New figures show the international air freight business is experiencing difficulties, with slowing volume growth and falling revenues.
The International Air Transport Association says freight revenue peaked at $US67 billion in 2011.
But it slipped last year and is forecast this year to be around $US51b.
In contrast with revenues, freight volumes are growing but slowly.
The figures show volumes grew 2.2 percent in 2015 compared with 5.0 percent in 2014.
The weakness reflects sluggish trade growth in Europe and Asia-Pacific, and lower amounts of freight carried per plane.