Volcanoes and Aviation: Part 1/2

Volcanoes and Volcanic Activity

Volcano is a natural phenomenon and it is caused by converging and diverging tectonic plates of the Earth’s crust. In the last couple of decades, volcanic activity has increased exponentially. There are many interesting theories as to why volcanic activity is increasing. One of the theories is self-evident and that is of David Wilcock. In his book “The Divine Cosmos”, he showed an increase in volcanic activity by correlating coronal mass ejections (CMEs) or sun activity. His theory suggested a violent increase in volcanic activity in 2003. University of North Dakota performed a research in 2003 revealing some staggering results. Seven months into 2003, there were 82 eruptions whereas there were only 12 eruptions in the entire year of 2002. That’s a 500% increase in volcanic activity worldwide.

Constant increase in volcanic activity around the world poses a direct threat to wildlife, populated surrounding areas and transportation routes. Since and even before 1875, volcanic activity was on the rise and the future according to Wilcock theories looks rather vicious. Many scientists believe Wilcock’s theories to be controversial however much of his theories have been proven through time.

Currently there are around 500 active and 1,500 potentially active volcanoes around the world. The question here is what does volcanoes and increasing volcanic activity have to do with aviation? The answer to this question lies in the composition of the lava. There are many types of erupted magma, each of them are categorised by percentage of silica it contains at the time of eruption. Felsic lava, which contains high percentage of silica (>63%), is predominantly the most lethal to aviation. The chemical compound silicon dioxide also known as silica is the major constituents of glass and when the volcano erupts, ash travels through the air very quickly and can rise to altitudes of around 70,000 ft. or more and travel more than 100 km. When silicon is exposed to the oxygen, silica is formed and at very high temperatures like the ones found in combustion chambers of aircraft engines, allow it to melt and stick to the turbine blades resulting in either reduced performance or engine failure.

Volcanic Areas and Flight Routes

The areas that are worst affected are the South Asia and Americas as both borders Ring of Fire, which is where the most earthquakes and volcanic eruptions take place. Ring of Fire is also a threat to neighboring countries and states. The North American and Eurasian tectonic plates meet in Iceland, resulting in several volcanoes. These volcanoes are a hazard to many countries including United Kingdom. As it became evident in April 2010, when volcano in South Iceland erupted disturbing the UK airspace. Guardian the newspaper on 9th May 2010 claims that British Airways lost over £100 million in six days because of volcanic ash and airport shutdowns.

Airspace around the globe is a very busy place with thousands of aircrafts in the matrix. Atlantic region and Ring of Fire, both airspaces accommodate hundreds of flights per day and thousands of lives are at risk while flying over these volatile territories.

As examples, we will use Air New Zealand and Philippine Airlines and their routes. Air New Zealand and Philippine Airlines, both operate at international and domestic levels. Air New Zealand fleet consists of Boeing 747, 777 and 787 with Airbus 320 and Philippine Airlines fleet consists of Airbus 319, 320 and 330 with Boeing 737 and 747. Operating fleet of both airlines use turbofan jet engines manufactured by Rolls-Royce and/or General Electric. To understand the problem, it’s imperative to know what is a turbofan and how does it operate. However, before we talk about turbofan engines, I want to discuss volcanic ash and its characteristics.

Composition of Volcanic Ash

Volcanic ash is made up of extremely fine particles of pulverized rock, the composition reflects the composition of the magma inside the volcano. The composition of volcanic ash varies from one volcano to another. However, it is mainly made up of silica (> 50%) together with smaller amounts of the oxides of aluminium, iron, calcium and sodium. The silica is in the form of glassy silicates and under the Scanning Electron Microscope (SEM) resembles sharp-edged glass shards. The glassy silicate material is very hard, typically of hardness level 6 on the Moh’s scale with a proportion of material of hardness equivalent to quartz level 7, all of which in pulverized form is extremely abrasive. As a matter of fact, volcanic ash is used as an abrasive powder in several commercial applications. The abrasive nature of volcanic ash results in damaging the aircraft structures, cockpit windows and engine parts.

In addition to the abrasive nature of volcanic ash, another important property is its melting point. Being made up predominantly of glassy silicates, the melting temperature, 1100°C, is below the temperature of jet engines operating temperature, 1400°C. Volcanic ash can melt and be deposited in the hot sections of the jet engine core and the nozzle guide vanes.

The solid particles from a volcanic eruption are extremely varied, ranging from extremely fine particles (< 5 μm) to large rocks. The term used by geologists to describe the whole range of particles is ‘tephra’ from the Greek word for ash. The particle size in a volcanic ash cloud decreases with time as the larger, heavier particles settle out from the cloud. The clouds of volcanic ash that are encountered by aircraft at some distance from the eruption are mainly comprised of the smallest particles (< 0.1 mm).

