The eruption of Icelandic volcano Eyjafjallajokull sent vast amounts of ash into the atmosphere posing a threat to aircraft jet engines.
So, what do pilots look out for when they're faced with the possibility of flying through it?
Nik Gowing spoke to Captain Fokko Doyen, a Lufthansa cargo pilot.
The Hub with Nik Gowing is broadcast Monday to Fridays at 1600, 1630 and 1700 GMT on BBC World News. Find out more information about regional broadcast times