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Plumes of volcanic ash near active volcanoes are an aviation 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 flameout. Its particles have low melting point, so they melt in the combustion chamber and the ceramic mass then sticks to turbine blades, fuel nozzles, and combustors—which can lead to total engine failure. Ash can also contaminate the cabin and damage avionics.〔(Danger to Aircraft from Volcanic Eruption Clouds )〕〔(Guidance for Flight Crews and Controllers )〕 In 1991, the aviation industry decided to set up Volcanic Ash Advisory Centers (VAACs) for liaison between meteorologists, volcanologists, and the aviation industry.〔(Volcanic Ash–Danger to Aircraft in the North Pacific )〕 Prior to 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.〔http://www.newscientist.com/article/dn18797-can-we-fly-safely-through-volcanic-ash.html〕 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.〔http://www.newscientist.com/article/dn18802-engine-stripdowns-establish-safe-volcanic-ash-levels.html〕 From May 2010, the CAA revised the safe limit upwards to 4 mg per cubic metre of air space. 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''.〔http://www.caa.co.uk/docs/7/Letter%20to%20NSAs%20re%20Volcanic%20Ash-%20Creation%20of%20TLZ.pdf〕 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. 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. Experts 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).〔〔(International Airways Volcano Watch Programme )〕 ==Volcanic hazards to aviation== Volcanic ash consists of small tephra, which are bits of pulverized rock and glass less than in diameter created by volcanic eruptions.〔(USGS Tephra: Volcanic Rock and Glass Fragments )〕 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 can drift away from the eruption point. The ash cloud can be dangerous to aviation if it reaches the heights of aircraft flight paths. Pilots can't see ash clouds at night. Also, ash particles are too small to return an echo to on-board weather radars on commercial airliners. Even when flying in daylight, pilots may interpret a visible ash cloud as a normal cloud of water vapour and not a danger—especially if the ash has travelled far from the eruption site.〔〔(Video on Dangers of Volcanic Ash by International Federation of Airline Pilots Associations )〕 In the image from the Chaitén volcano, the ash cloud has spread thousands of kilometers from the eruption site, crossing the width of South America from the Pacific coast and spreading over the Atlantic. Volcanic ash has a melting point of approximately , which is below the operating temperature of modern commercial jet engines, about . Volcanic ash can damage gas turbines in a number of ways. These can be categorised into those that pose an immediate hazard to the engines and those that present a maintenance problem. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Volcanic ash and aviation safety」の詳細全文を読む スポンサード リンク
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