The average length of a flight doubled from 903 kilometres in 1950 to 1,816 kilometres in 2000,29 but it has not changed much since then—with 1,827 kilometres in 2012. Longer flights and expanding passenger numbers generated a strong expansion of total passenger kilometres (pkm) travelled—up to 193-fold from the 28 billion pkm in 1950 to 5.4 trillion pkm in 2012.30
