[{"data":1,"prerenderedAt":323},["ShallowReactive",2],{"learn-\u002Flearn\u002Fvolcanic-ash-and-sigmet":3,"learn-nav-\u002Flearn\u002Fvolcanic-ash-and-sigmet":290},{"id":4,"title":5,"body":6,"date":226,"description":227,"draft":228,"extension":229,"faqs":230,"howTo":240,"keyTakeaways":241,"meta":246,"navigation":247,"path":248,"quiz":249,"seo":275,"series":276,"seriesOrder":277,"sources":278,"stem":287,"topic":288,"__hash__":289},"learn\u002Flearn\u002Fvolcanic-ash-and-sigmet.md","Volcanic ash and the SIGMET",{"type":7,"value":8,"toc":216},"minimark",[9,21,27,32,53,81,92,96,121,125,157,161,164,170,181,185,205,209],[10,11,12,13,20],"p",{},"Most weather hazards announce themselves. Volcanic ash does not: it is nearly invisible, it does not paint on radar, and it can stop every engine on an aircraft. Because you cannot see it coming, the warning system around it, the advisory centres and the ",[14,15,19],"a",{"href":16,"className":17},"\u002Flearn\u002Fglossary#gt-sigmet",[18],"glossary-link","SIGMET",", is the only thing standing between a flight and the hazard, which is why it is worth understanding how the warnings work.",[22,23,24],"blockquote",{},[10,25,26],{},"This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.",[28,29,31],"h2",{"id":30},"why-ash-is-uniquely-dangerous","Why ash is uniquely dangerous",[10,33,34,35,39,40,46,47,52],{},"Volcanic ash is not soft like soot; it is ",[36,37,38],"strong",{},"finely ground rock and glass",", hard and abrasive, with a melting point below the operating temperature inside a jet engine. As ",[14,41,45],{"href":42,"rel":43},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fmanual-on-volcanic-ash-radioactive-material-and-toxic-chemical-clouds-doc-9691",[44],"nofollow","ICAO Doc 9691"," and ",[14,48,51],{"href":49,"rel":50},"https:\u002F\u002Fskybrary.aero\u002Farticles\u002Fvolcanic-ash",[44],"SKYbrary"," describe, when ash is drawn into a running engine it:",[54,55,56,63,69,75],"ul",{},[57,58,59,62],"li",{},[36,60,61],{},"melts and fuses onto the turbine",", choking the airflow and potentially rolling the engine back or flaming it out;",[57,64,65,68],{},[36,66,67],{},"sandblasts"," the windscreen and landing lights to a frosted, opaque finish;",[57,70,71,74],{},[36,72,73],{},"blocks the pitot and static ports"," that feed the airspeed and altitude instruments, so the primary flight instruments become unreliable;",[57,76,77,80],{},[36,78,79],{},"contaminates"," the air-conditioning, fuel and electrical systems.",[10,82,83,84,87,88,91],{},"It is also ",[36,85,86],{},"nearly invisible",", especially in cloud or at night, and does ",[36,89,90],{},"not show on weather radar",", which is tuned to precipitation. The standing guidance is therefore simple: avoid it. There is no casual way to fly through ash safely.",[28,93,95],{"id":94},"how-you-are-warned-vaacs-and-advisories","How you are warned: VAACs and advisories",[10,97,98,99,102,103,106,107,110,111,116,117,120],{},"Because ash cannot be seen or radar-painted, the warning comes from forecasting. A global network of ",[36,100,101],{},"Volcanic Ash Advisory Centres (VAACs)",", each responsible for a region, tracks eruptions using satellite, ground reports and dispersion models, and issues ",[36,104,105],{},"volcanic ash advisories"," giving the observed position of the ash now and its forecast position over the hours ahead. The ",[36,108,109],{},"London VAAC",", run by the ",[14,112,115],{"href":113,"rel":114},"https:\u002F\u002Fwww.metoffice.gov.uk\u002Fservices\u002Ftransport\u002Faviation\u002Fregulated\u002Finternational-aviation\u002Fvaac\u002Findex",[44],"UK Met Office",", covers the north-east Atlantic and handled the 2010 Icelandic eruptions that closed much of European airspace. Volcano observatories add an ",[36,118,119],{},"aviation colour code",", green, yellow, orange or red, describing the state of a volcano.",[28,122,124],{"id":123},"how-the-sigmet-carries-it","How the SIGMET carries it",[10,126,127,128,130,131,136,137,140,141,144,145,148,149,152,153,156],{},"The advisory feeds the meteorological warning a crew reads in the briefing: the ",[36,129,19],{},". As set out in ",[14,132,135],{"href":133,"rel":134},"https:\u002F\u002Fstore.icao.int\u002Fen\u002Fannex-3-meteorological-service-for-international-air-navigation-1",[44],"ICAO Annex 3",", an ordinary ",[14,138,19],{"href":139},"\u002Flearn\u002Fhow-to-read-a-sigmet-and-airmet"," is valid for ",[36,142,143],{},"up to four hours",". A volcanic ash SIGMET, like a tropical cyclone SIGMET, is treated differently because the hazard moves slowly and persists: it can be valid for ",[36,146,147],{},"up to six hours"," and may be issued ",[36,150,151],{},"up