Airlines are used to ups and downs. The current problems facing the industry are sky-high fuel prices and the economic slowdown in some of the sector's most valuable markets.

But airlines have been collectively in profit only twice this millennium - in 2000 and again last year. The terror attacks of September 11 2001 helped push the industry into recession, and it took until 2007 until the airlines could report a net profit of $5.6bn between them.