MADRID, SPAIN - FEBRUARY 11:  Planes are parked as a Vueling plane arrives at Madrid Barajas Adolfo Suarez airport which is operated by group Aena on February 11, 2015 in Madrid, Spain. Shares in state-controlled Spanish airports operator, Aena, began trading today on the Madrid stock market. The Spanish government will retain a 51% controlling share in the firm, as it seeks to raise 3.9 billion Euros through the sale of the remaining 49%. Aena operates major hubs in Barcelona and Madrid, as well as a further 15 airports in Europe and America, including London Luton.  (Photo by Pablo Blazquez Dominguez/Getty Images)