Volcanism is a formidable natural phenomenon, which is studied by a special science named volcanology. Lava flows and scorching clouds burning everything in their path, floods, powerful all-destroying earthquakes, and tsunamis that devastate the entire coastlines were repeatedly described both in scientific and popular literature. However, there is one of the phenomena of volcanic activity, which is usually overshadowed by its other catastrophic manifestations. Until recently, it was of interest for professionals rather than the general public. This phenomenon is air emissions of fine particulate matter or the volcanic ash. In contrast to catastrophic consequences of eruptions that have local and across-the-Earth, dot coverage (except for tsunamis), volcanic haze and ash falls affect large regions and even influence the global climate. However, there is also another threat posed by them, which became clear to the general public after the eruption of the Icelandic volcano named Eyjafjallajökull (Siebert, Simkin, & Kimberly, 2010). Major releases of ash into the atmosphere have paralyzed air traffic over the entire Europe. More than 100 thousand flights were canceled or postponed worldwide, about ten million passengers were unable to get home in time, and the airlines suffered a damage of 2.5 billion euro (more than three billion dollars) (Papale & Shroder, 2014). Therefore, it is possible to say that volcanic ashes pose a serious threat to aviation. As a result, the following work is dedicated to the study of the mechanism of formation of volcanic ashes, their effect on the aircraft, and possible preventive and protective measures that are to be taken by pilots and air traffic controllers in order to reduce or eliminate this effect.
The Formation of Volcanic Ashes
As the name suggests, volcanic ashes are formed when a volcano erupts. During the volcanic eruption, from the depths of the earth to its surface and the atmosphere three types of products are obtained: lava (molten rock), pyroclastics, or tephra (solid particles of different sizes: ashes –pieces the size of a speck of dust, lapilli –small pebbles, and volcanic bombs –large fragments) and various gases (Kusky, 2008). It is estimated that, in general, volcanoes erupt six times more pyroclastics than lavas. When magma (future lava) is located in the depth under tremendous pressure, a large amount of gas is dissolved in it. This mixture operates in accordance with the physical law: solubility of any gas in the liquid is directly proportional to the pressure. As magma approaches the surface, pressure drop results in degassing and the surplus gas emerges from it in the form of bubbles. Through cracks, the gas migrates to the surface and into the air in the form of haze, which are called fumaroles and are considered as a sign of volcanic activity. The most dangerous situation is created in the event when the gas cannot dissipate and, instead, it is accumulated in the ground. Pressure build-up can lead to a powerful explosion with destruction of the top of the volcano and even the entire volcanic edifice. Another type of volcanic disasters is the collapse of the top of a volcano to underground cavities formed during the eruption due to magma withdrawal. This collapse forms a caldera, a huge (with a diameter of 15-20 kilometers) rounded dip with a depth of several hundred meters (Kusky, 2008).
On the surface of the boiling lava lake in the crater of the volcano, hot gas is constantly emerging, which is the reason why lava boils and bubbles. By rising up at a high speed, the gas carries away small droplets of lava that quickly harden and turn into particles of volcanic ash. As a result, the ash column or ash plume rises above the volcano at high altitude (sometimes up to the stratosphere). After that, it is spread by air currents for hundreds and thousands of kilometers from the epicenter of eruption. Later, the ashes are deposited from the air by precipitation. Near the volcano, a single eruption may create a layer of ash and larger pyroclastics a few tens of meters thick (Lockwood & Hazlett, 2010). With increasing distance from the volcano, ash concentration in the atmosphere decreases and thickness of the layers of ash as well as the size of ash grains falls rapidly (Fig. 1).
The volcanic ash primarily consists of hard vitreous particles with sharp edges and sprayed rock. Therefore, it has extremely high abrasive qualities. By having in its composition silicate materials, the ash starts melting at the temperature that is lower than inside of modern jet engines during their work. Moreover, the cloud of volcanic ash may contain gaseous solutions of sulfur dioxide (which forms sulfuric acid when combined with water), chlorine (which forms hydrochloric acid when combined with water), and other chemical elements that have a corrosive effect on the airframe and are dangerous to human health (Casadevall, 1991). Given these facts, it is clear that the volcanic ash in the air can present a serious threat to a flying aircraft. Therefore, planes should avoid contact with it.
The Effect of Volcanic Ashes on the Aircraft
The abrasive nature of volcanic ashes may lead to devastating consequences for the aircraft (Fig. 2). First of all, it may affect immediate safety of the aircraft by causing unstable operation or failure of one or more engines, which leads not only to a decrease or loss of traction, but also to a failure of electrical, pneumatic, and hydraulic systems of the plane. It should be noted that the majority of aircrafts in the world’s air fleet is equipped with turbojet engines and their varieties, which work on the same principle. The working medium for these engines is air that is sucked into engine in large quantities (the more air is sucked in, the higher the thrust). Of course, together with this air, anything that is dispersed in it gets into the engine. In turn, a volcano supplies tons of exhaust to the atmosphere during its eruption and volcanic ashes are the most hazardous for the engine. Of course, different stones and rocks produced by eruption are even more dangerous, but they do not hold in the atmosphere for a long time. Ash particles are light enough and can stay there for weeks and even months. In case a hard rock will get into the engine, which is unlikely, it may damage compressor blades and even break them. Consequences include off-nominal operation of the engine, its stopping, destruction, and fire (Casadevall, 1991). However, ash is much more harmful.
