Drifting clouds of fine silica-rich particles pose severe danger to all forms of aviation.
Earth’s tectonic plates are constantly in motion atop a molten sea of magma. As they collide, volcanoes along their edges provide vents to relieve built-up pressure. Some of these release valves can explode with tremendous force.
Boulders larger than houses may be cast a mile or more from the volcano’s crater. But the majority of material ejected by the volcano will be only a few millimeters or less in diameter.
A typical ash plume is composed of pulverized rock from the disintegrated volcano, small droplets of cooling lava and a mix of gases—including water vapor, sulfur dioxide, bromine oxide and other toxic compounds.
Depending on the force of the explosion, the debris can be launched upward of 50,000 ft MSL. Fortunately for aviation, most of the larger particles that might cause immediate physical damage to aircraft will fall out of the atmosphere almost immediately.
The real problem to aviation is that the rest of the ash can be suspended in the atmosphere for days or weeks, often fed fresh material by a volcano that may continue to erupt for weeks or longer. To remain suspended, only weak updrafts are needed.
Once aloft, the plume is at the mercy of the general circulation of the atmosphere. Near the top of the troposphere, winds can easily exceed 100 kts in places, rapidly transporting ash thousands of miles from its origin. Caught in these upper air currents, ash plumes have been known to circumnavigate the planet.
These upper air patterns are, however, somewhat predictable. In general, the atmosphere can be divided into latitudinal belts, so ash from a tropical volcano will generally remain in the tropics while ash from a midlatitude volcano will remain in the midlatitudes.
But, unlike the tropical atmosphere, middle and higher latitude winds are also affected by transient waves in the polar jet. These highs and lows can wrap the ash around them and spin it to higher or lower latitudes.
Small particles, big problems
Ash particles themselves are extremely bad for aircraft. Unlike dust or sand, the particles tend to be extremely abrasive. What might look like a thin brownish or gray cloud deck can disguise an industrial-strength sandblaster.
The crew of the British Airways 747 had to land on instruments, despite VFR conditions, because the ash had effectively scoured the windscreen to the point where they had zero forward vision.
In addition, as the plume spreads out downwind, it looks less and less like a cloud and more like a weak brownish haze that even an astute pilot may not recognize as a danger. Also, as it spreads out, any water vapor it may have contained initially will either have evaporated or precipitated out.
This means that the ash cloud may not show up on a weather radar tuned to look for precipitation. By far the most dangerous aspect of volcanic ash is that it is basically an abrasive glass. The high-silicate or felsic type of lava that emerges from a stratovolcano when it erupts is liquid at a relatively low temperature of around 700°C.
Thus, a solid ash particle of felsic rock will melt easily within the hot sections of a typical jet engine, which can achieve gas temperatures of up to 2000°C but are generally operating between 900° and 1700°C.
Even low-silicate lavas would remelt in these conditions. When this ash is ingested into the engines, it does indeed remelt and adhere to nozzles, fan blades and any other exposed surface. This is what caused the flameout of the 3 Boeing 747s mentioned earlier.
The amount of ash that can be ingested into a turbine engine can be significant. In the case of the KLM 747, over 180 lbs of ash were removed from each turbine. Fortunately, the seized engines cool rapidly in the subfreezing air and, as the lava ash resolidifies, enough of it may break free to permit a restart.
Turbine engines are especially vulnerable to volcanic ash. As the ash is ingested, it melts easily in the hot sections, clogging injection ports and resolidifying on exhaust fans. Larger particles can also cause physical damage to compressor blades.
Volcanic ash poses 4 additional problems for aircraft. First, given its fine size, it can easily clog pitot-static system ports, rendering traditional pressure-based avionics, such as airspeed and altitude indicators, inoperative without warning.
If an aircraft is bringing fresh air into the cabin during ash cloud penetration, filtration systems can quickly become overwhelmed and allow toxic gas to enter the aircraft. Third, ash often carries an electromagnetic charge, which in high concentrations of ash can affect radio communications and navigation.
Finally, although not an immediate danger, the ash mixture is highly corrosive. It is quite likely that an aircraft that encounters ash will have to undergo a major (and expensive) overhaul, with replacement of many key components.
To put that in perspective, the KLM 747-400 had only 900 total hours when it encountered Mount Redoubt’s ash cloud, but the subsequent overhaul included replacing engines, fuel tanks and hydraulic systems to the tune of $80 million (about $138 million in 2010 dollars).
Nine volcanic ash advisory centers (VAACs) were created around the world after Mount Pinatubo erupted in 1991. This was good news for pilots. These centers have the sole purpose of monitoring volcanic activity and issuing alerts about the ash clouds.
Located in Anchorage AK, Buenos Aires (Argentina), Darwin NT (Australia), London (England), Montreal QC (Canada), Tokyo (Japan), Toulouse (France), Washington DC and Wellington (New Zealand), they issue Volcanic Ash Sigmets to give pilots a good idea about where the ash cloud can be found and where it is headed.
Despite active, around-the-clock monitoring and the issuing of advisories, aircraft occasionally fly through volcanic ash. There have not been many test flights to determine the best courses of action if a pilot encounters volcanic ash, but based on both common sense and analysis of the actions of aircrews who have had encounters, there exist a number of recommendations.
The first step is, of course, to recognize that you have entered the ash cloud. This can include smelling an electrical fire or sulfuric odor in the cockpit. The air inside the aircraft can also become hazy and surfaces may become coated with dust.
Uncommanded changes in airspeed, cabin pressure or engine performance are also an indication that ash may be affecting the aircraft. Finally, pilots may notice St Elmo’s fire—a weak, bluish static discharge—around the wind screen.