In some parts of the world, volcanic ash can be encountered and is a severe threat to aviation. It can degrade engine performance and navigational instrumentation. It is extremely abrasive and mildly corrosive, and can conduct electricity when wet, and in fact can create a phenomenon called St. Elmo's fire, which is static electricity flashing on the cockpit windows - more on this in a later section. Engines have flamed out while encountering unforeseen volcanic ash, fortunately in most of these incidents, the engines have been able to be restarted. But if there is a chance of flying through it, all engines must be inspected, with the great potential of replacing these engines.
What is volcanic ash? It is small pieces of minerals, dirt, rock and volcanic glass that is formed from the eruption of a volcano. The size of sand and silt, it is hard and does not dissolve in water.
There are nine Volcanic Ash Advisory Centers (VAACs) worldwide, two cover the United States. These Centers provide forecasts and warnings, especially to aviation organizations.
In the United States, the Federal Aviation Administration (FAA) also helps provide these forecasts and warnings in their flight planning and reporting services. The Meteorological Watch Office is responsible to notify Air Traffic Control (ATC) in the U.S. of volcanic activity, possible ash, and it's resultant drift pattern. Finally, your respective airline's flight dispatch/operations will relay warnings on volcano activity and the direction, altitudes, and areas of any ash to your pilots in their preflight paperwork and enroute. However, because this information is a forecast at best, there can be a wide difference between the actual area of ash and where it should be.
For example, during the summer of 2008, several volcanoes in the Aleutian Island chain were erupting and the resultant ash drifted around the Anchorage area and was as far south as the Washington/Oregon border. Several airlines canceled or delayed flights into and out of Anchorage stranding thousands of passengers. Our flight was routed further north than usual to avoid the forecast ash pattern. However about an hour and half out of Anchorage we suddenly noticed a sulfur-like smell, and then you could see a very faint brown haze - and we were at 36,000 feet altitude! We immediately scanned our engine instruments to see if there was any fluctuations in them. There were none, and we complied with our checklist for "Volcanic Ash". Among the items that need to be done were turn our Start Switches to the Flight position so the engines would receive continuous ignition from both ignitors to enhance the engines' ability to avoid flameout. We also started up the Auxilary Power Unit (APU) as a backup for electrical power in case we lost an engine (and associated engine generator), and as a potential source of bleed air in case an engine needed to be restarted.
At the same time, we contacted ATC to alert them of the volcanic ash and tell other aircraft in the area of it, and requested a climb to 38,000 feet to get out of this thin layer. We alerted our flight dispatch and maintenance control of the encounter, and was probably the first flight to fly through this area. The flight continued without further incident, and upon landing in Anchorage, was inspected. Fortunately there was no physical evidence of engine damage, together with our observation of no fluctuations on the engine instruments. The following flight left a little bit late and I believe was the last flight to leave for a day or so!
Here is an example of what happened to a British Airways 747 when it encountered volcanic ash back in 1982. I remembered this incident because I was also traveling to Perth around this time and our flight from Singapore had to add a couple of hours of flight time to get around the area of the volcanic ash cloud.
Volcanic Ash back to Enroute