Effects of Volcanic Ash on Aircraft Systems
The latest volcanic eruptions have forced aircraft manufacturers to define specific limits on the acceptable concentration of ash in the atmosphere that causes no damage for jet engines after being ingested (Smith et.al, 2010). This issue was raised due to the creation of a new category of restricted airspace and differences in the particle’s size concentration, according to the vicinity of the eruption plume (ICAO, 2010a). Moreover, aircrafts’ inspections within two days of the ash-producing activity of eruption showed that the abrasive nature of the constituents of ash causes corrosive and erosive damage to aircraft components and negatively affects its performance (Vogel et. al., 2011).
Volcanic eruption columns contain such gases as water vapour, sulphur dioxide, oxides of nitrogen, chlorine hydrogen sulphides, and volcanic ash (Civil Aviation Safety Authority, 2011). These components are considered extremely fine particles of pulverised rock, which contains silica and oxides (Department of Defence, 2012). Sulphur dioxide forms sulphuric acid droplets, which are present above the lower troposphere and can be transferred to high altitudes and distributed over large distances (Grindle & Burcham, 2003). The resulting ash-acid mix of silicates, sulphur, and gaseous materials can form ice and develop electrostatic charges at high altitudes (Prata & Tupper, 2009). Moreover, these particles’ size is less than 15 microns, and they can be found more than 1000 mi from a volcano (Ahmed, Ishiguro & Akal, 2012).
The effect of volcanic ash on an aircraft is not considered life-threatening to passengers and crew; however, it is assumed as the cause of loss of performance characteristics (Vogel et. al, 2011). The issue of the abrasive nature of volcanic ash had not been so crucial until the latest volcanic eruption in April/May 2010, when the airline industry confirmed economic losses, ranging up to € 2, 5 billion for the period of regular flights’ delays (Vogel et. al, 2011).
Rapid dispersion of volcanic ash particles, regardless of the volcano’s ejection height, led to setting up Volcanic Advisory Centres in nine regions of the world, where the concentration of volcanic ash reaches ranges higher than 2 x 10³ gm ³ (Smith et. al, 2010; Vogel et. al, 2011). These areas are called “No Fly Zones”, and they stretch from the Pacific Ocean northwards, across South and North America, and the Aleutian and Kurile Island chains, falling into the chain through Kamchatka, the Philippines, and Japan, and crossing Indonesia, New Zealand, and Papua New Guinea, and reaching the islands of the South Pacific (Civil Aviation Safety Authority, 2011; Vogel et. al 2011). When entering the airspace, in which the ash density reaches the range between 2mg to 4 mg per cubic metre, airlines must produce the certificate of manufacturing compliance that an aircraft’s structural characteristics allow it to enter this Time Limited Zone (Smith et. al, 2010).
An aircraft’s exposure to ash-cloud hazards is defined by the ash concentration and time spent in a cloud (Prata & Tupper, 2009). The results of post-encounter inspections show that most damages can be identified within two days of eruptive activity of volcanic ash particles (Smith et. al, 2010). The pilot has to show an adequate response to a volcanic cloud encounter, taking into account threatening symptoms of the aircraft performance (ICAO, 2010b).
It should be taken into account that the airborne weather radar does not detect the volcanic ash concentration, since the size of its particles is too small (Department of Defence, 2012; Grindle & Burcham, 2003). The following is the list of symptoms that can be detected by the crew if volcanic ash is encountered:
1. Odour. Flight crew may notice the smell of sulphur or burned dust that is similar to the smell of electrical smoke (Prata & Tupper, 2009). It can derive from the airplane’s ventilation ductwork of some ingested gaseous ash particles that contaminated its interior (Grindle & Burcham, 2003).
2. Static discharges. The crew can notice the glow electrostatic phenomenon, similar to St. Elmo’s fire, which accompanies brush-like discharges of atmospheric electricity and can be observed along the wing tips and windshields, on the nose of an aircraft, and on the periphery of propellers (Moir & Seadbridge, 2008). In these instances, sparks similar to lightning can appear outside the windshields, while white glow can appear at the front of the engine inlets (Prata & Tupper, 2009). This phenomenon emerges due to the build-up of static charges on the airframe that must be discharged into the atmosphere to avoid its negative influence on the performance characteristics of avionics equipment (Wyatt & Tooley, 2012). Moreover, volcanic ash particles become ice-coated within less than 12 hours at the high altitudes up to 41, 000 ft and develop electrostatic charges (Grindle & Burcham, 2003). These electrostatic charges are dissipated from the surface of an aircraft during flight by the discharging of static electricity into the atmosphere (Wyatt & Tooley, 2012).