Volcanic clouds may also contain many gases including water vapour, sulphur dioxide, chlorine, hydrogen sulphide and oxides of nitrogen. While the proportion of each of these gases varies widely, the major constituent gases are water vapour, sulphur dioxide and chlorine. In their gaseous form these constituents are not harmful but oxidation and hydration of the sulphur dioxide forms sulphuric acid, which is very dangerous.

The resulting ash and acid mix is highly corrosive and can cause damage to jet engines and pitting on windscreens, and it will present a long-term maintenance expense for airliners operating regularly in contaminated airspace with even low concentration of such ash and acid particles.

Volcanic Ash Characteristics

The volcanic eruption is usually divided into three dynamic systems, gas thrust, convective thrust and umbrella region (or mushroom). In the system of umbrella region, the ascent of ash cloud begins to slow down in response to gravity and the temperature variations at the Tropopause, with the top spreading radially in the beginning and then in one direction. This is the region of most concern to aviation because this region is normal cruising level for most aircraft which is 30,000 to 45,000 ft.

Ash Cloud’s Electrical Phenomena

The occurrence of lightning in volcanic ash clouds has been reported since ancient times. Frequently such lightning displays can be quite spectacular and clearly indicate that volcanic ash clouds are highly charged electrically.

Moreover, one of the ways of recognizing that an aircraft has encountered volcanic ash is the static electricity discharge. The static electric charge on the aircraft also creates a ‘cocoon’ effect which may cause a temporary deterioration of VHF or HF communications with ground stations. It has been demonstrated that the potential gradient of the electric field in a volcanic ash cloud frequently reaches ±3 kv/m and can reach up to 10 kv/m.

The associated lightning can be of use in monitoring the location and extent of the cloud. It has also been suggested that the gradient of electric field strength could be monitored by equipment which could be installed on aircraft, thus providing an independent warning to aircraft of anomalous high electric charges in the atmosphere which then could be associated with volcanic ash or thunderstorms, both of which are to be avoided.

Cloud Movement

In the umbrella region, the extent of the cloud and its movement away from the volcano site depend on its natural dispersion and transport variables due to the upper tropospheric and stratospheric winds.

The strongest winds are usually in the higher levels of the troposphere, 30,000 to 45,000 ft., which is also the region in which most airliners cruise. This increases the probability of jet aircraft encountering volcanic ash cloud of significant concentration hundreds of kilometers from the volcano source.

The movement of a volcanic ash cloud from the volcano site and gradual deposition of the larger ash particles during the first few hours has important consequences for airports located within less than one hundred kilometres downwind of a volcano.

Effect on Aircraft

Volcanic ash damages the jet turbine engines, abrades cockpit windows, air frame and flight surfaces and clogs the pitot-static system. The ash also enters air conditioning and equipment cooling systems and contaminates electrical and avionics components, fuel and hydraulic systems and cargo and smoke-detection systems.

Effect on Turbofan Engines

The effect of volcanic ash on turbofan engines has been studied in detail from the inspection of old jet engines which are exposed to volcanic ash during flight and from ground tests of new jet engines into which volcanic ash mixtures have been introduced intentionally to study the effect.

Three effects contribute to the overall engine damage. The first, and most critical, is the fact that volcanic ash has a melting point below jet engine operating temperatures. The ash is made up predominantly of silica with a melting temperature of 1100°C, while at normal thrust the operating temperature of jet engines is 1400°C. The ash melts in the hot section of the engine and fuses on the high-pressure nozzle guide vanes and turbine blades. This reduces the guide vane’s throat area causing the static burner pressure and compressor discharge pressure to increase rapidly which causes engine surge. This effect alone can cause immediate thrust loss and engine flame-out. Earlier generations of jet engines, which operated at lower temperatures, were probably less vulnerable to this effect.

During the inspections, the volcanic ash deposits on the nozzle guide vanes were found to be very brittle at room temperature and easily broken and removed. These studies have led to interesting solutions such as shutting down the engines in flight and then restart them at a lower altitude. The sudden thermal and pressure shocks of the cold air during the restart process coupled with the cooling of the fused ash deposits seems to break off much of the deposits.

The volcanic ash also erodes compressor rotor paths and rotor blade tips, causing loss of turbine efficiency and engine thrust. The erosion also results in a decrease in the engine stall margin. The main factors that affect the extent of the erosion of the blades are the hardness, size, concentration, and impact velocity. Although the erosion takes longer than the melting of volcanic ash, the erosion damage is permanent and irreversible. Reduction of engine thrust to idle slows the rate of erosion of the blades but cannot eliminate it entirely because the engine is still ingesting contaminated air.

