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 SIGMET, is the only thing standing between a flight and the hazard, which is why it is worth understanding how the warnings work.
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.
Why ash is uniquely dangerous
Volcanic ash is not soft like soot; it is finely ground rock and glass, hard and abrasive, with a melting point below the operating temperature inside a jet engine. As ICAO Doc 9691 and SKYbrary describe, when ash is drawn into a running engine it:
- melts and fuses onto the turbine, choking the airflow and potentially rolling the engine back or flaming it out;
- sandblasts the windscreen and landing lights to a frosted, opaque finish;
- blocks the pitot and static ports that feed the airspeed and altitude instruments, so the primary flight instruments become unreliable;
- contaminates the air-conditioning, fuel and electrical systems.
It is also nearly invisible, especially in cloud or at night, and does 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.
How you are warned: VAACs and advisories
Because ash cannot be seen or radar-painted, the warning comes from forecasting. A global network of Volcanic Ash Advisory Centres (VAACs), each responsible for a region, tracks eruptions using satellite, ground reports and dispersion models, and issues volcanic ash advisories giving the observed position of the ash now and its forecast position over the hours ahead. The London VAAC, run by the UK Met Office, covers the north-east Atlantic and handled the 2010 Icelandic eruptions that closed much of European airspace. Volcano observatories add an aviation colour code, green, yellow, orange or red, describing the state of a volcano.
How the SIGMET carries it
The advisory feeds the meteorological warning a crew reads in the briefing: the SIGMET. As set out in ICAO Annex 3, an ordinary SIGMET is valid for 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 up to six hours and may be issued up to twelve hours before it takes effect. It gives the ash position now and a forecast (FCST) position later in the period, so the crew can see where the cloud is heading.
A worked example
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:
EGTT SIGMET 2 VALID 121200/121800 EGRR- LONDON FIR VA ERUPTION MT EYJAFJALLAJOKULL PSN N6338 W01937 VA CLD OBS AT 1200Z SFC/FL200 APRX ... MOV SE 25KT FCST 1800Z ...
Reading the parts that matter: it is a volcanic ash (VA) SIGMET, valid from 1200 to 1800 UTC, a 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.
Common pitfalls
- You will not see it or paint it. Do not rely on eyes or radar; the SIGMET and advisory are the warning.
- 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.
- 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.
In Pilot EFB
Pilot EFB keeps the 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.