If you’ve been following Solar Impulse’s mission flights, you might have wondered what a holding area is and why the HB-SIA prototype has done so many of them, particularly during this year’s Crossing Frontiers mission flights.

The definition of a holding area is literally to hold the position of the airplane in one region. There are multiple reasons why any aircraft must hold, including commercial ones:

• To wait for the weather conditions improve
• To wait for the air traffic control’s (ATC) green light to proceed

The above reasons also apply to the Solar Impulse prototype, but there are some additional situations, specific to HB-SIA, which also require holdings:

• Holding areas deliberately chosen by the Solar Impulse Mission Control Centre (MCC), avoiding too soon of an arrival at its final destination

For the first two points, one must not forget that there is more traffic in the skies than we might think. It is therefore essential that pilots remain in direct contact with the Control Centers along the leg to get the latest news on weather and traffic conditions. For HB-SIA this is even more important as it needs to ensure a landing after the main air traffic has slowed down.

For HB-SIA’s specifics holdings, the solar airplane usually needs to do an early take-off and late-night landing, waiting for winds and thermals to settle. Therefore, the MCC in Payerne has the responsibility to calculate the distance to the final destination in relation to the winds, weather conditions and air traffic, eventually choosing one or two appropriate holding areas along the way.

For example, in the flight from Ouarzazate to Rabat, André Borschberg was required to hold South of Casablanca. This strategy was meant to avoid too soon of an approach around Rabat which could have potentially pushed the aircraft away from its destination due to the wind currents that day.  

During today’s flight, Bertrand Piccard will most likely hold somewhere in the South of Spain to avoid arriving around Madrid too soon, and a second holding close to its destination to negotiate the best moment to start his final descent in the busy Madrid-Barajas airport.