Plumes of volcanic ash near active volcanoes are an aviation safety hazard, especially for night flights. The ash is hard and abrasive and can quickly cause significant wear on propellers and turbocompressor blades, and scratch the cockpit windows, impairing visibility. It contaminates fuel and water systems, can jam gears, and can cause a flameout of the engines. Its particles have low melting point, so they melt in the combustion chamber and the ceramic mass then sticks on the turbine blades, fuel nozzles, and the combustors, which can lead to a total engine failure. It 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]
Prior to 2010, aircraft engine manufacturers had not defined specific particle levels above which engines were considered to be at risk. The general approach taken by airspace regulators was that if the ash concentration rose above zero, then the airspace was considered unsafe and was consequently closed.[4]
The costs of disruption to air travel in Europe after a volcanic eruption in 2010 forced aircraft manufacturers to specify limits on how much ash is 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 to be 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]
In order to minimise the level of 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 experiencing severe weather conditions in that the restrictions are expected to be of a short duration; however, the key difference with TLZ airspace is that airlines must produce certificates of compliance in order for their aircraft to enter these areas. Any airspace in which the ash density exceeds 4 mg per cubic metre is categorised as a no fly zone.
Volcanic ash in the immediate vicinity of the eruption plume is of a different particle size range and density to that found in downwind dispersal clouds which contain only the finest grade of ash. The ash loading at which normal engine operation (other than engine lifetime and maintenance costs) is affected has not been established. Whether this silica-melt risk remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear.
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,[8] less than 2 millimetres in diameter. The ash enters the atmosphere from the force of the eruption and convection currents from the heated air and is then carried away from the volcano by winds. The ash with the smallest size can remain in the atmosphere for a considerable period of time and be carried well away from the eruption point. The ash cloud can be dangerous to aviation if it reaches the heights and enters the paths used by aircraft.
Volcanic ash has a melting point of approximately 1,100 °C (2,010 °F), which is below the operating temperature of modern commercial jet engines, about 1,400 °C (2,550 °F). If ash is ingested into the engines by an aircraft flying though the ash cloud then the ash melts in the engine on fuel nozzles and turbine blades rather than passing through the engine.[9] Sulfur dioxide, another product of volcanoes which is also carried within the ash clouds following an eruption is corrosive to aircraft that fly through it.[10]
When flying at night ash clouds are not visible to pilots, in addition they do not show up on radar, the small size of the ash particles do not return an echo to the onboard weather radars of commercial airliners. Even when flying in daylight if the ash cloud is visible it may be interpreted as a normal cloud formed by water vapour and not seen as a danger, especially if it has travelled well away from the eruption location.[10][11] In the image from the volcano Chaitén volcano the ash cloud has spread thousands of kilometers from the eruption site crossing the width South America from the Pacific coast and over the Atlantic.
It was recognised that there was an issue following the incident in 1982 with the British Airways Flight 9 and therefore the ICAO established the Volcanic Ash Warning Study Group in 1982. Due to the difficulty in forecasting accurate information out to 12 hours and beyond the ICAO later set up Volcanic Ash Advisory Centers (VAACs).[10][12]
In 1982, British Airways Flight 9 flew through an ash cloud, lost power from all four engines, and descended from 37,000 feet (11,000 m) to only 13,500 feet (4,100 m) before the flight crew managed to restart the engines. A similar incident occurred in 1989 involving KLM Flight 867.