Volcanic Ash

1. Severe volcanic eruptions which send ash and sulphur dioxide (SO2) gas into the upper atmosphere occur somewhere around the world several times each year. Flying into a volcanic ash cloud can be exceedingly dangerous. A B747−200 lost all four engines after such an encounter and a B747−400 had the same nearly catastrophic experience. Piston− powered aircraft are less likely to lose power but severe damage is almost certain to ensue after an encounter with a volcanic ash cloud which is only a few hours old.
2. Most important is to avoid any encounter with volcanic ash. The ash plume may not be visible, especially in instrument conditions or at night; and even if visible, it is difficult to distinguish visually between an ash cloud and an ordinary weather cloud. Volcanic ash clouds are not displayed on airborne or ATC radar. The pilot must rely on reports from air traffic controllers and other pilots to determine the location of the ash cloud and use that information to remain well clear of the area. Additionally, the presence of a sulphur-like odor throughout the cabin may indicate the presence of SO2 emitted by volcanic activity, but may or may not indicate the presence of volcanic ash. Every attempt should be made to remain on the upwind side of the volcano.

Source: Aeronautical Information Manual, ¶7-6-9

• Class 0
• Sulfur odor noted in cabin.
• Anomalous atmospheric haze observed.
• Electrostatic discharge (St. Elmo’s fire) on windshield, nose, or engine cowls.
• Ash reported or suspected by flight crew but no other effects or damage noted.
• Class 1
• Light dust observed in cabin.
• Ash deposits on exterior of aircraft.
• Fluctuations in exhaust gas temperature with return to normal values.
• Class 2
• Heavy cabin dust.
• Contamination of air handling and air conditioning systems requiring use of oxygen.
• Abrasion damage to exterior surfaces, engine inlet, and compressor fan blades.
• Pitting, frosting, or breaking of windscreen or windows.
• Minor plugging of pitot-static system, insufficient to affect instrument readings.
• Deposition of ash in engine.
• Class 3
• Vibration or surging of engine(s).
• Plugging of pitot-static system to give erroneous instrument readings.
• Contamination of engine oil or hydraulic system fluids.
• Damage to electrical or computer systems.
• Engine damage.
• Class 4
• Temporary engine failure requiring in-flight restart of engine.
• Class 5
• Engine failure or other damage leading to crash.

Source: USGS, Table 1

Melting and resolidification of ash within jet turbine engines have been identified as the primary mechanisms responsible for engine failure in an ash encounter. The melting temperature of the magmatic silicate glass in ash is lower than the operating temperatures of modern turbine engines; consequently, ingested ash particles can melt in hot sections and then accumulate as re-solidified deposits in cooler parts of the engine, causing ignition flame-out and engine shutdown. In two encounters ranked as class 4, climb out from the cloud at maximum thrust was identified as a key operational condition for engine shutdown. When engine power increased, more ash-laden air was ingested by the jet turbines and combustion temperatures were raised; thus, conditions favorable for substantial melting of ash particles were met. This lesson has been incorporated into guidance to pilots about recommended actions to take during an encounter.

Source: USGS, pg. 3

[The 17 June 1991 incident] is notable in that prolonged exposure to dilute ash may have contributed to significant damage. That incident involved the same aircraft (a B747–200B) as incident 1991–14; both flights were operating between South Africa and Southeast Asia in the aftermath of the June 1991 eruption of Mount Pinatubo. In both encounters, the aircraft was operating in airspace at distances in excess of 500 km from the volcano. In [the 15 June 1991 incident], the crew noted static discharge lasting approximately an hour, but all engine parameters were normal during flight, and no problems were experienced with any of the aircraft systems. Two days later, the aircraft flew for several hours through an area coincident with remote-sensing evidence of an ash cloud before one engine lost power and a second engine was shut down by the crew. During descent, the engine that had been shut down was restarted, and the aircraft made a successful landing. These events raise the possibility of significant damage from cumulative exposure to dilute ash.

