Volcanic ash and aviation safety

Plumes of volcanic ash near active volcanoes are a flight safety hazard, especially for night flights. Volcanic ash is hard and abrasive, and can quickly cause significant wear to propellers and turbocompressor blades, and scratch cockpit windows, impairing visibility. The ash contaminates fuel and water systems, can jam gears, and make engines flame out. Its particles have low melting points, so they melt in the engines' combustion chamber then the ceramic mass sticks to turbine blades, fuel nozzles, and combustors—which can lead to total engine failure. Ash can also contaminate the cabin and damage avionics.^[1][2]

In 1991, the aviation industry decided to set up Volcanic Ash Advisory Centers (VAACs) for liaison between meteorologists, volcanologists, and the aviation industry.^[3] Before 2010, aircraft engine manufacturers had not defined specific particle levels above which they considered engines at risk. Airspace regulators took the general approach that if ash concentration rose above zero, they considered airspace unsafe, and consequently closed it.^[4]

The costs of air travel disruption in Europe after a volcanic eruption in 2010 forced aircraft manufacturers to specify limits on how much ash they considered acceptable for a jet engine to ingest without damage. In April, the UK CAA, in conjunction with engine manufacturers, set the safe upper limit of ash density at 2 mg per cubic metre of air space.^[5] From May 2010, the CAA revised the safe limit upwards to 4 mg per cubic metre of air space.^[6]

To minimise further disruption that this and other volcanic eruptions could cause, the CAA created a new category of restricted airspace called a Time Limited Zone.^[7] Airspace categorised as TLZ is similar to airspace under severe weather conditions, in that restrictions should be of a short duration. However, a key difference with TLZ airspace is that airlines must produce certificates of compliance for aircraft they want to enter these areas. Any airspace where ash density exceeds 4 mg per cubic metre is prohibited airspace.^{[citation needed]}

Volcanic ash in the immediate vicinity of the eruption plume is different in particle size range and density than that in downwind dispersal clouds, which contain only the finest particle sizes of ash. Experts have not established the ash loading that affects normal engine operation (other than engine lifetime and maintenance costs). Whether this silica-melt risk remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear.^{[citation needed]}

Experts recognised that there was an issue following British Airways Flight 9 in 1982, and therefore the ICAO established the Volcanic Ash Warning Study Group. Due to the difficulty in forecasting accurate information out to 12 hours and beyond, the ICAO later set up Volcanic Ash Advisory Centers (VAACs).^[8][9]