The Weather Never Sleeps

Don't fly over volcanoes

The volcanic ash that Grace Swanson pours from a small box into a visitor's hand doesn't look particularly dangerous. Yet, it's the stuff that has stopped all four engines of at least three Boeing 747s, temporarily turning the airliners into the world's largest gliders. Unexpected encounters with volcanic ash have damaged many other aircraft, giving pilots tense moments. This is why the NOAA Volcanic Ash Advisory Center (VAAC) in Camp Springs, Maryland, which Swanson manages; the Anchorage, Alaska, VAAC; and seven similar centers in other nations keep a close watch on volcanoes.

When you put a little of the black ash from the Soufriere Hills volcano on the Caribbean island of Montserrat in the palm of your hand and lightly rub it with a finger it feels like flour. Press your finger down a little as you rub and you feel the grit; it's like a piece of emory cloth.

When a jet aircraft runs into a cloud of this stuff it quickly starts eroding all forward-facing surfaces, including the windshield, which in some cases has become too opaque to see through. That's only the beginning. The ash melts in the hot sections of a jet engine, and then fuses as a kind of dirty glass onto internal engine parts, causing the engines to surge and often flame out. Fortunately, this material becomes very brittle when it cools. This happens when the pilots shut down the engines and try to restart them. Repeated restart attempts can break free enough of the material for a successful restart. The ash can also clog the pitot tube, causing dangerously unreliable airspeed readings.

Unless pilots happen to see the cloud boiling out of a volcano or are alerted from the ground, they have no way of knowing that an ordinary-looking cloud they are about to fly into hides volcanic ash. Neither airplane nor ground weather radars detect it because ash particles are too small.

Volcanic ash won't damage a slower, piston-engine general aviation airplane as quickly as a fast-moving jet, but all pilots need to avoid volcanic ash for the sake of their airplanes and their lungs.

For example, when Alaska's Redoubt Volcano, which is about 110 miles southwest of Anchorage, erupted last March it sent an ash cloud across south-central Alaska, including Anchorage, where airlines cancelled several flights. The Los Angeles Times reported, Rural Alaskans depend on [general aviation] flights between hamlets, as do tourists. At Talkeetna Air Taxi, workers stayed inside their offices. Planes were covered and computers bagged to protect against the ash.

In addition to being abrasive, volcanic ash conducts electricity, which is bad news for electronic devices--whether aboard an aircraft or in an airport office. It is also why volcanic ash can cause spectacular displays of the bright blue or violet glow known as St. Elmo's fire.

Volcanic ash is nothing like the soft, fluffy material left behind by burning wood or paper. Instead it is pulverized rock, which can be harder than steel, and has sharp edges. It's usually mixed with corrosive gases such as sulfur dioxide that combines with water vapor in the air to create a sulfuric acid mist. This causes the odor that crews and passengers of jets that have flown into volcanic ash clouds think might be coming from burning electrical insulation.

Volcanic ash particles can be smaller than 0.001 millimeters in diameter with sulfuric acid adhering to them. They are tiny enough to travel deep into your lungs, to the end of the line in the alveoli where the oxygen you breathe goes into your blood and the carbon dioxide your body produces goes into the air to be exhaled. This is annoying at best to anyone and potentially fatal to someone suffering from respiratory illness.

The first 747 that volcanic ash endangered was British Airways Flight 9, which was cruising at 37,000 feet above the Indian Ocean after dark on January 24, 1982, when it flew into an ash cloud from the Mount Galunggung volcano in Indonesia.

Before all four engines failed, the crew and passengers saw St. Elmo's fire on the outside of the airplane and smoke and dust in the cabin. In fact, some passengers thought all four engines were on fire. The airplane descended to 13,500 feet above the ocean before the crew managed to restart all four engines and land safely at Jakarta, Indonesia. On approach the pilots could see through only a narrow strip of the windshield that the wipers had protected from blasting by the ash.

Three weeks later, it happened again when a Singapore Airlines 747 flew into ash from the same volcano, lost power from all four engines, and also managed to restart them in time to make a safe landing.

The third Boeing 747 to lose all four engines in an ash cloud was KLM Flight 867, which was descending through 27,000 feet on approach to Anchorage on December 15, 1989. The airplane--above the Talkeetna Mountains with elevations up to 11,000 feet--glided down 13,300 feet before the engines restarted and the jet landed at Anchorage. The ash was from Redoubt Volcano, approximately 150 miles away.

In 1987, prompted by the two Galunggung volcano encounters and other less dramatic incidents, NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) began supplying the FAA with global volcanic ash information. At the time of the KLM incident an ash alert was in effect and other flights had diverted.

In 1995 the International Civil Aviation Organization established the international system of Volcanic Ash Advisory Centers. Airlines and pilot organizations now offer training, including simulator training, in avoiding and, if necessary, coping with volcanic ash.

When a volcano observatory decides that an eruption seems likely, it sends the information to the FAA (or similar agencies in other nations), which issue notams to alert pilots. When one of the world's volcanic ash alert centers learns of an eruption, it begins issuing sigmets, which are alerts for atmospheric conditions that can be hazardous to all aircraft. Swanson explains that these are based primarily on infrared satellite images. If it's mixed with ordinary clouds, volcanic ash doesn't show up in visual satellite images.

Infrared satellites have different channels that detect various infrared frequencies. These channels detect the different infrared frequencies emitted by ice crystals at the tops of most clouds, and by volcanic ash.

In addition to locating volcanic ash, the sigmets also include 18-hour forecasts for ash locations. Computer models use meteorological data to forecast the winds at the ash's altitudes. Models using data on the eruption, including particle size, distribution, and how much mass has erupted, produce forecasts for how long the ash will remain in the air before settling to the surface or being scattered by the winds. Swanson says volcanic ash forecasting is very much a forecaster-computer mix. After all of these years quite a few of the people here, just by looking at the imagery, can sometimes say things such as 'This isn't going to last more than six hours.'

Except for Alaska, explosive volcanic eruptions that send ash high into the sky are rare in the United States. If you ever fly in Mexico, Central America, or South America, or on the air routes across the northern Pacific, you have to be more alert. An ordinary preflight weather briefing that includes notams will alert you to volcanic hazards and should keep you out of trouble, unless you see a volcano erupting and decide to fly over to take a look.