SELECT runway_sign as Landebahn, [Anzahl gelandet], [Anzahl gestartet]  from
    (
        SELECT runway_sign, COUNT(runway_sign) as [Anzahl gelandet], 0 as [Anzahl gestartet] FROM tbl_arriving_planes
            GROUP BY runway_sign
    ) as b
    UNION
    SELECT runway_sign as Landebahn, [Anzahl gelandet] ,[Anzahl gestartet]  from
    (
        SELECT runway_sign, 0 as [Anzahl gelandet], COUNT(runway_sign) as [Anzahl gestartet] FROM tbl_leaving_planes
            GROUP BY runway_sign
    ) as a

runway_sign | number of arrived planes | number of leaving planes

SELECT runway_sign as Landebahn,
       SUM([Anzahl gelandet]),
       SUM([Anzahl gestartet])
FROM ((SELECT runway_sign, COUNT(runway_sign) as [Anzahl gelandet], 0 as [Anzahl gestartet]
       FROM tbl_arriving_planes
       GROUP BY runway_sign
      ) UNION ALL
      (SELECT runway_sign as Landebahn, 0 as [Anzahl gelandet], COUNT(runway_sign) as [Anzahl gestartet]
       FROM tbl_leaving_planes
       GROUP BY runway_sign
      )
     ) al
GROUP BY runway_sign;