On 15 December 1989, a brand-new KLM Boeing 747-400 was descending towards Anchorage, Alaska, on its way from Amsterdam to Tokyo. The day before, Mount Redoubt, about 150 miles away, had erupted. Somewhere in the grey winter sky over the Alaskan mountains, the jumbo flew into its ash.

Within seconds, all four engines stopped. The 747, carrying 231 passengers, became a glider over Alaska. What happened in the next few minutes is one of the great saves in civil aviation, and one of the main reasons airlines now take volcanic ash so seriously.

Into the Cloud

According to the Anchorage Daily News, the 747 entered the ash cloud at around 25,000 feet, some 75 miles northwest of Anchorage. The US Geological Survey gives the altitude as 27,900 feet. Either way, the crew had little chance of spotting it: as volcanologists point out, ash plumes can be difficult or impossible to tell apart from ordinary clouds.

Within 10 to 15 seconds in the cloud, all four General Electric CF6 engines flamed out. The ash, made of fine rock and glass, melted in the engines' hot sections and coated the turbine parts with glass. That also fooled the engine temperature sensors, which helped trigger the automatic shutdowns.

Green Dot Aviation's breakdown of KLM Flight 867 and how the crew restarted a 747 with all engines out.

A 747 Without Engines

With no engine thrust, the 747 was running on standby electrical power for its critical systems, and many cockpit instruments went dark. Captain Karl van der Elst and his crew began working through the engine restart procedure while the jet descended. It took several attempts.

In the cabin, the passengers knew something was badly wrong.

According to the Anchorage Daily News, the crew got engines 1 and 2 running again after six or seven attempts, and the remaining two restarted at around 11,000 feet. By then the 747 had lost more than 14,000 feet. The USGS puts the drop at 27,900 to 13,300 feet, more than two miles.

Landing in Anchorage

With power back, the crew flew the damaged 747 to Anchorage and landed safely. Nobody among the 231 passengers and crew was hurt. The aircraft was not so lucky: repairs cost around $80 million, and all four engines had to be replaced.

PH-BFC went back into service and flew for KLM for almost three more decades, including with KLM Asia, before retiring on 14 March 2018.

The air traffic control recording of KLM Flight 867, with subtitles, published by Air Crash Daily.

Why Aviation Learned to Fear Ash

KLM 867 came seven years after a British Airways 747 lost all four engines over Indonesia in ash from Mount Galunggung, told in La nuit où un 747 a perdu ses quatre moteurs. In the years that followed, a worldwide network of Volcanic Ash Advisory Centres was set up to track ash clouds and warn aircraft, and volcano observatories now issue alerts specifically for aviation.

The North Pacific is one of the busiest volcanic regions for air traffic. The USGS says at least 15 aircraft had been damaged by volcanic ash along North Pacific routes since 1980, including KLM 867, and around 80 such encounters had been recorded worldwide. Every time a volcano like Etna closes an airport today, it is following lessons written over Alaska in 1989.

USGS aerial footage of Redoubt Volcano, Alaska.

Sources: Wikipedia (KLM Flight 867), US Geological Survey Fact Sheet 030-97, Anchorage Daily News, SMU Research