Figure 2. The zones of the aircraft most damaged by the volcanic ashes.
In a turbojet engine of the aircraft, namely in its hot zone, there is an intricate system of air cooling. Cold air passes through thin channels of very fine holes and creates a shielding layer that protects elements of the combustor and turbine from overheating. However, volcanic ash has a specific chemical composition and structure, which in contact with a hot surface turns into a glassy mass that can tightly seal channels and openings of the cooling system. As a result, the system stops. Furthermore, fuel injectors in the combustion chamber also contain thin gaps, which can be similarly sealed with the glassy mass. As a result, the engine may stall without the possibility to run it again in the air (Fig. 3). Moreover, some of its parts may break, resulting in burnouts of the motor housing and fire. It should be noted that in this situation aircrafts equipped with piston engines are in a somewhat better position. In these motors, air is not used entirely for the purposes such as in a turbojet engine and not in such quantities. Besides, it passes through filters before entering the engine, but the danger of malfunction due to the presence of volcanic ashes in the air still remains (Casadevall, 1991).
Another problem caused by abrasive volcanic ashes is electrification of the aircraft. In particular, planes having a large mass and a high speed of flight are subject to the influence of electrostatic discharges. The electric charge can be acquired by the aircraft when flying through a cloud of ash. It largely depends on properties of the medium through which the airplane is flying (size and number of particles, their phase, state, and electrical charges on them as well as the magnitude of the electric field of the atmosphere). Moreover, it is affected by characteristics of the aircraft (its structure, particularly coating material, and parameters of engines) and the flight regime (engine power, altitude, and speed of the flight). All these characteristics affect the magnitude of electric currents flowing between the aircraft and the atmosphere. The electric potential of the plane increases until the start of air ionization and the outflow of electricity in the atmosphere as sparks and glowing crown rims. Intensive electrification of the aircraft may lead to the following consequences (Casadevall, 1991):
During radio reception on very high frequencies, weak background noise increases gradually, being accompanied by crackles. In the intervals between these noises, radio communication is restored;
During radio reception in the range of short frequencies, individual crackles turn into a continuous background noise, making communication impossible;
Irregular angular movements, i.e. arrows of radiocompass deflect to 120 grades or more, being accompanied by their delay in this position. At the same time, there is an intensive increase in noise in the telephone channel of the compass, which precludes the possibility of receiving radio signals.
Therefore, intense electrification due to the contact with volcanic ashes may cut communication between the aircraft and the mission control center, so the crew will have to change the altitude in order to restore it (Fig. 4). As a result, the plane can deviate from its course. Moreover, ashes may result in blocking of pitot and static pressure sensors, which causes unreliable airspeed readings and false alarms. Due to the abrasive nature of ashes, windshields of the plane may lose their transparency, making it more difficult for the crew to navigate and especially land the aircraft. Finally, contamination of air in the cabin by ashes may force the crew to use oxygen masks (Casadevall, 1991).
The abovementioned consequences of the contact of a plane with clouds of ash pose an immediate threat to its safety. However, volcanic ashes may also affect efficient and safe operation of the aircraft in the long-term perspective. First of all, acids the ash contains may cause erosion of external units of the aircraft. The ash also reduces effectiveness of cooling of electronic components. Moreover, it actively absorbs moisture, provoking short circuits that lead to failure and abnormal operation of a number of systems on board. A severe contamination of the ventilation system of an aircraft by ash may require cleaning or replacement of equipment due to pollution of the air circulation system and an abrasive effect on circulating components, clogged air cleaners, and air filters blockage. In case the crew makes an attempt to maneuver to evade contact with the ash cloud, it may potentially lead to a collision with another nearby aircraft. Finally, in case the volcanic ash falls on the runway, it may lead to deterioration in braking performance of the aircraft, especially if it is wet. In extreme cases, it may even result in the closure of the runway (Lockwood & Hazlett, 2010).