Source: USGS, pg. 5

The 2006 Gulfstream II incident is notable for involving a different mechanism for engine shutdown than melted ash. The aircraft was flying over Papua New Guinea at 11.9 km (39,000 ft) in apparently clear air with no ash or sulfurous odors noted by the cockpit crew. Between 39,000 ft and descent to 24,000, both engines failed and then were restarted; the aircraft landed safely. The operator and engine manufacturer conducted a thorough investigation, including borescope analysis of the engine and fuel analysis. The manufacturer concluded that a cylindrical filter in each fuel-flow regulator may have become blocked by volcanic ash, which at that altitude could have caused loss of fuel flow and thus engine shutdown; on descent, the increasing pressure would have substantially cleared the filter and allowed the engine to restart. The likely source of the ash was an eruption of Manam Volcano in Papua New Guinea.

Source: USGS, pg. 5

The highest concentration of active volcanoes lies around the rim of the Pacific Ocean, the so-called "ring of fire", which stretches northwards, more or less continuously, along the western edge of South and North America, across the Aleutian and Kurile Island chains, down through Kamchatka, Japan and the Philippines and across Indonesia, Papua New Guinea and New Zealand to the islands of the South Pacific. Other active regions are to be found in Iceland, along the great rift valley in Central and East Africa, and in countries around the Mediterranean.

Source: ICAO Doc 9691, pg. I-i_1

• The eruption column which results is usually divided into three dynamic regimes: "gas thrust", "convective thrust", and "umbrella region" (or "mushroom") as shown in [the figure].
• The gas thrust region is produced by the sudden decompression of superheated volatile constituents dissolved in the ascending magma. This produces a jet of fluids and pulverized rock material of extremely high kinetic energy at the mouth of the volcano vent, the speed of which in extreme cases could exceed 500 kt. Such exit vent speeds may reach supersonic speed depending on the ambient conditions at the time.
• If insufficient air is entrained in the gas thrust region, the column remains denser than the surrounding atmosphere and, as the initial kinetic energy dissipates, the column collapses due to gravitation without forming a convective thrust region. The convective thrust region largely controls the ultimate height of the column and hence is critical to the eruption's potential concern to aviation. It is clear from the foregoing that the hotter the original jet of fluidized material at its release form the vent, the higher the thermal energy which can be carried through to the convective thrust region and the higher the column top.
• The third dynamic region of the volcanic ash column is the "umbrella region," to top of the mushroom-like ash cloud as its ascent begins to slow in response to gravity and the temperature inversions at the tropopause, with the top spreading radially to begin and then predominantly in one or more particular directions in response to the upper winds at different levels of the atmosphere. This region of most concern to aviation because vast volumes of airspace at normal jet aircraft cruising level of 10 to 14 km (30,000 to 45,000 ft) become contaminated with high concentrations of volcanic ash.

Source: ICAO Doc 9691, ¶2.2.1

Volcanic ash is mostly glass shards and pulverized rock, very abrasive and, being largely composed of siliceous materials, with a melting temperature below the operating temperature of jet engines at cruise thrust. The ash is accompanied by gaseous solutions of sulphur dioxide (sulphuric acid) and chlorine (hydrochloric acid). Given these stark facts, it is easy to imagine the serious hazard that volcanic ash poses to an aircraft which encounters it in the atmosphere. Volcanic ash damages the jet turbine engines, abrades cockpit windows, airframe and flight surfaces, clogs the pitot-static system, penetrates into air conditioning and equipment cooling systems and contaminates electrical and avionics units, fuel and hydraulic systems and cargo-hold smoke-detection systems. Moreover, the first two or three days following an explosive eruption are especially critical because high concentrations of ash comprising particles up to ~10 μm diameter could be encountered at cruise levels some considerable distance from the volcano. Beyond three days, it is assumed that if the ash is still visible by eye or from satellite data, it still presents a hazard to aircraft.

Source: ICAO Doc 9691, ¶4.1

• The occurrence of lightning in volcanic ash columns has been reported since antiquity. Moreover, one of the prime means of recognizing that an aircraft has encountered volcanic ash is the static electricity discharge exhibited by St. Elmo's fire at point on the airframe and the glow inside the jet engines. The static electric charge on the aircraft also creates a "cocoon" effect which may cause a temporary deterioration, or even complete loss, of VHF or HF communications with ground stations.