3. Changing engine conditions. The basic parameters of engine conditions may cause a loss of the aircraft’s performance and a possible flameout, leading to surging of the engine and torching of the tailpipe (Department of Defence, 2012). An engine performance is monitored by the following parameters: a) Engine Pressure Ratio (EPR), which is a primary indication system of the level of thrust that measures the exhaust and inlet pressures and drives the ratio between them (Wyatt & Tooley, 2012). When the engine’s thrust is low, the range of EPR indicates less than 1V (Moir & Seadbridge, 2008). This means that an aircraft may lose altitude due to possible engine failure (Allerton, 2009); b) Exhaust Gas Temperature (EGP), whose exceeding levels can lead to life cycle reduction of engines’ parts (Linke-Diesinger, 2008). Gas temperature limits are measured by the take-off thrust setting and indicated by Turbine Inlet Temperature (TIT), which is determined by a gas path in the hot section of the turbine (Linke-Diesinger, 2008). When the EGP level is reduced, it means that fuel is not efficiently mixed and there is a lack of air, passing through the engine and causing the loss of thrust (Grindle & Burcham, 2003). c) Fuel Flow, which is used to monitor the actual consumption of the engine and should match the engine thrust and aircraft climb (Linke-Diesinger, 2008). Therefore, when the engine’s thrust lowers, the fuel flow level falls down, causing the airplane to lose altitude (FAA, 2008a).
4) Slow restarts of the engine. Contaminated air results in the engine’s inability to maintain altitude or the Mach number (Department of Defence, 2012). The Mach number is considered as the ratio of the true airspeed of the aircraft, which corresponds to the level of the speed of sound and informs the pilot of an inadequate airspeed (FAA, 2008a). Shock waves form when the Mach number level reaches the speed of sound, and this means that the airplane has reached the required altitude (FAA 2008b). Therefore, terminal loss of thrust caused by damages of the engine’s parts make a successful restart possible, when clear air is regained (Department of Defence, 2012).
5) Emergence of haze. Dust settled on the aircraft’s surface can be observed by the crew and passengers (Prata & Tupper, 2009).
6) Decrease and erratic fluctuation of airspeed. This can cause an aircraft to emerge in the conditions of altitude losses or turbulence (FAA, 2011). The speed at which an aircraft is moving through the air is called the true airspeed (TAS) (FAA, 2008a). The instrument of the TAS indicator is navigational equipment that allows the pilot to rotate the subdial in order to make correspondence of the outside air temperature with the altitude pressure (Allerton, 2009). The pilot may experience lack of response to changes in throttle settings due to changes in the regulation of thrust, when the navigation system is contaminated with volcanic ash particles (Allerton, 2009). The control of airspeed requires the engine throttle input with the response by actuating subsystems and correspondence to a small change in forward force (Tewari, 2011). The engine capacity and fuel supply are predetermined by the changed engine conditions and engine readings (Moir & Seadbridge, 2008).
7) Loss of cabin pressurization. Another cause of the altitude loss can be depressurization of the aircraft, which results in a dangerous degree of hypoxia to the crew and passengers (Kundu, 2010). Regardless of the fact that modern commercial aircrafts can recycle 50% of cabin air through high-efficient particulate filters, volcanic ash consists of sharp rock fragments that can easily damage forward-facing surfaces of an aircraft (Grindle & Burcham, 2003; Kundu, 2010).
8) Loss of visibility through cockpit windows and sharp distinct shadows of landing lights (Department of Defence, 2012). Contaminated with sulphur dioxide, windscreens become opaque upon the encounter with volcanic ash (Vogel et. al, 2011). Although ice-coated particles are much less destructive to the airplane’s surface, the ice developed by them can cause sufficient damage to the engine, as it could melt in the compressor (Grindle & Burcham, 2003).
The abovementioned symptoms indicated that aircraft structures, aircraft systems, and propulsion systems are these areas that are damaged by the abrasive constituents of volcanic ash (Department of Defence, 2012). The minimum equipment lists, whose malfunction may alter the aircraft performance characteristics and should be considered by operators before dispatching an aircraft, are given in “Appendix A”.
As it has been mentioned before, the ash-acid mix is highly corrosive and can cause further damage to the aircraft structure and systems at high flight altitudes upon this mix’s rapid dispersal over great distances (Civil Aviation Safety Authority, 2011; Vogel et. al, 2011). Post-flight maintenance inspections have revealed contaminating, erosive, blocking, and plugging damages to the following aircraft systems and structures. See the table below:
-Wing, stabilizers and fins;
- Engine inlets and cowls;
-Propellers and rotor blades;
- Navigation and landing lights;
-Coverage with frost;
-Scratched by hard, sharp rock fragments;
- Corrosion and busting of the thin-walled sections;
- Gas path erosion, corrosion, and failure of a relevant component;
- Contamination, plugging, and casting of sharp distinct shadows;
- Air bleed system;
- Oil and hydraulic systems;
- Pilot static system;
-Electrical and avionics equipment;
- Contamination and blockage of supply nozzles through insufficient mixing of fuel and air;
- Contamination and deterioration;
- Blockage and electrostatic discharges;
- Arcing, short circuiting, and intermittent failures
-Cooling air system;
- Gas turbine (if applied)
-Erosion of the abradable seal coatings until its further liberation; hard carbon production; compressor discharge;
-Blockage of the rotating and static components and cooling holes;
-Erosion of the internal air-washed parts and glassification of sand particles.
Table 1. Affected Areas of an Aircraft by the Constituents of Volcanic Ash (Department of Defence, 2012, pp. 2-3; Smith et. al, 2010, p. 14; Gicquel, 2011, p. 327).
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