The walls of the mosque had turned black. In the villages beside the Sunda Strait, in the first days of September, people were out with brooms, sweeping grey dust from the floors of their houses and off the blackened walls of the mosque nearby. Reuters photographers found them at this work while the volcano was still erupting. At night, the lava from Anak Krakatau lit the sky red.
Mount Anak Krakatau began erupting on the Saturday, throwing up a fountain of lava, and people in several parts of the country reported booming sounds and tremors, according to Indonesia’s geology agency. At half past one on Sunday morning, after ash was detected in the airspace around it, Soekarno-Hatta International Airport stopped all flights. The transport ministry explained the decision in the flat language of procedure: the latest paper test showed positive results indicating the spread of volcanic ash in the airport area, so the closure was extended until half past nine that morning. By the ministry’s count, 209 flights and more than 22,000 passengers were affected. Services to and from Singapore were hit hardest, and routes to Doha, Sydney and Kuala Lumpur were disrupted too. Local airport authorities counted at least sixteen domestic flights to Bali cancelled.
The reopening did not last. By Monday morning seven Indonesian airports, two of them serving Jakarta, were still closed because of the ash, the transport minister said. Ash had covered the capital and several surrounding areas since the eruption began late on Friday, and the airports were shut for safety reasons.
Safety is a word with a particular memory in that airspace. On the night of 24 June 1982, a British Airways Boeing 747 named City of Edinburgh, flying from Kuala Lumpur towards Perth, flew into ash from the eruption of Mount Galunggung, about 110 miles south east of Jakarta. The cloud was dry, and the weather radar, which is built to find moisture, showed clear skies. Captain Eric Moody was on his way to the lavatory when the crew noticed a glow on the windscreen. Smoke began to collect in the cabin, at first mistaken for cigarette smoke, then smelling of sulphur. Passengers who could see the engines saw them shining an unusual bright blue. At 13:42 UTC the number four engine failed, and within a minute the other three had gone too. The flight engineer said what everyone was thinking: he could not believe it, all four engines had failed.
What happened next is the reason ash warnings exist. The crew calculated that they could glide for twenty three minutes and 91 nautical miles from 37,000 feet, and sent a mayday that Jakarta control misheard as a single engine failure until a Garuda Indonesia flight passed the message on. Moody told the cabin: “Ladies and gentlemen, this is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.” Many passengers wrote notes to their relatives. At 13,500 feet, close to the height at which the crew would have had to turn out to sea and attempt a landing on water that no one had ever tried in a 747, engine four restarted, then three, then one and two. The windscreen had been sandblasted opaque, so the approach into Halim Perdanakusuma was flown almost entirely on instruments, with the first officer calling out heights to build a glide path out of nothing. At the bottom of the steps the flight engineer knelt and kissed the ground. Asked why, he said the Pope does it, and Moody replied that the Pope flies Alitalia.
The investigation found the mechanism. The ash starved the engines of oxygen, sandblasted the windscreen and the landing light covers, and clogged the machinery, and inside the combustion chambers it melted and stuck to the engine. Laboratory work has since put numbers on this. Jet engines run at 1,200 to 2,000 degrees Celsius, hot enough to melt the ash they swallow and make it stick to turbine parts, and because the chemical composition of ash varies widely from volcano to volcano, so does the temperature at which it turns liquid. One group of researchers heated samples in order to build a model that predicts when ash will melt and stick. Their experiments also destroyed a convenient shortcut: using sand or dust instead of real ash is wholly inadequate, because it overestimates the sticking temperature and so understates the danger.
The other reason airports close, instead of simply routing planes around the cloud, is that nobody can say with confidence where the cloud is. Forecasts depend heavily on the size of the particles, because a large grain falls far faster than a small one, and a smaller grain can travel up to five times further. The size of the particles leaving the volcano is hard to know, and operational systems do not model how particles stick together, because the computing cost is too high.
Europe learned what that uncertainty costs in April 2010. On 14 April, meltwater from the Eyjafjallajökull glacier met hot magma, and the explosive eruption pushed unusually fine ash into the jet stream, where it spread quickly over the continent. Air traffic was grounded for days. On the Thursday, between 5,000 and 6,000 of Europe’s 28,000 daily flights were cancelled, and at Heathrow’s Terminal 4 every check in counter was closed. The next day Eurocontrol expected 11,000 flights instead of the usual 28,000, around sixty flights between Asia and Europe were called off, stranded passengers at Frankfurt were given camp beds, and in Copenhagen the guest list for Queen Margrethe’s seventieth birthday grew shorter as the flight bans kept dignitaries away, including Spain’s King Juan Carlos and Iceland’s own president. Jerry O’Callaghan, fifty, from Cork, on his way home from a holiday in America, had been about to board when his flight was cancelled. They should have better equipment, he said. It was only a load of ash.
The volcano now dusting Jakarta has a history of a different order. Krakatoa’s explosions of 26 and 27 August 1883 destroyed more than seventy per cent of the island, and the third blast of that morning remains the loudest sound known to history, heard 3,110 kilometres away in Perth and 4,800 kilometres away near Mauritius. At least 36,417 deaths are attributed to the eruption and its tsunamis. The cone that grew out of that crater first appeared in 1927 and stood permanently above the sea from 11 August 1930, and up to September 2018 its summit was rising by seven to nine metres a year. On 22 December 2018, two minutes after a small earthquake, its south western flank slid away, cutting off the top of the cone and sending a tsunami that killed 430 people.
In 2010, asked how long the disruption would last, an Icelandic geophysicist named Einar Kjartansson gave the only honest answer available then, and now. The ash would keep coming for days or weeks, he said, but whether it ruined anyone’s travel was a matter of weather: “It depends how the wind carries the ash.”