As frustration mounts over the economic cloud an erupting volcano in Iceland has cast over the global airline industry, a handful of European airlines have started to fly empty airliners through Europe’s skies to see if it’s safe to resume air travel. But a case study by the US National Aeronautics and Space Administration (NASA) suggests that even tenuous volcano ash clouds can inflict serious damage to an airliner. While the damage may not necessarily threaten the immediate flight, one cruise through an invisible ash plume can run up a multimillion-dollar repair tab. The plume in the NASA study was so thin that the flight crew had none of the cues they ordinarily might rely on – odd engine readings, the smell of smoke of sulfur in the cockpit, or even outside electrical phenomena such as St. Elmo’s Fire – to alert them to a plume’s presence. And they had no visual clues on the aircraft after landing to tell them they’d encountered a plume. The case study concludes that the engines sustained enough damage that key components could well have started to fail with only another 100 hours of flying time.