public FlightFilter()
        {
            InitializeComponent();

            using (FlightServiceClient fsc = new FlightServiceClient())
            {
                foreach (string s in fsc.getCountries())
                {
                    cb_a_land.Items.Add(s);
                    cb_v_land.Items.Add(s);
                }

                foreach (string s in fsc.getAirPlaneClasses())
                    cb_klasse.Items.Add(s);
            }
        }