswell…

The landing at Dallas is an extraordinary moment. As expected, the low-level jet is there, but with a reduced intensity, thanks to the late arrival at Dallas. We see that the aircraft is very difficult to maneuver during the one hour holding and the final approach, due to a wind almost the same speed as the plane. Thanks to André’s exceptional flying skills, the landing is great, absolutely perfect, but I must confess that everyone in the Mission Control Centre is holding its breath while watching the great moment, live from Dallas. At 06:16 GMT (01:16 local time), the plane safely lands in Dallas Fort Worth airport, breaking the record of the longest distance flown by a solar airplane!"

 

Christophe Béesau

Altran expert on Solar Impulse for Advanced Modelling and Simulation