A red warning includes the danger of plumes of volcanic ash injected into the higher layers of the atmosphere, possibly interfering with flight paths. A precautionary step, as volcanic ash can damage plane engines. The strange light noted by pilots and passengers around the plane was also due to the volcanic ash. In the last thirty years, 80 modern jets were damaged by volcanic ash. In ten cases the volcanic ash stopped temporarily the jet engines.

Source: Forbes November 26, 2017 21:21 UTC