In addition to the melting of the volcanic ash and the blade erosion problems, the ash clogs flow holes in the fuel and cooling systems. In ground tests of jet engines subjected to forced volcanic ash ingestion, a deposition of black carbon-like material was found on the fuel nozzles. Analysis confirmed that the contaminating material was predominantly carbon, and although the main fuel nozzle appeared to remain clear, the swirl vanes which atomize the fuel were clogged. Such a condition would render engine restart very difficult, because there is no way for the material to break off during restart attempts.

High Bypass Turbofan Engine

The turbofan is a type of aircraft jet engine based on the similar principals of gas turbine engine. It produces thrust by using a combination of a ducted fan and jet exhaust nozzle. Air stream is divided into two parts; first one passing through the core providing oxygen to burn the fuel and second stream passes over the core and re-joins the high-speed air produced by the core. By this method, acoustic pollution is also reduced. The slow bypass air produces thrust more efficiently than the high-speed air from the core which results in reduced specific fuel consumption.

Net exhaust speed of a turbofan engine is much lower than of a turbojet, which makes turbofan more efficient at subsonic speeds. Subsonic efficiency and reduced noise makes turbofan jet engines the perfect choice for commercial airliners and they are indeed used in every commercial airliner up to date.

The air is drawn in by the huge fan at the inlet, which slows down the velocity and increases the pressure in accordance with Bernoulli’s Equation. Part of the air continues its path into the core where it is compressed. Then the fuel is added in the combustion chamber to ignite the high-speed air, which then passes through the turbine, propelling them consequently propelling the whole of the engine. Then the air leaves at the nozzle where it also mixes with the bypass air resulting in thrust.

Low specific thrust and high bypass ratio are achieved by replacing the multi-stage fan with a single stage unit. The inlet fan is scaled to achieve the desired net thrust. The core must generate enough power to drive the fan and its design flow and pressure ratio. Although there have been many improvements in turbine and fan blades design and materials, single stage fan limits the specific thrust, therefore limiting the jet velocity. High bypass ratio and low specific thrust turbofan has a high thrust lapse rate with rising flight speed. Therefore, engine will be oversized to provide sufficient thrust at climb and cruise speeds. These huge engines add extra weight to the aircraft demanding a reasonable length runaway to take off safely.

Engine Failure and Reduced Performance

Volcanic ash has threatened aviation since early jet flights and because of the amount of damaged caused, engineers have divided the encounters into several categories to tackle the situation more efficiently.

Severity of Encounters

  • Class 0 – Acrid odour, electrostatic discharge
  • Class 1 – Light cabin dust, EGT fluctuations
  • Class 2 – Heavy cabin dust, exterior and interior abrasion damage, windows frosting
  • Class 3 – Engine vibration, erroneous instrument readings, hydraulic-fluid contamination, damage to engine and electrical systems
  • Class 4 – Engine failure requiring in-flight restart
  • Class 5 – Engine failure or other damage leading to crash

Every class has its own unknown variables that require in-depth research and development to prevent it in future. This research is solely based upon engine failure and reduced performance, which covers Classes 4 and 5.

The common theory among the people was that volcanic ash clogs up the engine as an air filter gets clogged up when exposed to heavy smog. However, this is partially true and this also sets a basis of the research key areas. BBC News revealed the fact in one of its videos published on YouTube and BBC website around 2010. The fact is that the volcanic ash contains high quantity of silica. This substance melts when it goes through the combustion chamber because of the high temperature. This melted glass like substance sticks to the turbine blades and hardens because of the temperature drop. Turbine blades are manufactured with microns of accuracy and when this substance sticks to the surface, it obstructs the hot air from passing through which results in reduced performance of the engine. If the engine is exposed to high concentration of volcanic ash for a certain period, then turbine ends up choking, resulting in engine failure.

The theory behind restarting the engine in-flight is that the engines are shut down completely. Then the aircraft glides down to a lower altitude while allowing the cold air to pass through the engine at high velocity. This cold air allows the hardened silica to breakoff which allows the engine to function again.

Concluding Remarks

I hope this post has given you a good idea as to why volcanic activity is so dangerous for aviation and why airports shutdown at the time of such events. In the next post (part 2/2), I will list several methods that are used in present day for monitoring volcanic activity for the purposes of aviation safety, and methods that will help prevent engine failures.

*list of information sources is available on request

Note: I recognize and appreciate the respective owners of this information and I do not intend to take credit for someone else’s work. This post intends to bring the useful information together for the avid readers.

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