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15 June: Cincinnati-Washington DC

Reminder: Our last challenge consists in flying from Saint-Louis to Washington. But we had to separate this ultimate step into 2 different flights because of unfavorable weather conditions in the state of Virginia. So, we planned two flights: a first one from Saint-Louis to Cincinnati and a second one until Washington. While André arrived at Cincinnati yesterday where we had a quick stopover, today it is up to Bertrand to fly until Washington. 

“After four or five hours of sleep, everyone is back to the Mission Control Centre (MCC) and ready for another 24 hours of continuous work. Unfortunately, we immediately understand that today’s flight will be a race against time.


A heavy fog prevents the Solar Impulse from flying

The culprit’s name holds in three letters: FOG. Three letters that we don’t like at all, here… On the runway 21L where the plane waits for take-off, quite a heavy fog is visible. As the airport is near a river, the humidity of air and the change of temperature at sunrise cause this fog. In these conditions, a take-off is impossible for two reasons: the lack of visibility is the major difficulty, but also moisture which has to be removed from every critical part of the plane, especially from the solar panels and the electrical engines.

After checking the weather situation once again, Luc and Wim reassure Bertrand and the whole team: the fog will disappear as soon as the sun will be high enough in the sky to warm up this layer of moisture. During this time, the ATC team and ourselves analyse the possible consequences of a late departure: how can we ensure Dulles Airport that we will be on time on the planned track? Do we have to modify the flight plan? What about the energy collected by the plane? What about the maximum altitude to reach, and above all, its impact when we will cross the Appalachian Mountains? Will this late take-off modify the crosswind? Because of these many questions, we have to use the ten processors of our computers to elaborate all the answers as quickly as possible. And here is what we find out: yes, a late departure is possible without serious impact on the collected solar energy. Yes, we will have to be careful with crosswind while crossing the Appalachian Mountains. And no, this flight will not be very simple, because our delayed take-off increased our chances to meet cumulus clouds during the flight. 

Finally, after 2 hours of delay and some extended tests and run-up of the engines, Bertrand takes-off at 10h11 local time, 16h11 in Payerne. As Washington asked us not to arrive in the vicinity of Dulles Airport before midnight (local time), three holdings are planned so as to wait for the landing slot: the first one above the Appalachian Mountains, the second one at around 60 nm from Dulles, and the last, short one, at only 8 nm, in order to wait for the final landing authorisation given by the control tower. 

Keeping a low altitude

Bertrand climbs up to 10,000 ft. in less than two hours, as expected, and we decide not to reach the 12,000 ft. initially planned as long as there are still turbulences and risk of clouds. This new strategy enables him not to use the oxygen mask, which is of course more comfortable and less tiring. And like for André’s flight yesterday, the impact of keeping a low altitude instead of having a normal “mission climb” is reduced: batteries are full at noon by the sun, and the expected landing slot at Washington leaves a very comfortable margin of electrical energy in batteries.

The aircraft reaches the first holding around 20h00 GMT, and we watch carefully the telemetry, as Bertrand will encounter the maximum crosswind at this place. You will have noticed that I talk about crosswind at each flight. Why is that? Because this only one parameter can change our take-off decision. Indeed as the wind pushes the aircraft in a wrong direction, the pilot has to turn the plane into the wind so as to maintain its course. But the more he turns into the wind (to avoid being pushed far from the planned track), the less he can actually fly towards the chosen direction. One can see that everything here is a matter of balance: if the speed of crosswind is close to the speed of the plane, most of the available energy is spent to prevent the aircraft from drifting, and the part of the power remaining to move it towards the chosen direction is tiny. So, the crosswind is calculated with a great attention and the value we find is one of the critical parameters we take in account so as to give “our” take-off decision to Ray, the Flight Director.

Fortunately Bertrand keeps the track and the aircraft doesn’t drift despite a crosswind up to 18 kt. Then, he leaves the holding but he soon notices some cumulus clouds appearing in front of him, lower than the plane. Even if they are not dangerous, as they will not turn into a thick layer and will remain segregated, we definitely feel that it’s time to cross the Appalachian Mountains.

Yes, we can!

The second holding, at 60 nm from Dulles, seems very long to us. We are eager to have landing authorisation, and Bertrand patiently draws “eights” in the sky while slowly as the sun goes down over the horizon. Finally, the American air traffic controllers decide at sunset to authorise the aircraft to join the last holding point. At this moment, once again, we analyse the results of our simulations compared to the updated weather data.

The last two hours of the flight are an indescribable mix of concentration, relief, and happiness to reach Washington. Yes, there is this last holding, but it is so close to Dulles airport that we know Bertrand can (almost!) see the runways… And we also know that the hardest part of the flight is behind us, too. But everyone keeps focusing on his work: the ATC team is negotiating the conditions of the final approach with the control tower, Luc and Wim are computing and double-checking the wind at low altitude, and Stephane and myself are comparing the results of our updated computations with the telemetry data, in order to give Ray the last parameters.

Finally, André appears on the Live TV. On the runway 19L, he is smiling as, the elegant, horizontal long line of lights of the plane, illuminates the sky. Here, in the Mission Control Centre, we are all holding our breath, smiling in silence. Even the surface wind has calmed down to welcome the plane. Fifteen minutes after midnight local time (6h15 in Switzerland), we hear the short signal meaning that the plane has stopped. At this moment, we all share the same idea: ‘Yes, we can!’”


Christophe Béesau

Altran expert on Solar Impulse for Advanced Modelling and Simulation