to twelve hours"," before it takes effect. It gives the ash position now and a ",[36,154,155],{},"forecast (FCST) position"," later in the period, so the crew can see where the cloud is heading.",[28,158,160],{"id":159},"a-worked-example","A worked example",[10,162,163],{},"An Icelandic volcano erupts and the London VAAC issues an advisory tracking the ash south-east toward the UK. A SIGMET follows, of the form:",[10,165,166],{},[167,168,169],"code",{},"EGTT SIGMET 2 VALID 121200\u002F121800 EGRR- LONDON FIR VA ERUPTION MT EYJAFJALLAJOKULL PSN N6338 W01937 VA CLD OBS AT 1200Z SFC\u002FFL200 APRX ... MOV SE 25KT FCST 1800Z ...",[10,171,172,173,176,177,180],{},"Reading the parts that matter: it is a ",[36,174,175],{},"volcanic ash (VA)"," SIGMET, valid from 1200 to 1800 UTC, a ",[36,178,179],{},"six-hour"," window. The ash cloud is observed at 1200 UTC from the surface up to FL200, moving south-east at 25 knots, with a forecast position given for 1800 UTC. A flight planned through that block of airspace and altitude has a clear instruction: the ash will be there, and the route or the levels need to change to stay out of it.",[28,182,184],{"id":183},"common-pitfalls","Common pitfalls",[54,186,187,193,199],{},[57,188,189,192],{},[36,190,191],{},"You will not see it or paint it."," Do not rely on eyes or radar; the SIGMET and advisory are the warning.",[57,194,195,198],{},[36,196,197],{},"The cloud moves and grows."," The forecast position matters as much as the current one, which is why the validity is longer; check where it is heading, not just where it is.",[57,200,201,204],{},[36,202,203],{},"Avoid means avoid."," Unlike turbulence or icing, there is no recognised technique for flying through ash; the plan is to stay clear of it entirely.",[28,206,208],{"id":207},"in-pilot-efb","In Pilot EFB",[10,210,211,212,215],{},"Pilot EFB keeps the ",[14,213,214],{"href":139},"SIGMETs"," and warnings you have pulled together with the rest of your briefing, so an active ash advisory is in front of you while you plan rather than buried. A briefing you have already pulled stays readable with no signal; fetching the latest SIGMET needs a connection. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.",{"title":217,"searchDepth":218,"depth":218,"links":219},"",2,[220,221,222,223,224,225],{"id":30,"depth":218,"text":31},{"id":94,"depth":218,"text":95},{"id":123,"depth":218,"text":124},{"id":159,"depth":218,"text":160},{"id":183,"depth":218,"text":184},{"id":207,"depth":218,"text":208},"2026-06-22","Why volcanic ash is so dangerous to aircraft, how Volcanic Ash Advisory Centres and SIGMETs warn you about it, and why an ash SIGMET runs longer and is issued further ahead than an ordinary one.",false,"md",[231,234,237],{"q":232,"a":233},"Why is volcanic ash so dangerous to aircraft?","Volcanic ash is finely ground rock and glass. Drawn into a hot jet engine it melts and fuses onto the turbine, which can roll the engine back or flame it out; it sandblasts the windscreen and landing lights to a frosted finish, blocks the pitot and static ports that drive the airspeed and altitude instruments, and contaminates the air and fuel systems. It is also nearly invisible and does not show on weather radar, so a crew can fly into it without warning.",{"q":235,"a":236},"What is a Volcanic Ash Advisory Centre?","A Volcanic Ash Advisory Centre, or VAAC, is one of a global network of specialist centres, each responsible for a region, that track ash clouds and forecast where they will spread. They issue volcanic ash advisories with the observed and forecast position of the ash. The London VAAC, run by the UK Met Office, covers the north-east Atlantic and is the centre that handled the 2010 Icelandic eruptions.",{"q":238,"a":239},"How is a volcanic ash SIGMET different from an ordinary one?","A standard SIGMET is valid for up to four hours. Because a volcanic ash cloud, like a tropical cyclone, moves and persists over a long time, an ash SIGMET can be valid for up to six hours and may be issued up to twelve hours before it takes effect, so crews have more notice. It carries the position of the ash now and a forecast position later in the validity period.",null,[242,243,244,245],"Volcanic ash is ground rock and glass that melts in a hot engine and fuses to the turbine, sandblasts the windscreen, and blocks the pitot and static ports.","Ash is nearly invisible and does not show on weather radar, so warnings come from forecasts, not from what the crew can see.","Volcanic Ash Advisory Centres track and forecast ash clouds and issue advisories; the London VAAC is run by the UK Met Office.","An ash SIGMET can be valid up to six hours and