Source: ICAO Doc 9691, ¶2.3.1

• There are basically three effects which 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 with thrust settings above idle. The ash is made up predominantly of silicates with a melting temperature of 1 100°C, while at normal thrust the operating temperature of jet engines is 1 400°C. The ash melts in the hot section of the engine and fuses on the high pressure nozzle guide vanes and turbine blades. This drastically reduces the high pressure turbine inlet guide-vane throat area causing the static burner pressure and compressor discharge pressure to increase rapidly which, in turn, causes engine surge. This effect alone can cause immediate thrust loss and possible engine flame-out. Earlier generations of jet engines, which operated at lower temperatures, were probably less susceptible to this effect. The general tendency, however, is to increase engine operating temperature as each successive family of jet engines is introduced, thereby increasing power but with improved specific fuel consumption. The melting/fusing effect of volcanic ash in jet engines will, therefore, continue to be a hazard in the future.
• During the strip-down inspections, the fused volcanic ash deposits on the high pressure nozzle guide vanes were found to be very brittle at room temperature and easily broke up and fell off the nozzle guide vanes. It seems clear that this can also happen when contaminated engines are shut down in flight and then restarted. The sudden thermal and pressure shocks of the ram air during the restart process, coupled with the cooling of the fused ash deposit when the engine is reduced to idle, seem to break off much of the deposit. Moreover, subsequent operation of the engines after restart, in the clearer air outside the ash cloud, also seems to further dislodge and evacuate some of the fused ash deposits.
• The volcanic ash being abrasive also erodes compressor rotor paths and rotor blade tips (mostly high pressure section), causing loss of high pressure 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 compressor blades are the hardness of the volcanic ash, particle size and concentration, the ash particle impact velocity, and thrust setting and core protection. Although this abrasion effect takes longer than the melting fusion of volcanic ash to shut down an engine, the abrasion damage is permanent and irreversible. Reduction of engine thrust to idle slows the rate of erosion of the compressor blades but cannot eliminate it entirely while the engine is still ingesting air contaminated by volcanic ash.
• In addition to the melting/fusing of the volcanic ash and the blade erosion problems referred to above, the ash can clog flow holes in the fuel and cooling systems, although these particular effects appear to be rather variable. 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 if not impossible, because there seems to be no tendency for the material to break off during restart attempts. Strip-down of the engines involved in two flame-out incidents (British Airways B747, 1982, and KLM B747, 1989), however, did not in fact show such extreme clogging of fuel nozzles and cooling systems, possibly due to insufficient exposure time to produce this effect.

Source: ICAO Doc 9691, ¶4.2

• In day visual meteorological conditions (VMC) a precursor to a volcanic ash encounter will likely be a visual indication of a volcanic ash cloud or haze. If a flight crew observes a cloud or haze suspected of containing volcanic ash they should be aware that a volcanic ash encounter is imminent and they should take action to avoid the contaminated airspace.
• Indicators that an aircraft is encountering volcanic ash are related principally to the following:
• Odour. When encountering volcanic ash, flight crews usually notice a smoky or acrid odour that can smell like electrical smoke, burnt dust or sulphur.
• Haze. Most flight crews, as well as cabin crew or passengers, see a haze develop within the aircraft cockpit and/or cabin. Dust can settle on surfaces.
• Changing engine conditions. Surging, torching from the tailpipe, and flameouts can occur. Engine temperatures can change unexpectedly, and a white glow can appear at the engine inlet.
• Airspeed. If volcanic ash fouls the pitot tube, the indicated airspeed can decrease or fluctuate erratically.
• Pressurization. Cabin pressure can change, including possible loss of cabin pressurization.
• Static discharges. A phenomenon similar to St. Elmo’s fire or glow can occur. In these instances, blue- coloured sparks can appear to flow up the outside of the windshield or a white glow can appear at the leading edges of the wings or at the front of the engine inlets.
• Any of these indicators should suffice to alert the flight crew of an ash encounter, and appropriate action should be taken to vacate the contaminated airspace as safely and expeditiously as possible.

Source: ICAO Doc 9974, ¶1.3 and 1.4