Military TAF Volcanic Ash: The VA Group and What It Warns
Military TAF volcanic ash is one of the few weather codes where the right response is not to plan around it but to stay out of it entirely.
The code itself is simple. VA in the weather group, read like any other
present weather code. What sits behind it is the reason it gets its own page.
Where VA appears in the forecast
Ash is present weather, so it goes in the weather slot. That is the same slot that holds rain, snow and mist.
A forecast might read FM120600 VRB05KT 3SM VA BKN020. Light and variable wind,
three miles visibility, volcanic ash, broken cloud at 2000 feet.
It can sit inside a change group like anything else. Under TEMPO, ash is
expected in bursts. Under FM, it becomes the new steady state.
It can also take an intensity marker. Ash is normally written plain, but heavier falls near a vent can carry a plus.
Why ash is not like other weather
Most weather in a TAF limits what you can do. Ash breaks the aircraft.
Ash is ground rock. The particles are hard, sharp and very fine. They melt at temperatures a jet engine reaches easily.
Once melted, the glass coats the turbine blades and the fuel nozzles. Air flow drops. The engine can surge and flame out. This has happened to airliners, and crews have restarted engines only after gliding down into cooler air.
Ash also strips paint, frosts a windscreen until you cannot see through it, and blocks the small holes that feed the pitot and static system. Your airspeed and altitude readings can go wrong at the same time as the engines.
The visibility trap
Ash is hard to see and hard to detect, which is what makes it so risky.
Weather radar looks for water. Ash is dry, so it gives back almost nothing. A radar screen can look clean while ash sits directly ahead.
At night it is close to invisible. Crews have flown into ash clouds with no warning at all, and the first sign has been a smell like sulphur, a haze in the cabin, and a glow around the engines.
By day the cloud can look like ordinary haze or thin cirrus. Nothing about the look of it tells you what it is.
So the forecast is doing work your eyes and your radar cannot. When a TAF says
VA, that is most of the warning you are going to get.
Where the forecast comes from
Ash forecasts are not built from the airfield outward. They come down from a wider system.
Nine Volcanic Ash Advisory Centres cover the world. Each watches the volcanoes in its region, tracks any plume, and models where the ash will drift with the upper winds.
Their advisories feed the SIGMETs and the aerodrome forecasts. So a VA in a
military TAF usually means an advisory already exists for the area, with far
more detail on the height and spread of the cloud.
That is worth chasing. The TAF tells you ash is expected at the field. The advisory tells you how thick the layer is and which way it is going.
What to do with an ash forecast
The guidance here is unusually blunt, and it is the same across every operator.
Do not fly into ash. There is no light ash the way there is light rain. The avoidance is total rather than graded.
If a forecast puts ash over your destination, the answer is a different destination or a different time. Route around the advisory area with a wide margin, because the edges of an ash cloud are not sharp and the models carry real error.
If you meet it in flight, the drill is to reduce thrust, turn back, and descend out of it. You came in through clean air, so the way you came is the known way out.
How long ash stays a problem
Ash does not clear the way rain clears. It can sit in the air for days.
A big eruption puts ash high, where the wind is fast and the air is dry. There is no rain up there to wash it out. So the cloud drifts, thins slowly, and covers ground a long way from the vent.
Ash from one volcano has closed airspace on the far side of an ocean. Distance from the source is weak comfort.
The plume also breaks into patches. Gaps open and close. A route that was clean this morning may not be clean tonight, and the reverse is true as well.
Check the advisory again before you go, even if you checked it a few hours ago.
Verdict on military TAF volcanic ash
Read VA as a stop rather than a limit. Every other weather code in a TAF tells
you how hard the flight will be. This one tells you not to make it.
The code is easy. The habit that matters is chasing the advisory behind it, so you know how high the ash goes and where it is heading, and giving the whole area far more room than the forecast box suggests.
Frequently asked questions
How does volcanic ash appear in a TAF?
As the code VA in the weather group, read the same way as any other present weather code. It can carry an intensity and it can sit inside a change group, so VA under a TEMPO means ash is expected in bursts.
Why is volcanic ash treated separately from other weather?
Because the damage is different in kind. Ash melts in a jet engine and coats the turbine, which can flame it out. It also sandblasts windscreens and blocks pitot tubes. No amount of ash is a safe amount.
Can a weather radar see volcanic ash?
Not reliably. Ash particles are dry and small, so they return very little energy. A radar picture can look clear while an ash cloud sits in front of you, which is why forecast and advisory products carry the load.
What is a VAAC?
A Volcanic Ash Advisory Centre. Nine of them cover the world, and they issue the advisories that ash forecasts are built from. Their products track where the ash is now and where it is expected to drift.
Does ash show up in a METAR too?
Yes, as the same VA code in the present weather group. In a METAR it means ash was observed at the field. In a TAF it means ash is forecast, which is the more common of the two.