WEATHER BRIEF

Volcanic ash

Often invisible, fallout from volcanic eruptions poses risks to aircraft at any altitude. The ash can cause widespread flight ops disruption.

By Karsten Shein
Comm-Inst, Climate Scientist


With forces that can exceed a nuclear blast, a volcanic ash plume shoots into the stratosphere. Close to the volcano, the cloud may contain larger bits of rock; moving further out, it will mainly hold small highly abrasive ash particles (<2 mm) that can melt inside engine combustion chambers.

Departing DPS (Ngurah Rai Intl, Bali, Indonesia), the globe-hopping business jet had just received clearance to FL410 and the crew members were setting up the autopilot for the long cruise back to their company's base at BOI (Boise ID). About 4 hours into their flight, as they slid through clear skies between the Philippines and Guam, the pilot got up to use the lavatory.

Just a few seconds thereafter, both engines began to surge. Before the pilot could return to his seat, engine 2 had flamed out. The copilot had already started the shutdown/restart procedure. Engine 1 flamed out moments later.

While the pilots were ascertaining what had happened, both of them detected a faint odor in the cockpit like burning wires and noticed a trace of St Elmo's fire moving along the edge of their windscreen. "Ash!" they both exclaimed simultaneously. Knowing they needed to either get the engines restarted or glide to safety, the crew divided the duties. The PIC turned the aircraft toward ROR (Roman Tmetuchl Intl, Koror, Palau) about 100 km to their south, while the copilot worked on restarting the engines.

On the radio with Manila Center, the PIC declared an emergency and their intentions to try to reach Palau. With their altitude, they might just make it without engines. Fortunately, at FL320 the copilot managed to get engine 1 restarted. But suddenly the cockpit began to fill with a fine dust and a strong sulfur smell. Engine 1 flamed out again.

Koror approach had cleared the airspace around the business jet turned glider. It appeared the aircraft had just enough altitude to make it to the airport. At 12,000 ft, the dust in the cabin had cleared but the windscreen had taken on a very milky opacity and they couldn't see the island directly ahead of them. With the pilot still flying the aircraft on instruments, the copilot repeated the restart process and managed to get engine 2 started. The aircraft limped to the runway for a safe landing with just 1 operating engine.

The aircraft manufacturer immediately dispatched a crew of mechanics to inspect the aircraft before ferrying it back to the US for repair. They found that the ash had completely clogged the combustion chambers and blocked the air filters. Engine 2 only restarted because enough of the ash had come loose as the inop engine had cooled.

In addition, the windscreen had been abraded to the point where the pilots couldn't see through it, and ash had even been pulled into the fuel tanks through the pressurization ports, contaminating the fuel. It had been very fortunate that a restart had worked at all.

Volcanoes


Volcanic Ash Advisory Center (VAAC) regions. Each VAAC uses satellite imagery, weather forecasts, and numerical dispersion models to monitor and forecast ash clouds that may impact aviation.

We live on a dynamic planet. The earth's crust is a thin hard shell that rests above an immense layer of viscous liquefied rock called the mantle. Compared to the crust, which is anywhere from around 3 to 25 km thin, the mantle is around 1800 km thick. It is heated from below by ever increasing pressure on the core with convection currents in the mantle gradually moving the crust around as a series of continental plates.

At their edges these plates may collide, move alongside each other, or spread apart. This movement creates the potential for releasing some of this molten material through holes or weak spots in the crust, along with residual gases and other debris. These holes in the crust are volcanoes.

There are 4 major categories of volcanoes:
Cinder cone volcanoes tend to be small channels through which lava erupts from time to time. The lava is also accompanied by copious ash and rock which form a cone-shaped mountain that is generally not more than 1000 ft (300 m) above the surrounding landscape.

Shield volcanoes are formed by very fluid lava pouring out of a vent. The low viscosity keeps the lava from clogging the vent and allows it to travel great distances after it emerges. This type of process accounts for some of the largest mountains on earth, including the Hawaiian Islands.

Lava domes form as high-viscosity lava erupts from a vent. It only travels a very small distance before solidifying, resulting in a large dome of lava building up around the vent. Lava domes can become rather large and are prone to violent eruptions because highly viscous lava can clog the vent, building pressure within.

Composite volcanoes are so named because of the layers set down over millennia of activity. These are normally the most dangerous to aviation. They occur mostly near the edges of continental plates where a plate has been forced beneath another. The collision creates numerous cracks and fissures in the top plate while the bottom plate is slowly pushed into the mantle, where it melts and increases pressure on the top plate.

Through the cracks in the top plate, the magma can erupt to the surface. In some places the vent remains an open, weak spot, and frequent eruptions occur. In these cases, the composite volcano slowly grows over thousands of years of eruptions, each depositing a new layer of lava and debris to further build the mountain.

Most iconic mountains in the world are composite "strato" volcanoes. Examples include Mt Fuji in Japan, Mt St Helens in Washington and Mt Pinatubo in the Philippines. While they may appear bucolic when dormant (between eruptions), most are not extinct (in that the vent is closed and they won't erupt again) and have a history of frequent and violent eruptions throughout geologic time.

Effects of eruptions

After an eruption, the volcano's main vent may become blocked by debris and cooled lava. In the case of shield volcanoes and cinder cones, this is not normally an issue. For a cinder cone, the debris that may fall back into the vent often remains loose, allowing the frequent release of pressure through relatively minor eruptions. On the other hand, the low viscosity lava of a shield volcano ensures that most of it flows away before it can solidify in the vent.

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