The Volcanic Ashes in the History of Aviation
The clouds of volcanic ashes pose an immediate threat to aviation. However, it should be noted that the abovementioned malfunctions can occur in case the aircraft flies right through the ash cloud that is formed during eruption of the volcano. Thus, it is possible to say that its destructive effect largely depends on concentration of ash particles in the air. However, the threat posed by volcanic ashes is not to be underestimated as there are many examples of aircraft malfunctions and accidents caused by them in the history of the aviation. For example, in June 1982, the passenger plane Boeing 747, which belonged to the British airlines, was flying at an altitude of twelve thousand meters from the city of Kuala Lumpur (Malaysia) to Perth (Australia) when it suffered the engine failure (Alcantra-Ayala & Goudie, 2010). The plane began to fall and only at a height of four thousand meters the crew was able to run one engine and later the other two. As a result, the plane made an emergency landing in Jakarta (Indonesia). It should be noted that the incident was caused by ashes thrown into the atmosphere in April 1982 during the eruption of Galunggung volcano, which is adjacent to the airline routes between South Asia and Australia (Alcantra-Ayala & Goudie, 2010). Three weeks later, a similar incident occurred at about the same place with the Boeing 747of Singapore Airlines, which got in the cloud of volcanic ash. The plane made an emergency landing in Jakarta due to the failure of two of the four engines. On December 15, 1989, the plane flying from Amsterdam with 231 passengers and 13 crew members on board started preparations for landing in Anchorage (Alaska, USA) (Alcantra-Ayala & Goudie, 2010). At an altitude of seven thousand meters, at a distance of 240 kilometers from the Redoubt volcano, the plane hit the ash cloud ejected ninety minutes before. The aircraft was trying to climb to get out of the volcanic ash cloud. However, the ash formed a glassy layer on the turbine blades. As a result, all four engines stalled and the aircraft was falling for about eight minutes. When pilots were able to restart the engines, the ground was less than two thousand meters away. However, the plane landed safely in Anchorage on the two engines. It should be noted that from each of the turbines, the mechanics extracted about 80 kilos of volcanic ash. All the four engines, as well as the navigation and electrical systems of the aircraft, had to be replaced. As a result, financial losses of the company owning the plane have amounted to about eighty million dollars (Alcantra-Ayala & Goudie, 2010).
In conclusion, it is possible to say that clouds of volcanic ashes pose an immediate threat to aviation. Therefore, it is imperative to develop measures to avoid it. By summarizing the listed information, it is possible to provide certain recommendations on preventing the abovementioned malfunctions caused by volcanic ashes by aircraft pilots as well as owners of air companies. First of all, the range of behavior of volcanoes may vary from a calm, steady outpouring of lava to highly explosive eruptions. During larger eruptions, many cubic kilometers of glass particles and the spray rock (volcanic ash) as well as corrosive and harmful gas may be thrown into the atmosphere. They can stay there for several hours, weeks, or even months. Volcanic eruptions may pose a direct threat to safety of an aircraft in the flight and create significant operational difficulties on the ground and in the air, which are located on the leeward side of the volcanic ash cloud. The situation may become even more difficult in case eruptions occur with high intensity and are of a continuous nature. Thus, timely receipt of reliable, full information on volcanic ash (observations and forecasts) is important in terms of reducing the risk to safety of an aircraft that is about to contact the volcanic ash. Having this information is important for strategic pre-flight planning and tactical plan changes during the flight when the likelihood of contact with the ash cloud may be assessed.
However, the mentioned preventive measures cannot guarantee complete safety from contacts with volcanic ashes. Therefore, pilots must know how to determine if an aircraft has entered the ash cloud. During daytime, contact with volcanic ash most likely may be preceded by visual detection of clouds or haze. In case flight crew members observe clouds or haze, which may contain volcanic ash, they should take necessary measures (maneuvers) to evade flying through polluted air. In general, signs that an aircraft is in contact with volcanic ash are as follows:
- Smell. Upon contact with volcanic ash, the flight crew may notice smoke or pungent odor, which may resemble a smell that occurs in case of an electric discharge or a smell of burnt dust and sulfur;
- Haze. Most of the flight crew and cabin crew or passengers may observe the emergence of haze in the cockpit of the aircraft;
- Changes in the operating mode of jet engines. In particular, there may be surging, afterburning of fuel in the exhaust pipe, and sudden stops. Temperature of the engine can suddenly change and white glow may appear in the zone of air intake;
- Changes in the airspeed. In case volcanic ash gets into the air pressure receiver, indicated airspeed may decrease or fluctuate randomly.
- Changes in the air pressure. The air pressure inside the plane may vary, including possible depressurization of the cabin.
- Static discharges. There may be a phenomenon analogous to the St. Elmo’s fire (a natural phenomenon that appears as a ghostly blue flame on the tops of ship masts, airplane wings, flagpoles, street lamps, and other high-pointed objects). In these cases, there may be a bluish flash on the outer side of the windshield or white light on the leading edge or the outside air intake.
Presence of any of these signs should be sufficient to alert the flight crew about dealing with ashes. As a result, appropriate measures should be taken in order to provide secure and speedy evacuation from the contaminated airspace. As a result, these measures will help to reduce or even avoid damage of the aircraft and prevent both financial and human losses.