issued up to twelve hours ahead, longer than the four-hour standard SIGMET, because ash persists and moves.",{},true,"\u002Flearn\u002Fvolcanic-ash-and-sigmet",[250,259,267],{"q":251,"options":252,"answer":257,"explanation":258},"What is the main reason volcanic ash can flame out a jet engine?",[253,254,255,256],"It is magnetic and jams the compressor","It melts in the hot engine and fuses onto the turbine","It is too cold for combustion","It increases the fuel flow",1,"Ash is ground rock and glass that melts in the heat of the engine and fuses onto the turbine, which can roll the engine back or flame it out.",{"q":260,"options":261,"answer":257,"explanation":266},"What does a Volcanic Ash Advisory Centre (VAAC) do?",[262,263,264,265],"Issue landing clearances near volcanoes","Track ash clouds and forecast where they will spread, issuing volcanic ash advisories","Close airports automatically","Repair ash-damaged engines","A VAAC is a regional specialist centre that tracks ash clouds and forecasts their spread, issuing volcanic ash advisories with observed and forecast positions.",{"q":268,"options":269,"answer":257,"explanation":274},"How long can a volcanic ash SIGMET be valid, compared with an ordinary SIGMET?",[270,271,272,273],"Up to one hour, versus four","Up to six hours, versus the standard four","Exactly the same, four hours","Up to 24 hours, versus four","An ordinary SIGMET is valid for up to four hours; because ash persists and moves, an ash SIGMET can be valid up to six hours and issued up to twelve hours ahead.",{"title":5,"description":227},"decode-the-weather",8,[279,281,283,285],{"label":280,"url":133},"ICAO Annex 3: Meteorological Service for International Air Navigation",{"label":282,"url":42},"ICAO Doc 9691: Manual on Volcanic Ash, Radioactive Material and Toxic Chemical Clouds",{"label":284,"url":113},"UK Met Office: London Volcanic Ash Advisory Centre",{"label":286,"url":49},"SKYbrary: Volcanic Ash","learn\u002Fvolcanic-ash-and-sigmet","Weather","hZCc8hxkc9VN_EYgMdeIutQ3qTYt3S1Ae5kqqYtyQrw",{"related":291,"newer":306,"older":311,"series":316},[292,297,302],{"path":293,"title":294,"description":295,"date":226,"topic":288,"draft":228,"minutes":296,"series":240,"seriesOrder":240},"\u002Flearn\u002Fcloud-types-explained","Cloud types and what they tell a pilot","The ten cloud genera grouped into high, middle and low, the difference between heaped and layered cloud, and how to read what each type is telling you about stability, moisture and the weather ahead.",3,{"path":298,"title":299,"description":300,"date":226,"topic":288,"draft":228,"minutes":301,"series":240,"seriesOrder":240},"\u002Flearn\u002Fjet-stream-and-clear-air-turbulence","The jet stream and clear-air turbulence","What a jet stream is, where it sits near the tropopause, and why the wind shear around it produces clear-air turbulence that arrives with no cloud to warn you.",4,{"path":303,"title":304,"description":305,"date":226,"topic":288,"draft":228,"minutes":301,"series":240,"seriesOrder":240},"\u002Flearn\u002Fmountain-waves-and-rotor","Mountain waves and rotor","How stable air flowing over high ground sets up standing mountain waves and the violent rotor beneath them, the cloud signs that give them away, and why the turbulence and downdraughts deserve respect.",{"path":307,"title":308,"description":309,"date":226,"topic":310,"draft":228,"minutes":301,"series":240,"seriesOrder":240},"\u002Flearn\u002Fv-speeds-explained","V-speeds explained","What the V in V-speeds means, the difference between a design limit and an operating speed, and a plain-language tour of the speeds a pilot lives by, from the stall and the airspeed indicator's colour code to V1, Vr, V2, Vref and Vne.","Operations",{"path":312,"title":313,"description":314,"date":315,"topic":310,"draft":228,"minutes":301,"series":240,"seriesOrder":240},"\u002Flearn\u002Fcold-temperature-altimeter-corrections","Cold-temperature altimeter corrections","Why a pressure altimeter over-reads in cold air, leaving you lower than indicated, and how to correct minimum altitudes for temperature on an approach in mountainous or freezing conditions.","2026-06-21",{"slug":276,"title":317,"part":277,"total":277,"prev":318,"next":240},"Decode the weather",{"path":319,"title":320,"description":321,"date":226,"topic":288,"draft":228,"minutes":296,"series":276,"seriesOrder":322},"\u002Flearn\u002Fturbulence-reporting-and-intensity","Turbulence reporting and intensity scales","What light, moderate, severe and extreme turbulence actually mean, how the intensity is judged by the aircraft's reaction rather than a guess, and how pilots report it so the aircraft behind benefits.",7,1782089964086]