At 23:07 on Friday night, local time, Anak Krakatau started throwing lava into the sky and did not stop for twenty-five hours. People heard the booming 700 kilometres away.

By Sunday morning the biggest airport in Southeast Asia’s fourth-largest country was shut. By Monday, Indonesia’s transport ministry was counting more than a quarter of a million passengers who could not get where they were going.

Nobody has been killed. No village has been evacuated. The damage from this eruption is being done almost entirely in the air.

Eight Airports, One Volcano

The closure list on Saturday read like a map of Java’s western half. Soekarno-Hatta, Jakarta’s main international gateway. Halim Perdanakusuma, the capital’s second airport. Radin Inten II in Lampung, across the Sunda Strait. Husein Sastranegara at Bandung. Budiarto at Curug, Pondok Cabe in South Tangerang, Taufik Kiemas at Pesisir Barat and Atung Bungsu at Pagar Alam.

Six of those closed first. The seventh and eighth were added later on 6 September for a reason that will be familiar to anyone who has ever watched a volcanic-ash forecast: the wind changed. Indonesia’s meteorological agency BMKG reported that at around 5 km altitude the flow had swung eastward at roughly 9 km/h, dragging ash over airfields that had been upwind an hour earlier.

BMKG issued eighteen SIGMETs. The Volcanic Ash Advisory Centre in Darwin, which is responsible for this slice of the planet, went through a rapid sequence of advisories on Friday night: ash at 7,000 feet, then a high-level eruption to roughly 45,000 feet, then within ten minutes an assessment of ash near 48,000 feet moving west at 93 km/h. Darwin was reading cloud-top temperatures of minus 68 degrees Celsius off Himawari-9 satellite imagery to get those numbers.

Then came the detail that turns a geology story into an aviation story. At 05:30 UTC on 6 September, the routine weather observation issued by Soekarno-Hatta itself carried the code VA. Volcanic ash. Not forecast, not advisory — observed, at the field.

Why Ash Is the One Thing Nobody Argues About

Airlines will fly in thunderstorms, in crosswinds, in weather that soaks a runway and turns an approach into a fight. They will not fly through volcanic ash, and the reason is a Boeing 747 that fell out of the sky over this exact stretch of Indonesia in 1982.

British Airways Flight 9, en route from Kuala Lumpur to Perth on 24 June that year, flew into an invisible ash cloud from Mount Galunggung in West Java. All four engines flamed out. The aircraft descended for around sixteen minutes as an unpowered glider before the crew managed to relight three of them and divert to Jakarta. Everyone survived. Nobody in the industry has ever forgotten it.

The physics is unforgiving. Volcanic ash is pulverised rock, and it melts at a temperature well below the operating temperature inside a modern turbine. It enters the engine as dust and leaves the combustor as molten glass, which then solidifies on the cooler turbine surfaces and progressively chokes the airflow. The engine does not warn you. It simply stops.

Ash also sandblasts windscreens opaque, blocks pitot-static ports and strips paint. There is no procedure for flying through it. There is only avoidance.

A Quarter of a Million People Going Nowhere

The numbers moved quickly, and it is worth being precise about which is which.

By 17:06 local time on Saturday 6 September, Indonesia’s Ministry of Transportation counted 1,558 flights delayed or cancelled across the eight closed airports and roughly 170,000 passengers affected, more than 900 of those flights at Soekarno-Hatta alone. The airport operator InJourney gave a tighter slice: 209 flights cancelled at Soekarno-Hatta between 01:30 and 09:30 on Sunday morning, made up of 160 domestic, 39 international and 10 cargo services.

By Monday the ministry was quoting a different, cumulative figure — more than 270,000 passengers affected since 5 September, counting everything cancelled, delayed or diverted. On Monday morning alone, Soekarno-Hatta reported over 240 flights cancelled or delayed and nearly 30,000 passengers caught up in it.

Singapore Airlines, Scoot, Malaysia Airlines, AirAsia, the Lion Air Group and Garuda Indonesia all cancelled services. Qantas dropped four flights and offered refunds, credits or date changes for Jakarta bookings between 5 and 9 September. Australians heading to Bali were unaffected — that traffic runs well east of the ash.

Indonesia designated five alternates — Kertajati, Juanda at Surabaya, Yogyakarta, Ahmad Yani at Semarang and Solo — and laid on free buses and trains for passengers whose flights were shifted. The government has guaranteed full refunds for ash cancellations.

Soekarno-Hatta opened a command post in Terminal 1B, kept its commercial tenants running around the clock and handed out food. Police were distributing masks. Jakarta moved its schools online for Monday.

Nobody Knows When It Ends

The continuous phase stopped just after midnight on Sunday. Activity has since reverted to what volcanologists call Strombolian — discrete, rhythmic bursts rather than a sustained fountain. Three of those were logged in the following day. Seismic energy peaked on Saturday and has been falling since.

That is encouraging but not decisive. The alert level stays at III. The three-kilometre exclusion zone stays in place on land and on the water. Indonesia’s disaster agency has begun cloud seeding to try to wash ash out of the air over Jakarta, and BMKG confirmed that all twenty tsunami gauges around the Sunda Strait recorded no unusual sea-level change — a check that matters here, because the 2018 flank collapse of this same volcano killed hundreds along these coasts.

As of Monday afternoon, six airports were still closed, Soekarno-Hatta among them.

That is the honest answer, and it is the same answer Europe got in 2010 and Iceland got again after that. An airline can plan around almost anything. It cannot plan around a mountain.

Footage of the eruption shot by anglers in the Sunda Strait, filed by the Associated Press.

Sources: Indonesian Ministry of Transportation, Badan Geologi (PVMBG), BMKG, AirNav Indonesia, InJourney Airports, Darwin VAAC / Bureau of Meteorology, ANTARA, Al Jazeera, ABC News, RTÉ, CIMSS Satellite Blog.