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14 June: Saint Louis-Cincinnati

"So, at last, we are facing our last challenge: reaching the East coast and Washington, this mythical destination…


Nature is queen

But this time, things don’t seem to be really simple. Weather has turned to a quite unstable situation, something which sounds to our ears almost like “hostile”. Of course, this is frustrating… However, there is also something important to understand when we can’t do exactly what we want with this plane because of weather conditions. It’s as if clouds and winds want to remind us that they are more or less directly caused by the Sun, our best energy supplier for Solar Impulse. It reminds us that natural elements themselves remain the strongest power even if we may feel like submitting them sometimes, and this principle is actually part of the message of the whole Solar Impulse project.

Solar Impulse Saint Louis - Cincinnati

We have been elaborating a two-step flight strategy for the evening of Friday 14th and the following night because wind speed and direction are not favourable in the state of Virginia. This special strategy, called “pit-stop”, will consist in a first flight followed by a quick stopover, just before the Appalachian Mountains, then a second flight to reach Washington, a couple of hours afterwards. Thus, we get the benefit of leaving Saint-Louis not too late - as clouds are approaching, - and also the opportunity to be in Virginia not too early, at a time when wind will have become more favourable.

Managing two flights in parallel

This strategy represents quite a big challenge for the whole Mission Control Centre team, by requiring an almost continuous work during 48 hours. For Stephane and me from the “Modelling and Simulation team”, it also means managing two flights in parallel.

The place chosen for the pit-stop is Cincinnati, at 366 nm from Saint-Louis, roughly in the middle of our route between Saint-Louis and Washington. The distance given takes into account the actual route André has to follow; as the crow flies, the distance between these two cities is 279 nm only.

The take-off is planned at 4.00 am local time, 1 hour and a half before sunrise. And as André suddenly sends all the electrical power to the four engines of the aircraft, at only five seconds to leave the ground of the 12L runway in the early morning of Saint-Louis, we are once again optimising his flight by processing the last weather data available and also computing the flight conditions for Bertrand’s flight between Cincinnati and Washington. Two flights to manage at the same time, as I said…

The profile of this first flight doesn’t look like the others, especially because of particular weather conditions we have to take into account today: wind is too strong at high altitude, and for this reason the aircraft will not go upper than 10,000 ft. This could be a difficulty in case of a long flight, because this type of profile doesn’t allow collecting much potential energy. But fortunately, Nik, Yves-André and Michel have negotiated an early landing at Cincinnati, so this “loosen” potential energy will not miss.

In this area of the USA, between Missouri and Ohio, the terrain is quite flat. André doesn’t fly over relief higher than 1,000 ft, and doesn’t meet “mountain waves” (a type of turbulences) as he did during his flight between Phoenix and Dallas.


Waiting for the landing slot

Half an hour before noon by the sun, André reaches 10,000 ft, its maximum altitude. Batteries are already at 85%, and if there are some cirrus clouds high above him, our computations show that their extent is not sufficient to significantly slow down the battery charge. 

What could be more annoying are the wind conditions in the middle of his afternoon (3.30 pm local time, 8.30 pm GMT). At this time, André has to start a 3-hours holding, which can be something difficult when the wind is quite strong. Even if we are at only 35 nm from Cincinnati, this holding is necessary so as to wait for the landing slot given by the American air traffic controllers. The speed of the aircraft varies from 25 kt when André turns the plane into the wind, to 40 kt when he flies in the opposite direction, but the correction angle he gives to the aircraft doesn’t exceed 35 degrees, a value which remains “reasonable” according to our flight criteria.

At 11.00 pm GMT (7.00 pm local time, 01.00 am in Payerne), the American controllers confirm the landing slot and then authorise the final approach. At last, we can see on our screens the plane which is landing before the sunset for the first time during this “Across America” adventure. Images are extraordinary! It is 8:15 pm local time in Cincinnati, 2.15 am in Payerne, and everyone applauds once again in the Mission Control Centre. But soon, we all go back to our work, as we also know that, this time, we just have a couple of hours before the next take-off. Rendez-vous in Washington!"


Christophe Béesau

Altran expert on Solar Impulse for Advanced Modelling and Simulation