This is a historical moment for the Solar Impulse team and for solar aviation. Bertrand Piccard just crossed the Strait of Gibraltar, the passage separating Europe from Africa, in the cockpit of the HB-SIA. He crossed around 16:00 (UTC+2) and is now heading straight into the direction of Tangier at an altitude of approximately 7’000m.

Elâ had a quick talk with Bertrand: “The scenery is just incredible, I see clouds on one side which reminds me of the « Aletsch Glacier » and the Atlantic see on the other side.” Photos will be available when Bertrand comes back on earth!

Most of you might think that flying towards warmer and sunnier climates is ideal for the solar aircraft. Yet, there are other meteorological aspects that need to be taken into consideration in these regions. The summer solstice is approaching (21 June), and with it, the intensity of direct sunlight. The heating of the ground results in an increase in vertical winds, better known as turbulence, which can destabilize the aircraft or, worse, disengage it.

For this reason the team of meteorologists, modeling and simulation specialists are key to the smooth progress of the flight. Aside from obvious and visible climate obstacles like fog and rain, the most important data that needs to be analyzed for a given flight are the:

• Take-off and landing conditions. Crosswinds and headwinds must not exceed 5 knots and 7 knots respectively. 
• Reliability of meteorological data
• Surface conditions (thermal inversion and low level jets, i.e. strong winds at less than 400m)
• Conditions of altitude (change in winds at each altitude profile)

This flight is faced with very favorable weather conditions; there are no storms in sight and turbulence is at a minimum throughout the flight. Nevertheless, the meteorologists have to prepare detailed weather predictions for two alternate airports along the way, in this case Seville (Spain) and Tangier (Morocco).

The final challenge for this flight will be faced at the time of landing. The wind situation will improve during the evening while the Meteorological Team and the Mission Control Center work closely with the pilot to find the perfect approach and landing window. 

It will be a long day for the Solar Impulse team, some of whom were up at midnight when the hangar doors swung open.

The HB-SIA emerged from the dark hangar at 05:30 with the final preparations and checks completed and with its batteries fully charged from its time in the sun on Friday morning. André boarded at 06:10 and the aircraft was towed to the end of the runway. Then came engine and control checks and at last the take off!

Mission director, Raymond Clerc, and his team are based in Payerne, Switzerland, and are coordinating the flight from the ground. He explained that the first challenge of the day will be to skirt Charles de Gaulle airport. André Borschberg will follow exactly the same flight path out as he flew in on, skirting CdG to the South, and  flying at low altitude (600 metres above sea level) for approximately two hours. Once clear, the HB-SIA will regain its cruising altitude of 3,500 metres and head for Troyes before following a path between Dijon and Belfort. If everything goes to plan, the aircraft will cross the French Swiss border at Brevine at approximately 15:00. It will approach from the direction of Besancon.

From then it becomes a waiting game and the HB-SIA will circle around Payerne and Morat while it waits for the local wind 'La Bise' to drop and for any thermal turbulency to fade with the sunset.

As Raymond Clerc explains, the forecast is absolutely perfect: "We have a light easterly up to 2,000 metres and a westerly above that which is so light it is negligable." As for the 'Bise' that is blowing over the Swiss plateau: "It will drop out towards the end of the afternon."

All in all dreamy conditions for the last flight of the Solar Impulse European campaign.

Raymond Clerc, the head of mission, explains that the final flight procedure is firstly to organise the precise timing of HB-SIA's approach to Le Bourget and also the allocated runway. Once this information has been established Andre will be brief for the landing procedure. This is a real dress rehearsal, which has already been done but will also be repeated shortly before landing.

I am convinced that Andre and HB-SIA will be instructed to land on runway 21" Raymond Clerc commented, "but this may change and continue to be on runway 03 as previously planned. Ideally we would prefer runway 03 but it all depends on Air Traffic Control as they will take precedence over all other considerations. The final decision will be taken at 20:30pm".

Andre Borschberg and the Mission team saw an increase in their workload after they discovered that the clouds had not dispersed as previously anticipated. André had to descend into a gap to cross the upper layer of clouds, and is now flying with conviction between the upper and lower cloud formations, which have gradually been fragmenting.

The battery level is also encouraging as there will still be 100% charge for another half an hour thanks to the few rays of sunshine that have managed to pierce the clouds. This will drop to about 80% during the landing procedure at 21:00hrs.